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<!-- Author(s):    nickg, bartv, markg                               -->
<!-- Contributors: eCos team                                         -->
<!-- Date:        2002/02/13                                         -->
<!-- Version:     0.02                                               -->
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<part id="kernel">
  <title>The eCos Kernel</title>

<!-- {{{ Overview                       -->

  <refentry id="kernel-overview">

    <refmeta>
    <refentrytitle>Kernel Overview</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>Kernel</refname>
      <refpurpose>Overview of the eCos Kernel</refpurpose>
    </refnamediv>

<!-- {{{ Description                    -->

    <refsect1 id="kernel-overview-description">
      <title>Description</title>
      <para>
The kernel is one of the key packages in all of eCos. It provides the
core functionality needed for developing multi-threaded applications:
      </para>
      <orderedlist>
        <listitem><para>
The ability to create new threads in the system, either during startup
or when the system is already running.
        </para></listitem>
        <listitem><para>
Control over the various threads in the system, for example
manipulating their priorities.
        </para></listitem>
        <listitem><para>
A choice of schedulers, determining which thread should currently be
running. 
        </para></listitem>
        <listitem><para>
A range of synchronization primitives, allowing threads to interact
and share data safely.
        </para></listitem>
        <listitem><para>
Integration with the system's support for interrupts and exceptions.
        </para></listitem>
      </orderedlist>
      <para>
In some other operating systems the kernel provides additional
functionality. For example the kernel may also provide memory
allocation functionality, and device drivers may be part of the kernel
as well. This is not the case for eCos. Memory allocation is handled
by a separate package. Similary each device driver will typically be a
separate package. Various packages are combined and configured using
the eCos configuration technology to meet the requirements of the
application.
      </para>
      <para>
The eCos kernel package is optional. It is possible to write
single-threaded applications which do not use any kernel
functionality, for example RedBoot. Typically such applications are
based around a central polling loop, continually checking all devices
and taking appropriate action when I/O occurs. A small amount of
calculation is possible every iteration, at the cost of an increased
delay between an I/O event occurring and the polling loop detecting
the event. When the requirements are straightforward it may well be
easier to develop the application using a polling loop, avoiding the
complexities of multiple threads and synchronization between threads.
As requirements get more complicated a multi-threaded solution becomes
more appropriate, requiring the use of the kernel. In fact some of the
more advanced packages in eCos, for example the TCP/IP stack, use
multi-threading internally. Therefore if the application uses any of
those packages then the kernel becomes a required package, not an
optional one.
      </para>
      <para>
The kernel functionality can be used in one of two ways. The kernel
provides its own C API, with functions like
<function>cyg_thread_create</function> and
<function>cyg_mutex_lock</function>. These can be called directly from
application code or from other packages. Alternatively there are a
number of packages which provide compatibility with existing API's,
for example POSIX threads or &micro;ITRON. These allow application
code to call standard functions such as
<function>pthread_create</function>, and those functions are
implemented using the basic functionality provided by the eCos kernel.
Using compatibility packages in an eCos application can make it much
easier to reuse code developed in other environments, and to share
code.
      </para>
      <para>
Although the different compatibility packages have similar
requirements on the underlying kernel, for example the ability to
create a new thread, there are differences in the exact semantics. For
example, strict &micro;ITRON compliance requires that kernel
timeslicing is disabled. This is achieved largely through the
configuration technology. The kernel provides a number of
configuration options that control the exact semantics that are
provided, and the various compatibility packages require particular
settings for those options. This has two important consequences.
First, it is not usually possible to have two different compatibility
packages in one eCos configuration because they will have conflicting
requirements on the underlying kernel. Second, the semantics of the
kernel's own API are only loosely defined because of the many
configuration options. For example <function>cyg_mutex_lock</function>
will always attempt to lock a mutex, but various configuration options
determine the behaviour when the mutex is already locked and there is
a possibility of priority inversion.
      </para>
      <para>
The optional nature of the kernel package presents some complications
for other code, especially device drivers. Wherever possible a device
driver should work whether or not the kernel is present. However there
are some parts of the system, especially those related to interrupt
handling, which should be implemented differently in multi-threaded
environments containing the eCos kernel and in single-threaded
environments without the kernel. To cope with both scenarios the
common HAL package provides a driver API, with functions such as
<function>cyg_drv_interrupt_attach</function>. When the kernel package
is present these driver API functions map directly on to the
equivalent kernel functions such as
<function>cyg_interrupt_attach</function>, using macros to avoid any
overheads. When the kernel is absent the common HAL package implements
the driver API directly, but this implementation is simpler than the
one in the kernel because it can assume a single-threaded environment. 
      </para>
    </refsect1>

<!-- }}} -->
<!-- {{{ Schedulers                     -->

    <refsect1 id="kernel-overview-schedulers">
      <title>Schedulers</title>
      <para>
When a system involves multiple threads, a scheduler is needed to
determine which thread should currently be running. The eCos kernel
can be configured with one of two schedulers, the bitmap scheduler and
the multi-level queue (MLQ) scheduler. The bitmap scheduler is
somewhat more efficient, but has a number of limitations. Most systems
will instead use the MLQ scheduler. Other schedulers may be added in
the future, either as extensions to the kernel package or in separate
packages.
      </para>
      <para>
Both the bitmap and the MLQ scheduler use a simple numerical priority
to determine which thread should be running. The number of priority
levels is configurable via the option
<varname>CYGNUM_KERNEL_SCHED_PRIORITIES</varname>, but a typical
system will have up to 32 priority levels. Therefore thread priorities
will be in the range 0 to 31, with 0 being the highest priority and 31
the lowest. Usually only the system's idle thread will run at the
lowest priority. Thread priorities are absolute, so the kernel will
only run a lower-priority thread if all higher-priority threads are
currently blocked.
      </para>
      <para>
The bitmap scheduler only allows one thread per priority level, so if
the system is configured with 32 priority levels then it is limited to
only 32 threads &mdash; still enough for many applications. A simple
bitmap can be used to keep track of which threads are currently
runnable. Bitmaps can also be used to keep track of threads waiting on
a mutex or other synchronization primitive. Identifying the
highest-priority runnable or waiting thread involves a simple
operation on the bitmap, and an array index operation can then be used
to get hold of the thread data structure itself. This makes the
bitmap scheduler fast and totally deterministic.
      </para>
      <para>
The MLQ scheduler allows multiple threads to run at the same priority.
This means that there is no limit on the number of threads in the
system, other than the amount of memory available. However operations
such as finding the highest priority runnable thread are a little bit
more expensive than for the bitmap scheduler.
      </para>
      <para>
Optionally the MLQ scheduler supports timeslicing, where the scheduler
automatically switches from one runnable thread to another when some
number of clock ticks have occurred. Timeslicing only comes into play
when there are two runnable threads at the same priority and no higher
priority runnable threads. If timeslicing is disabled then a thread
will not be preempted by another thread of the same priority, and will
continue running until either it explicitly yields the processor or
until it blocks by, for example, waiting on a synchronization
primitive. The configuration options
<varname>CYGSEM_KERNEL_SCHED_TIMESLICE</varname> and
<varname>CYGNUM_KERNEL_SCHED_TIMESLICE_TICKS</varname> control
timeslicing. The bitmap scheduler does not provide timeslicing
support. It only allows one thread per priority level, so it is not
possible to preempt the current thread in favour of another one with
the same priority.
      </para>
      <para>
Another important configuration option that affects the MLQ scheduler
is <varname>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</varname>. This
determines what happens when a thread blocks, for example by waiting
on a semaphore which has no pending events. The default behaviour of
the system is last-in-first-out queuing. For example if several
threads are waiting on a semaphore and an event is posted, the thread
that gets woken up is the last one that called
<function>cyg_semaphore_wait</function>. This allows for a simple and
fast implementation of both the queue and dequeue operations. However
if there are several queued threads with different priorities, it may
not be the highest priority one that gets woken up. In practice this
is rarely a problem: usually there will be at most one thread waiting
on a queue, or when there are several threads they will be of the same
priority. However if the application does require strict priority
queueing then the option
<varname>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</varname> should be
enabled. There are disadvantages: more work is needed whenever a
thread is queued, and the scheduler needs to be locked for this
operation so the system's dispatch latency is worse. If the bitmap
scheduler is used then priority queueing is automatic and does not
involve any penalties.
      </para>
      <para>
Some kernel functionality is currently only supported with the MLQ
scheduler, not the bitmap scheduler. This includes support for SMP
systems, and protection against priority inversion using either mutex
priority ceilings or priority inheritance.
      </para>
    </refsect1>

<!-- }}} -->
<!-- {{{ Synch primitives               -->

    <refsect1 id="kernel-overview-synch-primitives">
      <title>Synchronization Primitives</title>
      <para>
The eCos kernel provides a number of different synchronization
primitives: <link linkend="kernel-mutexes">mutexes</link>,
<link linkend="kernel-condition-variables">condition variables</link>,
<link linkend="kernel-semaphores">counting semaphores</link>,
<link linkend="kernel-mail-boxes">mail boxes</link> and
<link linkend="kernel-flags">event flags</link>.
      </para>
      <para>
Mutexes serve a very different purpose from the other primitives. A
mutex allows multiple threads to share a resource safely: a thread
locks a mutex, manipulates the shared resource, and then unlocks the
mutex again. The other primitives are used to communicate information
between threads, or alternatively from a DSR associated with an
interrupt handler to a thread.
      </para>
      <para>
When a thread that has locked a mutex needs to wait for some condition
to become true, it should use a condition variable. A condition
variable is essentially just a place for a thread to wait, and which
another thread, or DSR, can use to wake it up. When a thread waits on
a condition variable it releases the mutex before waiting, and when it
wakes up it reacquires it before proceeding. These operations are
atomic so that synchronization race conditions cannot be introduced.
      </para>
      <para>
A counting semaphore is used to indicate that a particular event has
occurred. A consumer thread can wait for this event to occur, and a
producer thread or a DSR can post the event. There is a count
associated with the semaphore so if the event occurs multiple times in
quick succession this information is not lost, and the appropriate
number of semaphore wait operations will succeed.
      </para>
      <para>
Mail boxes are also used to indicate that a particular event has
occurred, and allows for one item of data to be exchanged per event.
Typically this item of data would be a pointer to some data structure.
Because of the need to store this extra data, mail boxes have a
finite capacity. If a producer thread generates mail box events
faster than they can be consumed then, to avoid overflow, it will be
blocked until space is again available in the mail box. This means
that mail boxes usually cannot be used by a DSR to wake up a
thread. Instead mail boxes are typically only used between threads.
      </para>
      <para>
Event flags can be used to wait on some number of different events,
and to signal that one or several of these events have occurred. This
is achieved by associating bits in a bit mask with the different
events. Unlike a counting semaphore no attempt is made to keep track
of the number of events that have occurred, only the fact that an
event has occurred at least once. Unlike a mail box it is not
possible to send additional data with the event, but this does mean
that there is no possibility of an overflow and hence event flags can
be used between a DSR and a thread as well as between threads.
      </para>
      <para>
The eCos common HAL package provides its own device driver API which
contains some of the above synchronization primitives. These allow
the DSR for an interrupt handler to signal events to higher-level
code. If the configuration includes the eCos kernel package then
the driver API routines map directly on to the equivalent kernel
routines, allowing interrupt handlers to interact with threads. If the
kernel package is not included and the application consists of just a
single thread running in polled mode then the driver API is
implemented entirely within the common HAL, and with no need to worry
about multiple threads the implementation can obviously be rather
simpler. 
      </para>
    </refsect1>

<!-- }}} -->
<!-- {{{ Threads and interrupts         -->

    <refsect1 id="kernel-overview-threads-interrupts">
      <title>Threads and Interrupt Handling</title>
      <para>
During normal operation the processor will be running one of the
threads in the system. This may be an application thread, a system
thread running inside say the TCP/IP stack, or the idle thread. From
time to time a hardware interrupt will occur, causing control to be
transferred briefly to an interrupt handler. When the interrupt has
been completed the system's scheduler will decide whether to return
control to the interrupted thread or to some other runnable thread.
      </para>
      <para>
Threads and interrupt handlers must be able to interact. If a thread
is waiting for some I/O operation to complete, the interrupt handler
associated with that I/O must be able to inform the thread that the
operation has completed. This can be achieved in a number of ways. One
very simple approach is for the interrupt handler to set a volatile
variable. A thread can then poll continuously until this flag is set,
possibly sleeping for a clock tick in between. Polling continuously
means that the cpu time is not available for other activities, which
may be acceptable for some but not all applications. Polling once
every clock tick imposes much less overhead, but means that the thread
may not detect that the I/O event has occurred until an entire clock
tick has elapsed. In typical systems this could be as long as 10
milliseconds. Such a delay might be acceptable for some applications,
but not all.
      </para>
      <para>
A better solution would be to use one of the synchronization
primitives. The interrupt handler could signal a condition variable,
post to a semaphore, or use one of the other primitives. The thread
would perform a wait operation on the same primitive. It would not
consume any cpu cycles until the I/O event had occurred, and when the
event does occur the thread can start running again immediately
(subject to any higher priority threads that might also be runnable).
      </para>
      <para>
Synchronization primitives constitute shared data, so care must be
taken to avoid problems with concurrent access. If the thread that was
interrupted was just performing some calculations then the interrupt
handler could manipulate the synchronization primitive quite safely.
However if the interrupted thread happened to be inside some kernel
call then there is a real possibility that some kernel data structure
will be corrupted. 
      </para>
      <para>
One way of avoiding such problems would be for the kernel functions to
disable interrupts when executing any critical region. On most
architectures this would be simple to implement and very fast, but it
would mean that interrupts would be disabled often and for quite a
long time. For some applications that might not matter, but many
embedded applications require that the interrupt handler run as soon
as possible after the hardware interrupt has occurred. If the kernel
relied on disabling interrupts then it would not be able to support
such applications.
      </para>
      <para>
Instead the kernel uses a two-level approach to interrupt handling.
Associated with every interrupt vector is an Interrupt Service Routine
or ISR, which will run as quickly as possible so that it can service
the hardware. However an ISR can make only a small number of kernel
calls, mostly related to the interrupt subsystem, and it cannot make
any call that would cause a thread to wake up. If an ISR detects that
an I/O operation has completed and hence that a thread should be woken
up, it can cause the associated Deferred Service Routine or DSR to
run. A DSR is allowed to make more kernel calls, for example it can
signal a condition variable or post to a semaphore.
      </para>
      <para>
Disabling interrupts prevents ISRs from running, but very few parts of
the system disable interrupts and then only for short periods of time.
The main reason for a thread to disable interrupts is to manipulate
some state that is shared with an ISR. For example if a thread needs
to add another buffer to a linked list of free buffers and the ISR may
remove a buffer from this list at any time, the thread would need to
disable interrupts for the few instructions needed to manipulate the
list. If the hardware raises an interrupt at this time, it remains
pending until interrupts are reenabled.
      </para>
      <para>
Analogous to interrupts being disabled or enabled, the kernel has a
scheduler lock. The various kernel functions such as
<function>cyg_mutex_lock</function> and
<function>cyg_semaphore_post</function> will claim the scheduler lock,
manipulate the kernel data structures, and then release the scheduler
lock. If an interrupt results in a DSR being requested and the
scheduler is currently locked, the DSR remains pending. When the
scheduler lock is released any pending DSRs will run. These may post
events to synchronization primitives, causing other higher priority
threads to be woken up.
      </para>
      <para>
For an example, consider the following scenario. The system has a high
priority thread A, responsible for processing some data coming from an
external device. This device will raise an interrupt when data is
available. There are two other threads B and C which spend their time
performing calculations and occasionally writing results to a display
of some sort. This display is a shared resource so a mutex is used to
control access.
      </para>
      <para>
At a particular moment in time thread A is likely to be blocked,
waiting on a semaphore or another synchronization primitive until data
is available. Thread B might be running performing some calculations,
and thread C is runnable waiting for its next timeslice. Interrupts
are enabled, and the scheduler is unlocked because none of the threads
are in the middle of a kernel operation. At this point the device
raises an interrupt. The hardware transfers control to a low-level
interrupt handler provided by eCos which works out exactly which
interrupt occurs, and then the corresponding ISR is run. This ISR
manipulates the hardware as appropriate, determines that there is now
data available, and wants to wake up thread A by posting to the
semaphore. However ISR's are not allowed to call
<function>cyg_semaphore_post</function> directly, so instead the ISR
requests that its associated DSR be run and returns. There are no more
interrupts to be processed, so the kernel next checks for DSR's. One
DSR is pending and the scheduler is currently unlocked, so the DSR can
run immediately and post the semaphore. This will have the effect of
making thread A runnable again, so the scheduler's data structures are
adjusted accordingly. When the DSR returns thread B is no longer the
highest priority runnable thread so it will be suspended, and instead
thread A gains control over the cpu.
      </para>
      <para>
In the above example no kernel data structures were being manipulated
at the exact moment that the interrupt happened. However that cannot
be assumed. Suppose that thread B had finished its current set of
calculations and wanted to write the results to the display. It would
claim the appropriate mutex and manipulate the display. Now suppose
that thread B was timesliced in favour of thread C, and that thread C
also finished its calculations and wanted to write the results to the
display. It would call <function>cyg_mutex_lock</function>. This
kernel call locks the scheduler, examines the current state of the
mutex, discovers that the mutex is already owned by another thread,
suspends the current thread, and switches control to another runnable
thread. Another interrupt happens in the middle of this
<function>cyg_mutex_lock</function> call, causing the ISR to run
immediately. The ISR decides that thread A should be woken up so it
requests that its DSR be run and returns back to the kernel. At this
point there is a pending DSR, but the scheduler is still locked by the
call to <function>cyg_mutex_lock</function> so the DSR cannot run
immediately. Instead the call to <function>cyg_mutex_lock</function>
is allowed to continue, which at some point involves unlocking the
scheduler. The pending DSR can now run, safely post the semaphore, and
thus wake up thread A.
      </para>
      <para>
If the ISR had called <function>cyg_semaphore_post</function> directly
rather than leaving it to a DSR, it is likely that there would have
been some sort of corruption of a kernel data structure. For example
the kernel might have completely lost track of one of the threads, and
that thread would never have run again. The two-level approach to
interrupt handling, ISR's and DSR's, prevents such problems with no
need to disable interrupts.
      </para>
    </refsect1>

<!-- }}} -->
<!-- {{{ Calling contexts               -->

    <refsect1 id="kernel-overview-contexts">
      <title>Calling Contexts</title>
      <para>
eCos defines a number of contexts. Only certain calls are allowed from
inside each context, for example most operations on threads or
synchronization primitives are not allowed from ISR context. The
different contexts are initialization, thread, ISR and DSR.
      </para>
      <para>
When eCos starts up it goes through a number of phases, including
setting up the hardware and invoking C++ static constructors. During
this time interrupts are disabled and the scheduler is locked. When a
configuration includes the kernel package the final operation is a
call to <link
linkend="kernel-schedcontrol"><function>cyg_scheduler_start</function></link>.
At this point interrupts are enabled, the scheduler is unlocked, and
control is transferred to the highest priority runnable thread. If the
configuration also includes the C library package then usually the C
library startup package will have created a thread which will call the
application's <function>main</function> entry point.
      </para>
      <para>
Some application code can also run before the scheduler is started,
and this code runs in initialization context. If the application is
written partly or completely in C++ then the constructors for any
static objects will be run. Alternatively application code can define
a function <function>cyg_user_start</function> which gets called after
any C++ static constructors. This allows applications to be written
entirely in C.
      </para>
      <programlisting width=72>
void
cyg_user_start(void)
{
    /* Perform application-specific initialization here */
}
      </programlisting>
      <para>
It is not necessary for applications to provide a
<function>cyg_user_start</function> function since the system will
provide a default implementation which does nothing.
      </para>
      <para>
Typical operations that are performed from inside static constructors
or <function>cyg_user_start</function> include creating threads,
synchronization primitives, setting up alarms, and registering
application-specific interrupt handlers. In fact for many applications
all such creation operations happen at this time, using statically
allocated data, avoiding any need for dynamic memory allocation or
other overheads.
      </para>
      <para>
Code running in initialization context runs with interrupts disabled
and the scheduler locked. It is not permitted to reenable interrupts
or unlock the scheduler because the system is not guaranteed to be in
a totally consistent state at this point. A consequence is that
initialization code cannot use synchronization primitives such as
<function>cyg_semaphore_wait</function> to wait for an external event.
It is permitted to lock and unlock a mutex: there are no other threads
running so it is guaranteed that the mutex is not yet locked, and
therefore the lock operation will never block; this is useful when
making library calls that may use a mutex internally.
      </para>
      <para>
At the end of the startup sequence the system will call
<function>cyg_scheduler_start</function> and the various threads will
start running. In thread context nearly all of the kernel functions
are available. There may be some restrictions on interrupt-related
operations, depending on the target hardware. For example the hardware
may require that interrupts be acknowledged in the ISR or DSR before
control returns to thread context, in which case
<filename>cyg_interrupt_acknowledge</filename> should not be called
by a thread.
      </para>
      <para>
At any time the processor may receive an external interrupt, causing
control to be transferred from the current thread. Typically a VSR
provided by eCos will run and determine exactly which interrupt
occurred. Then the VSR will switch to the appropriate ISR, which can
be provided by a HAL package, a device driver, or by the application.
During this time the system is running at ISR context, and most of the
kernel function calls are disallowed. This includes the various
synchronization primitives, so for example an ISR is not allowed to
post to a semaphore to indicate that an event has happened. Usually
the only operations that should be performed from inside an ISR are
ones related to the interrupt subsystem itself, for example masking an
interrupt or acknowledging that an interrupt has been processed. On
SMP systems it is also possible to use spinlocks from ISR context.
      </para>
      <para>
When an ISR returns it can request that the corresponding DSR be run
as soon as it is safe to do so, and that will run in DSR context. This
context is also used for running alarm functions, and threads can
switch temporarily to DSR context by locking the scheduler. Only
certain kernel functions can be called from DSR context, although more
than in ISR context. In particular it is possible to use any
synchronization primitives which cannot block.  These include
<function>cyg_semaphore_post</function>,
<filename>cyg_cond_signal</filename>,
<function>cyg_cond_broadcast</function>,
<function>cyg_flag_setbits</function>, and
<function>cyg_mbox_tryput</function>. It is not possible to use any
primitives that may block such as
<function>cyg_semaphore_wait</function>,
<function>cyg_mutex_lock</function>, or
<function>cyg_mbox_put</function>. Calling such functions from inside
a DSR may cause the system to hang.
      </para>
      <para>
The specific documentation for the various kernel functions gives more
details about valid contexts.
      </para>
    </refsect1>

<!-- }}} -->
<!-- {{{ Error handling                 -->

    <refsect1 id="kernel-overview-errors">
      <title>Error Handling and Assertions</title>
      <para>
In many APIs each function is expected to perform some validation of
its parameters and possibly of the current state of the system. This
is supposed to ensure that each function is used correctly, and that
application code is not attempting to perform a semaphore operation on
a mutex or anything like that. If an error is detected then a suitable
error code is returned, for example the POSIX function
<function>pthread_mutex_lock</function> can return various error codes
including <literal>EINVAL</literal> and <literal>EDEADLK</literal>.
There are a number of problems with this approach, especially in the
context of deeply embedded systems:
      </para>
      <orderedlist>
        <listitem><para>
Performing these checks inside the mutex lock and all the other
functions requires extra cpu cycles and adds significantly to the code
size. Even if the application is written correctly and only makes
system function calls with sensible arguments and under the right
conditions, these overheads still exist.
        </para></listitem>
        <listitem><para>
Returning an error code is only useful if the calling code detects
these error codes and takes appropriate action. In practice the
calling code will often ignore any errors because the programmer
<emphasis>&ldquo;knows&rdquo;</emphasis> that the function is being
used correctly. If the programmer is mistaken then an error condition
may be detected and reported, but the application continues running
anyway and is likely to fail some time later in mysterious ways.
        </para></listitem>
        <listitem><para>
If the calling code does always check for error codes, that adds yet
more cpu cycles and code size overhead. 
        </para></listitem>
        <listitem><para>
Usually there will be no way to recover from certain errors, so if the
application code detected an error such as <literal>EINVAL</literal>
then all it could do is abort the application somehow.
        </para></listitem>
      </orderedlist>
      <para>
The approach taken within the eCos kernel is different. Functions such
as <function>cyg_mutex_lock</function> will not return an error code.
Instead they contain various assertions, which can be enabled or
disabled. During the development process assertions are normally left
enabled, and the various kernel functions will perform parameter
checks and other system consistency checks. If a problem is detected
then an assertion failure will be reported and the application will be
terminated. In a typical debug session a suitable breakpoint will have
been installed and the developer can now examine the state of the
system and work out exactly what is going on. Towards the end of the
development cycle assertions will be disabled by manipulating
configuration options within the eCos infrastructure package, and all
assertions will be eliminated at compile-time. The assumption is that
by this time the application code has been mostly debugged: the
initial version of the code might have tried to perform a semaphore
operation on a mutex, but any problems like that will have been fixed
some time ago. This approach has a number of advantages:
      </para>
      <orderedlist>
        <listitem><para>
In the final application there will be no overheads for checking
parameters and other conditions. All that code will have been
eliminated at compile-time.
        </para></listitem>
        <listitem><para>
Because the final application will not suffer any overheads, it is
reasonable for the system to do more work during the development
process. In particular the various assertions can test for more error
conditions and more complicated errors. When an error is detected
it is possible to give a text message describing the error rather than
just return an error code.
        </para></listitem>
        <listitem><para>
There is no need for application programmers to handle error codes
returned by various kernel function calls. This simplifies the
application code.
        </para></listitem>
        <listitem><para>
If an error is detected then an assertion failure will be reported
immediately and the application will be halted. There is no
possibility of an error condition being ignored because application
code did not check for an error code.
        </para></listitem>
      </orderedlist>
      <para>
Although none of the kernel functions return an error code, many of
them do return a status condition. For example the function
<function>cyg_semaphore_timed_wait</function> waits until either an
event has been posted to a semaphore, or until a certain number of
clock ticks have occurred. Usually the calling code will need to know
whether the wait operation succeeded or whether a timeout occurred.
<function>cyg_semaphore_timed_wait</function> returns a boolean: a
return value of zero or false indicates a timeout, a non-zero return
value indicates that the wait succeeded.
      </para>
      <para>
In conventional APIs one common error conditions is lack of memory.
For example the POSIX function <function>pthread_create</function>
usually has to allocate some memory dynamically for the thread stack
and other per-thread data. If the target hardware does not have enough
memory to meet all demands, or more commonly if the application
contains a memory leak, then there may not be enough memory available
and the function call would fail. The eCos kernel avoids such problems
by never performing any dynamic memory allocation. Instead it is the
responsibility of the application code to provide all the memory
required for kernel data structures and other needs. In the case of
<function>cyg_thread_create</function> this means a
<structname>cyg_thread</structname> data structure to hold the thread
details, and a <type>char</type> array for the thread stack.
      </para>
      <para>
In many applications this approach results in all data structures
being allocated statically rather than dynamically. This has several
advantages. If the application is in fact too large for the target
hardware's memory then there will be an error at link-time rather than
at run-time, making the problem much easier to diagnose. Static
allocation does not involve any of the usual overheads associated with
dynamic allocation, for example there is no need to keep track of the
various free blocks in the system, and it may be possible to eliminate
<function>malloc</function> from the system completely. Problems such
as fragmentation and memory leaks cannot occur if all data is
allocated statically. However, some applications are sufficiently
complicated that dynamic memory allocation is required, and the
various kernel functions do not distinguish between statically and
dynamically allocated memory. It still remains the responsibility of
the calling code to ensure that sufficient memory is available, and
passing null pointers to the kernel will result in assertions or
system failure.
      </para>
    </refsect1>

<!-- }}} -->

  </refentry>

<!-- }}} -->
<!-- {{{ SMP                            -->

  <refentry id="kernel-SMP">

    <refmeta>
    <refentrytitle>SMP Support</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>SMP</refname>
      <refpurpose>Support Symmetric Multiprocessing Systems</refpurpose>
    </refnamediv>

    <refsect1 id="kernel-smp-description">
      <title>Description</title>
      <para>
eCos contains support for limited Symmetric Multi-Processing (SMP).
This is only available on selected architectures and platforms.
The implementation has a number of restrictions on the kind of
hardware supported. These are described in <xref linkend="hal-smp-support">.
    </para>

    <para>
The following sections describe the changes that have been made to the
eCos kernel to support SMP operation.
    </para>
    </refsect1>

    <refsect1 id="kernel-smp-startup">
      <title>System Startup</title>
      <para>
The system startup sequence needs to be somewhat different on an SMP
system, although this is largely transparent to application code. The
main startup takes place on only one CPU, called the primary CPU. All
other CPUs, the secondary CPUs, are either placed in suspended state
at reset, or are captured by the HAL and put into a spin as they start
up. The primary CPU is responsible for copying the DATA segment and
zeroing the BSS (if required), calling HAL variant and platform
initialization routines and invoking constructors. It then calls
<function>cyg_start</function> to enter the application. The
application may then create extra threads and other objects.
      </para>
      <para>
It is only when the application calls
<function>cyg_scheduler_start</function> that the secondary CPUs are
initialized. This routine scans the list of available secondary CPUs
and invokes <function>HAL_SMP_CPU_START</function> to start each
CPU. Finally it calls an internal function
<function>Cyg_Scheduler::start_cpu</function> to enter the scheduler
for the primary CPU.
      </para>
      <para>
Each secondary CPU starts in the HAL, where it completes any per-CPU
initialization before calling into the kernel at
<function>cyg_kernel_cpu_startup</function>. Here it claims the
scheduler lock and calls
<function>Cyg_Scheduler::start_cpu</function>.
      </para>
      <para>
<function>Cyg_Scheduler::start_cpu</function> is common to both the
primary and secondary CPUs. The first thing this code does is to
install an interrupt object for this CPU's inter-CPU interrupt. From
this point on the code is the same as for the single CPU case: an
initial thread is chosen and entered.
      </para>
      <para>
From this point on the CPUs are all equal, eCos makes no further
distinction between the primary and secondary CPUs. However, the
hardware may still distinguish between them as far as interrupt
delivery is concerned.
      </para>
    </refsect1>

    <refsect1 id="kernel-smp-scheduling">
      <title>Scheduling</title>
      <para>
To function correctly an operating system kernel must protect its
vital data structures, such as the run queues, from concurrent
access. In a single CPU system the only concurrent activities to worry
about are asynchronous interrupts. The kernel can easily guard its
data structures against these by disabling interrupts. However, in a
multi-CPU system, this is inadequate since it does not block access by
other CPUs.
      </para>
      <para>
The eCos kernel protects its vital data structures using the scheduler
lock. In single CPU systems this is a simple counter that is
atomically incremented to acquire the lock and decremented to release
it. If the lock is decremented to zero then the scheduler may be
invoked to choose a different thread to run. Because interrupts may
continue to be serviced while the scheduler lock is claimed, ISRs are
not allowed to access kernel data structures, or call kernel routines
that can. Instead all such operations are deferred to an associated
DSR routine that is run during the lock release operation, when the
data structures are in a consistent state.
      </para>
      <para>
By choosing a kernel locking mechanism that does not rely on interrupt
manipulation to protect data structures, it is easier to convert eCos
to SMP than would otherwise be the case. The principal change needed to
make eCos SMP-safe is to convert the scheduler lock into a nestable
spin lock. This is done by adding a spinlock and a CPU id to the
original counter.
      </para>
      <para>
The algorithm for acquiring the scheduler lock is very simple. If the
scheduler lock's CPU id matches the current CPU then it can just increment
the counter and continue. If it does not match, the CPU must spin on
the spinlock, after which it may increment the counter and store its
own identity in the CPU id.
      </para>
      <para>
To release the lock, the counter is decremented. If it goes to zero
the CPU id value must be set to NONE and the spinlock cleared.
      </para>
      <para>
To protect these sequences against interrupts, they must be performed
with interrupts disabled. However, since these are very short code
sequences, they will not have an adverse effect on the interrupt
latency.
      </para>
      <para>
Beyond converting the scheduler lock, further preparing the kernel for
SMP is a relatively minor matter. The main changes are to convert
various scalar housekeeping variables into arrays indexed by CPU
id. These include the current thread pointer, the need_reschedule
flag and the timeslice counter.
      </para>
      <para>
At present only the Multi-Level Queue (MLQ) scheduler is capable of
supporting SMP configurations. The main change made to this scheduler
is to cope with having several threads in execution at the same
time. Running threads are marked with the CPU that they are executing on.
When scheduling a thread, the scheduler skips past any running threads
until it finds a thread that is pending. While not a constant-time
algorithm, as in the single CPU case, this is still deterministic,
since the worst case time is bounded by the number of CPUs in the
system.
      </para>
      <para>
A second change to the scheduler is in the code used to decide when
the scheduler should be called to choose a new thread. The scheduler
attempts to keep the <property>n</property> CPUs running the
<property>n</property> highest priority threads. Since an event or
interrupt on one CPU may require a reschedule on another CPU, there
must be a mechanism for deciding this. The algorithm currently
implemented is very simple. Given a thread that has just been awakened
(or had its priority changed), the scheduler scans the CPUs, starting
with the one it is currently running on, for a current thread that is
of lower priority than the new one. If one is found then a reschedule
interrupt is sent to that CPU and the scan continues, but now using
the current thread of the rescheduled CPU as the candidate thread. In
this way the new thread gets to run as quickly as possible, hopefully
on the current CPU, and the remaining CPUs will pick up the remaining
highest priority threads as a consequence of processing the reschedule
interrupt.
      </para>
      <para>
The final change to the scheduler is in the handling of
timeslicing. Only one CPU receives timer interrupts, although all CPUs
must handle timeslicing. To make this work, the CPU that receives the
timer interrupt decrements the timeslice counter for all CPUs, not
just its own. If the counter for a CPU reaches zero, then it sends a
timeslice interrupt to that CPU. On receiving the interrupt the
destination CPU enters the scheduler and looks for another thread at
the same priority to run. This is somewhat more efficient than
distributing clock ticks to all CPUs, since the interrupt is only
needed when a timeslice occurs.
      </para>
      <para>
All existing synchronization mechanisms work as before in an SMP
system. Additional synchronization mechanisms have been added to
provide explicit synchronization for SMP, in the form of
<link linkend="kernel-spinlocks">spinlocks</link>.
      </para>
    </refsect1>

    <refsect1 id="kernel-smp-interrupts">
      <title>SMP Interrupt Handling</title>
      <para>
The main area where the SMP nature of a system requires special
attention is in device drivers and especially interrupt handling. It
is quite possible for the ISR, DSR and thread components of a device
driver to execute on different CPUs. For this reason it is much more
important that SMP-capable device drivers use the interrupt-related
functions correctly. Typically a device driver would use the driver
API rather than call the kernel directly, but it is unlikely that
anybody would attempt to use a multiprocessor system without the
kernel package.
      </para>
      <para>
Two new functions have been added to the Kernel API
to do <link linkend="kernel-interrupts-smp">interrupt
routing</link>: <function>cyg_interrupt_set_cpu</function> and
<function>cyg_interrupt_get_cpu</function>. Although not currently
supported, special values for the cpu argument may be used in future
to indicate that the interrupt is being routed dynamically or is
CPU-local. Once a vector has been routed to a new CPU, all other
interrupt masking and configuration operations are relative to that
CPU, where relevant.
      </para>

      <para>
There are more details of how interrupts should be handled in SMP
systems in <xref linkend="devapi-smp-support">.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->

<!-- {{{ cyg_thread_create()            -->

  <refentry id="kernel-thread-create">

    <refmeta>
    <refentrytitle>Thread creation</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_create</refname>
      <refpurpose>Create a new thread</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
	<funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;    
	</funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_thread_create</function></funcdef>
          <paramdef>cyg_addrword_t <parameter>sched_info</parameter></paramdef>
          <paramdef>cyg_thread_entry_t* <parameter>entry</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>entry_data</parameter></paramdef>
          <paramdef>char* <parameter>name</parameter></paramdef>
          <paramdef>void* <parameter>stack_base</parameter></paramdef>
          <paramdef>cyg_ucount32 <parameter>stack_size</parameter></paramdef>
          <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef>
          <paramdef>cyg_thread* <parameter>thread</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-thread-create-description"><title>Description</title>
      <para>
The <function>cyg_thread_create</function> function allows application
code and eCos packages to create new threads. In many applications
this only happens during system initialization and all required data
is allocated statically.  However additional threads can be created at
any time, if necessary. A newly created thread is always in suspended
state and will not start running until it has been resumed via a call
to <function>cyg_thread_resume</function>. Also, if threads are
created during system initialization then they will not start running
until the eCos scheduler has been started.
      </para>
      <para>
The <parameter class="function">name</parameter> argument is used
primarily for debugging purposes, making it easier to keep track of
which <structname>cyg_thread</structname> structure is associated with
which application-level thread. The kernel configuration option
<varname>CYGVAR_KERNEL_THREADS_NAME</varname> controls whether or not
this name is actually used.
      </para>
      <para>
On creation each thread is assigned a unique handle, and this will be
stored in the location pointed at by the <parameter
class="function">handle</parameter> argument. Subsequent operations on
this thread including the required
<function>cyg_thread_resume</function> should use this handle to
identify the thread.
      </para>
      <para>
The kernel requires a small amount of space for each thread, in the
form of a <structname>cyg_thread</structname> data structure, to hold
information such as the current state of that thread. To avoid any
need for dynamic memory allocation within the kernel this space has to
be provided by higher-level code, typically in the form of a static
variable. The <parameter class="function">thread</parameter> argument
provides this space.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-create-entry"><title>Thread Entry Point</title>
      <para>
The entry point for a thread takes the form:
      </para>
      <programlisting width=72>
void
thread_entry_function(cyg_addrword_t data)
{
    &hellip;
}
      </programlisting>
      <para>
The second argument to <function>cyg_thread_create</function> is a
pointer to such a function. The third argument <parameter
class="function">entry_data</parameter> is used to pass additional
data to the function. Typically this takes the form of a pointer to
some static data, or a small integer, or <literal>0</literal> if the
thread does not require any additional data.
      </para>
      <para>
If the thread entry function ever returns then this is equivalent to
the thread calling <function>cyg_thread_exit</function>. Even though
the thread will no longer run again, it remains registered with the
scheduler. If the application needs to re-use the
<structname>cyg_thread</structname> data structure then a call to
<function>cyg_thread_delete</function> is required first.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-create-priorities"><title>Thread Priorities</title>
      <para>
The <parameter class="function">sched_info</parameter> argument
provides additional information to the scheduler. The exact details
depend on the scheduler being used. For the bitmap and mlqueue
schedulers it is a small integer, typically in the range 0 to 31, with
0 being the highest priority. The lowest priority is normally used
only by the system's idle thread. The exact number of priorities is
controlled by the kernel configuration option
<varname>CYGNUM_KERNEL_SCHED_PRIORITIES</varname>. 
      </para>
      <para>
It is the responsibility of the application developer to be aware of
the various threads in the system, including those created by eCos
packages, and to ensure that all threads run at suitable priorities.
For threads created by other packages the documentation provided by
those packages should indicate any requirements.
      </para>
      <para>
The functions <function>cyg_thread_set_priority</function>,
<function>cyg_thread_get_priority</function>, and
<function>cyg_thread_get_current_priority</function> can be used to
manipulate a thread's priority.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-create-stack"><title>Stacks and Stack Sizes</title>
      <para>
Each thread needs its own stack for local variables and to keep track
of function calls and returns. Again it is expected that this stack is
provided by the calling code, usually in the form of static data, so
that the kernel does not need any dynamic memory allocation
facilities. <function>cyg_thread_create</function> takes two arguments
related to the stack, a pointer to the base of the stack and the total
size of this stack. On many processors stacks actually descend from the
top down, so the kernel will add the stack size to the base address to
determine the starting location.
      </para>
      <para>
The exact stack size requirements for any given thread depend on a
number of factors. The most important is of course the code that will
be executed in the context of this code: if this involves significant
nesting of function calls, recursion, or large local arrays, then the
stack size needs to be set to a suitably high value. There are some
architectural issues, for example the number of cpu registers and the
calling conventions will have some effect on stack usage. Also,
depending on the configuration, it is possible that some other code
such as interrupt handlers will occasionally run on the current
thread's stack. This depends in part on configuration options such as
<varname>CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK</varname>
and <varname>CYGSEM_HAL_COMMON_INTERRUPTS_ALLOW_NESTING</varname>.
      </para>
      <para>
Determining an application's actual stack size requirements is the
responsibility of the application developer, since the kernel cannot
know in advance what code a given thread will run. However, the system
does provide some hints about reasonable stack sizes in the form of
two constants: <varname>CYGNUM_HAL_STACK_SIZE_MINIMUM</varname> and
<varname>CYGNUM_HAL_STACK_SIZE_TYPICAL</varname>. These are defined by
the appropriate HAL package. The <varname>MINIMUM</varname> value is
appropriate for a thread that just runs a single function and makes
very simple system calls. Trying to create a thread with a smaller
stack than this is illegal. The <varname>TYPICAL</varname> value is
appropriate for applications where application calls are nested no
more than half a dozen or so levels, and there are no large arrays on
the stack.
      </para>
      <para>
If the stack sizes are not estimated correctly and a stack overflow
occurs, the probably result is some form of memory corruption. This
can be very hard to track down. The kernel does contain some code to
help detect stack overflows, controlled by the configuration option
<varname>CYGFUN_KERNEL_THREADS_STACK_CHECKING</varname>: a small
amount of space is reserved at the stack limit and filled with a
special signature: every time a thread context switch occurs this
signature is checked, and if invalid that is a good indication (but
not absolute proof) that a stack overflow has occurred. This form of
stack checking is enabled by default when the system is built with
debugging enabled. A related configuration option is
<varname>CYGFUN_KERNEL_THREADS_STACK_MEASUREMENT</varname>: enabling
this option means that a thread can call the function
<function>cyg_thread_measure_stack_usage</function> to find out the
maximum stack usage to date. Note that this is not necessarily the
true maximum because, for example, it is possible that in the current
run no interrupt occurred at the worst possible moment.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-create-context"><title>Valid contexts</title>
      <para>
<function>cyg_thread_create</function> may be called during
initialization and from within thread context. It may not be called
from inside a DSR.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-create-example"><title>Example</title>
      <para>
A simple example of thread creation is shown below. This involves
creating five threads, one producer and four consumers or workers. The
threads are created in the system's
<function>cyg_user_start</function>: depending on the configuration it
might be more appropriate to do this elsewhere, for example inside
<function>main</function>.
      </para>
      <programlisting width=72>
#include &lt;cyg/hal/hal_arch.h&gt;
#include &lt;cyg/kernel/kapi.h&gt;

// These numbers depend entirely on your application
#define NUMBER_OF_WORKERS    4
#define PRODUCER_PRIORITY   10
#define WORKER_PRIORITY     11
#define PRODUCER_STACKSIZE  CYGNUM_HAL_STACK_SIZE_TYPICAL
#define WORKER_STACKSIZE    (CYGNUM_HAL_STACK_SIZE_MINIMUM + 1024)

static unsigned char producer_stack[PRODUCER_STACKSIZE];
static unsigned char worker_stacks[NUMBER_OF_WORKERS][WORKER_STACKSIZE];
static cyg_handle_t producer_handle, worker_handles[NUMBER_OF_WORKERS];
static cyg_thread_t producer_thread, worker_threads[NUMBER_OF_WORKERS];

static void
producer(cyg_addrword_t data)
{
    &hellip;
}

static void
worker(cyg_addrword_t data)
{
    &hellip;
}

void
cyg_user_start(void)
{
    int i;

    cyg_thread_create(PRODUCER_PRIORITY, &amp;producer, 0, "producer",
                      producer_stack, PRODUCER_STACKSIZE,
                      &amp;producer_handle, &amp;producer_thread);
    cyg_thread_resume(producer_handle);
    for (i = 0; i < NUMBER_OF_WORKERS; i++) {
        cyg_thread_create(WORKER_PRIORITY, &amp;worker, i, "worker",
                          worker_stacks[i], WORKER_STACKSIZE,
                          &amp;(worker_handles[i]), &amp;(worker_threads[i]));
        cyg_thread_resume(worker_handles[i]);
    }
}
      </programlisting>
    </refsect1>


    <refsect1 id="kernel-thread-create-cxx"><title>Thread Entry Points and C++</title>
      <para>
For code written in C++ the thread entry function must be either a
static member function of a class or an ordinary function outside any
class. It cannot be a normal member function of a class because such
member functions take an implicit additional argument
<varname>this</varname>, and the kernel has no way of knowing what
value to use for this argument. One way around this problem is to make
use of a special static member function, for example:
      </para>
      <programlisting width=72>
class fred {
  public:
    void thread_function();
    static void static_thread_aux(cyg_addrword_t);
};

void
fred::static_thread_aux(cyg_addrword_t objptr)
{
    fred* object = static_cast&lt;fred*&gt;(objptr);
    object-&gt;thread_function();
}

static fred instance;

extern "C" void
cyg_start( void )
{
    &hellip;
    cyg_thread_create( &hellip;,
                      &amp;fred::static_thread_aux,
                      static_cast&lt;cyg_addrword_t&gt;(&amp;instance),
                      &hellip;);
    &hellip;
}
      </programlisting>
      <para>
Effectively this uses the <parameter
class="function">entry_data</parameter> argument to
<function>cyg_thread_create</function> to hold the
<varname>this</varname> pointer. Unfortunately this approach does
require the use of some C++ casts, so some of the type safety that can
be achieved when programming in C++ is lost.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Thread info                    -->

  <refentry id="kernel-thread-info">

    <refmeta>
    <refentrytitle>Thread information</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_self</refname>
      <refname>cyg_thread_idle_thread</refname>
      <refname>cyg_thread_get_stack_base</refname>
      <refname>cyg_thread_get_stack_size</refname>
      <refname>cyg_thread_measure_stack_usage</refname>
      <refname>cyg_thread_get_next</refname>
      <refname>cyg_thread_get_info</refname>
      <refname>cyg_thread_find</refname>
      <refpurpose>Get basic thread information</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
	<funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;    
	</funcsynopsisinfo>
        <funcprototype>
          <funcdef>cyg_handle_t <function>cyg_thread_self</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_handle_t <function>cyg_thread_idle_thread</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_addrword_t <function>cyg_thread_get_stack_base</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_uint32 <function>cyg_thread_get_stack_size</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_uint32 <function>cyg_thread_measure_stack_usage</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>        
        <funcprototype>
          <funcdef>cyg_bool <function>cyg_thread_get_next</function></funcdef>
          <paramdef>cyg_handle_t *<parameter>thread</parameter></paramdef>
          <paramdef>cyg_uint16 *<parameter>id</parameter></paramdef>
        </funcprototype>        
        <funcprototype>
          <funcdef>cyg_bool <function>cyg_thread_get_info</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
          <paramdef>cyg_uint16 <parameter>id</parameter></paramdef>
          <paramdef>cyg_thread_info *<parameter>info</parameter></paramdef>
        </funcprototype>        
        <funcprototype>
          <funcdef>cyg_handle_t <function>cyg_thread_find</function></funcdef>
          <paramdef>cyg_uint16 <parameter>id</parameter></paramdef>
        </funcprototype>        
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-thread-info-description"><title>Description</title>
      <para>
These functions can be used to obtain some basic information about
various threads in the system. Typically they serve little or no
purpose in real applications, but they can be useful during debugging.
      </para>
      <para>
<function>cyg_thread_self</function> returns a handle corresponding
to the current thread. It will be the same as the value filled in by
<function>cyg_thread_create</function> when the current thread was
created. This handle can then be passed to other functions such as
<function>cyg_thread_get_priority</function>.
      </para>
      <para>
<function>cyg_thread_idle_thread</function> returns the handle
corresponding to the idle thread. This thread is created automatically
by the kernel, so application-code has no other way of getting hold of
this information.
      </para>
      <para>
<function>cyg_thread_get_stack_base</function> and
<function>cyg_thread_get_stack_size</function> return information
about a specific thread's stack. The values returned will match the
values passed to <function>cyg_thread_create</function> when this
thread was created.
      </para>
      <para>
<function>cyg_thread_measure_stack_usage</function> is only available
if the configuration option
<varname>CYGFUN_KERNEL_THREADS_STACK_MEASUREMENT</varname> is enabled.
The return value is the maximum number of bytes of stack space used so
far by the specified thread. Note that this should not be considered a
true upper bound, for example it is possible that in the current test
run the specified thread has not yet been interrupted at the deepest
point in the function call graph. Never the less the value returned
can give some useful indication of the thread's stack requirements.
      </para>
      <para>
<function>cyg_thread_get_next</function> is used to enumerate all the
current threads in the system. It should be called initially with the
locations pointed to by <parameter>thread</parameter> and
<parameter>id</parameter> set to zero. On return these will be set to
the handle and ID of the first thread. On subsequent calls, these
parameters should be left set to the values returned by the previous
call.  The handle and ID of the next thread in the system will be
installed each time, until a <literal>false</literal> return value
indicates the end of the list.
      </para>
      <para>
<function>cyg_thread_get_info</function> fills in the
<type>cyg_thread_info</type> structure with information about the
thread described by the <parameter>thread</parameter> and
<parameter>id</parameter> arguments. The information returned includes
the thread's handle and id, its state and name, priorities and stack
parameters. If the thread does not exist the function returns
<literal>false</literal>.
    </para>
    <para>
The <type>cyg_thread_info</type> structure is defined as follows by
&lt;<filename class=headerfile>cyg/kernel/kapi.h</filename>&gt;, but may
be extended in future with additional members, and so its size should
not be relied upon:
<programlisting>
typedef struct
{
    <type>cyg_handle_t</type>        <structfield>handle</structfield>;
    <type>cyg_uint16</type>          <structfield>id</structfield>;
    <type>cyg_uint32</type>          <structfield>state</structfield>;
    <type>char</type>                <structfield>*name</structfield>;
    <type>cyg_priority_t</type>      <structfield>set_pri</structfield>;
    <type>cyg_priority_t</type>      <structfield>cur_pri</structfield>;
    <type>cyg_addrword_t</type>      <structfield>stack_base</structfield>;
    <type>cyg_uint32</type>          <structfield>stack_size</structfield>;
    <type>cyg_uint32</type>          <structfield>stack_used</structfield>;
} cyg_thread_info;
</programlisting>
    </para>
    <para>
<function>cyg_thread_find</function> returns a handle for the thread
whose ID is <parameter>id</parameter>. If no such thread exists, a
zero handle is returned.
    </para>
    </refsect1>

    <refsect1 id="kernel-thread-info-context"><title>Valid contexts</title>
      <para>
<function>cyg_thread_self</function> may only be called from thread
context. <function>cyg_thread_idle_thread</function> may be called
from thread or DSR context, but only after the system has been
initialized. <function>cyg_thread_get_stack_base</function>,
<function>cyg_thread_get_stack_size</function> and
<function>cyg_thread_measure_stack_usage</function> may be called
any time after the specified thread has been created, but measuring
stack usage involves looping over at least part of the thread's stack
so this should normally only be done from thread context.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-info-examples"><title>Examples</title>
      <para>
A simple example of the use of the
<function>cyg_thread_get_next</function> and
<function>cyg_thread_get_info</function> follows:      
      </para>
      <programlisting width=72>

#include &lt;cyg/kernel/kapi.h&gt;
#include &lt;stdio.h&gt;

void show_threads(void)
{
    cyg_handle_t thread = 0;
    cyg_uint16 id = 0;

    while( cyg_thread_get_next( &amp;thread, &amp;id ) )
    {
        cyg_thread_info info;

        if( !cyg_thread_get_info( thread, id, &amp;info ) )
            break;

        printf("ID: %04x name: %10s pri: %d\n",
                info.id, info.name?info.name:"----", info.set_pri );
    }
}

      </programlisting>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Thread control                 -->

  <refentry id="kernel-thread-control">

    <refmeta>
    <refentrytitle>Thread control</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_yield</refname>
      <refname>cyg_thread_delay</refname>
      <refname>cyg_thread_suspend</refname>
      <refname>cyg_thread_resume</refname>
      <refname>cyg_thread_release</refname>
      <refpurpose>Control whether or not a thread is running</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_thread_yield</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_thread_delay</function></funcdef>
          <paramdef>cyg_tick_count_t <parameter>delay</parameter></paramdef>
        </funcprototype>
        <funcprototype>
           <funcdef>void <function>cyg_thread_suspend</function></funcdef>
           <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
           <funcdef>void <function>cyg_thread_resume</function></funcdef>
           <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
           <funcdef>void <function>cyg_thread_release</function></funcdef>
           <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1><title id="kernel-thread-control-description">Description</title>
      <para>
These functions provide some control over whether or not a particular
thread can run. Apart from the required use of
<function>cyg_thread_resume</function> to start a newly-created
thread, application code should normally use proper synchronization
primitives such as condition variables or mail boxes.
      </para>
    </refsect1>

    <refsect1><title id="kernel-thread-control-yield">Yield</title>
      <para>
<function>cyg_thread_yield</function> allows a thread to relinquish
control of the processor to some other runnable thread which has the
same priority. This can have no effect on any higher-priority thread
since, if such a thread were runnable, the current thread would have
been preempted in its favour. Similarly it can have no effect on any
lower-priority thread because the current thread will always be run in
preference to those. As a consequence this function is only useful
in configurations with a scheduler that allows multiple threads to run
at the same priority, for example the mlqueue scheduler. If instead
the bitmap scheduler was being used then
<function>cyg_thread_yield()</function> would serve no purpose.
      </para>
      <para>
Even if a suitable scheduler such as the mlqueue scheduler has been
configured, <function>cyg_thread_yield</function> will still rarely
prove useful: instead timeslicing will be used to ensure that all
threads of a given priority get a fair slice of the available
processor time. However it is possible to disable timeslicing via the
configuration option <varname>CYGSEM_KERNEL_SCHED_TIMESLICE</varname>,
in which case <function>cyg_thread_yield</function> can be used to
implement a form of cooperative multitasking.
      </para>
    </refsect1>

    <refsect1><title id="kernel-thread-control-delay">Delay</title>
      <para>
<function>cyg_thread_delay</function> allows a thread to suspend until
the specified number of clock ticks have occurred. For example, if a
value of 1 is used and the system clock runs at a frequency of 100Hz
then the thread will sleep for up to 10 milliseconds. This
functionality depends on the presence of a real-time system clock, as
controlled by the configuration option
<varname>CYGVAR_KERNEL_COUNTERS_CLOCK</varname>.
      </para>
      <para>
If the application requires delays measured in milliseconds or similar
units rather than in clock ticks, some calculations are needed to
convert between these units as described in <xref
linkend="kernel-clocks">. Usually these calculations can be done by
the application developer, or at compile-time. Performing such
calculations prior to every call to
<function>cyg_thread_delay</function> adds unnecessary overhead to the
system. 
      </para>
    </refsect1>

    <refsect1><title id="kernel-thread-control-suspend">Suspend and Resume</title>
      <para>
Associated with each thread is a suspend counter. When a thread is
first created this counter is initialized to 1.
<function>cyg_thread_suspend</function> can be used to increment the
suspend counter, and <function>cyg_thread_resume</function> decrements
it. The scheduler will never run a thread with a non-zero suspend
counter. Therefore a newly created thread will not run until it has
been resumed.
      </para>
      <para>
An occasional problem with the use of suspend and resume functionality
is that a thread gets suspended more times than it is resumed and
hence never becomes runnable again. This can lead to very confusing
behaviour. To help with debugging such problems the kernel provides a
configuration option
<varname>CYGNUM_KERNEL_MAX_SUSPEND_COUNT_ASSERT</varname> which
imposes an upper bound on the number of suspend calls without matching
resumes, with a reasonable default value. This functionality depends
on infrastructure assertions being enabled.
      </para>
    </refsect1>

    <refsect1><title id="kernel-thread-control-release">Releasing a Blocked Thread</title>
      <para>
When a thread is blocked on a synchronization primitive such as a
semaphore or a mutex, or when it is waiting for an alarm to trigger,
it can be forcibly woken up using
<function>cyg_thread_release</function>. Typically this will call the
affected synchronization primitive to return false, indicating that
the operation was not completed successfully. This function has to be
used with great care, and in particular it should only be used on
threads that have been designed appropriately and check all return
codes. If instead it were to be used on, say, an arbitrary thread that
is attempting to claim a mutex then that thread might not bother to
check the result of the mutex lock operation - usually there would be
no reason to do so. Therefore the thread will now continue running in
the false belief that it has successfully claimed a mutex lock, and
the resulting behaviour is undefined. If the system has been built
with assertions enabled then it is possible that an assertion will
trigger when the thread tries to release the mutex it does not
actually own.
      </para>
      <para>
The main use of <function>cyg_thread_release</function> is in the
POSIX compatibility layer, where it is used in the implementation of
per-thread signals and cancellation handlers.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-control-context"><title>Valid contexts</title>
      <para>
<function>cyg_thread_yield</function> can only be called from thread
context, A DSR must always run to completion and cannot yield the
processor to some thread. <function>cyg_thread_suspend</function>,
<function>cyg_thread_resume</function>, and
<function>cyg_thread_release</function> may be called from thread or
DSR context. 
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Thread termination             -->

  <refentry id="kernel-thread-termination">

    <refmeta>
    <refentrytitle>Thread termination</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_exit</refname>
      <refname>cyg_thread_kill</refname>
      <refname>cyg_thread_delete</refname>
      <refpurpose>Allow threads to terminate</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_thread_exit</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_thread_kill</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_thread_delete</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1><title id="kernel-thread-termination-description">Description</title>
      <para>
In many embedded systems the various threads are allocated statically,
created during initialization, and never need to terminate. This
avoids any need for dynamic memory allocation or other resource
management facilities. However if a given application does have a
requirement that some threads be created dynamically, must terminate,
and their resources such as the stack be reclaimed, then the kernel
provides the functions <function>cyg_thread_exit</function>,
<function>cyg_thread_kill</function>, and
<function>cyg_thread_delete</function>.
      </para>
      <para>
<function>cyg_thread_exit</function> allows a thread to terminate
itself, thus ensuring that it will not be run again by the scheduler.
However the <structname>cyg_thread</structname> data structure passed
to <function>cyg_thread_create</function> remains in use, and the
handle returned by <function>cyg_thread_create</function> remains
valid. This allows other threads to perform certain operations on the
terminated thread, for example to determine its stack usage via
<function>cyg_thread_measure_stack_usage</function>. When the handle
and <structname>cyg_thread</structname> structure are no longer
required, <function>cyg_thread_delete</function> should be called to
release these resources. If the stack was dynamically allocated then
this should not be freed until after the call to
<function>cyg_thread_delete</function>.
      </para>
      <para>
Alternatively, one thread may use <function>cyg_thread_kill</function>
on another This has much the same effect as the affected thread
calling <function>cyg_thread_exit</function>. However killing a thread
is generally rather dangerous because no attempt is made to unlock any
synchronization primitives currently owned by that thread or release
any other resources that thread may have claimed. Therefore use of
this function should be avoided, and
<function>cyg_thread_exit</function> is preferred.
<function>cyg_thread_kill</function> cannot be used by a thread to
kill itself.
      </para>
      <para>
<function>cyg_thread_delete</function> should be used on a thread
after it has exited and is no longer required. After this call the
thread handle is no longer valid, and both the
<structname>cyg_thread</structname> structure and the thread stack can
be re-used or freed. If <function>cyg_thread_delete</function> is
invoked on a thread that is still running then there is an implicit
call to <function>cyg_thread_kill</function>.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-termination-context"><title>Valid contexts</title>
      <para>
<function>cyg_thread_exit</function>,
<function>cyg_thread_kill</function> and
<function>cyg_thread_delete</function> can only be called from thread
context. 
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Thread priorities              -->

  <refentry id="kernel-thread-priorities">

    <refmeta>
    <refentrytitle>Thread priorities</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_get_priority</refname>
      <refname>cyg_thread_get_current_priority</refname>
      <refname>cyg_thread_set_priority</refname>
      <refpurpose>Examine and manipulate thread priorities</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>cyg_priority_t <function>cyg_thread_get_priority</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_priority_t <function>cyg_thread_get_current_priority</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_thread_set_priority</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
          <paramdef>cyg_priority_t <parameter>priority</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1><title id="kernel-thread-priorities-description">Description</title>
      <para>
Typical schedulers use the concept of a thread priority to determine
which thread should run next. Exactly what this priority consists of
will depend on the scheduler, but a typical implementation would be a
small integer in the range 0 to 31, with 0 being the highest priority.
Usually only the idle thread will run at the lowest priority. The
exact number of priority levels available depends on the
configuration, typically the option
<varname>CYGNUM_KERNEL_SCHED_PRIORITIES</varname>.
      </para>
      <para>
<function>cyg_thread_get_priority</function> can be used to determine
the priority of a thread, or more correctly the value last used in a
<function>cyg_thread_set_priority</function> call or when the thread
was first created. In some circumstances it is possible that the
thread is actually running at a higher priority. For example, if it
owns a mutex and priority ceilings or inheritance is being used to
prevent priority inversion problems, then the thread's priority may
have been boosted temporarily.
<function>cyg_thread_get_current_priority</function> returns the real
current priority.
      </para>
      <para>
In many applications appropriate thread priorities can be determined
and allocated statically. However, if it is necessary for a thread's
priority to change at run-time then the
<function>cyg_thread_set_priority</function> function provides this
functionality. 
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-priorities-context"><title>Valid contexts</title>
      <para>
<function>cyg_thread_get_priority</function> and
<function>cyg_thread_get_current_priority</function> can be called
from thread or DSR context, although the latter is rarely useful.
<function>cyg_thread_set_priority</function> should also only be
called from thread context.
      </para>
    </refsect1>
  </refentry>

<!-- }}} -->
<!-- {{{ Per-thread data                -->

  <refentry id="kernel-thread-data">

    <refmeta>
    <refentrytitle>Per-thread data</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_new_data_index</refname>
      <refname>cyg_thread_free_data_index</refname>
      <refname>cyg_thread_get_data</refname>
      <refname>cyg_thread_get_data_ptr</refname>
      <refname>cyg_thread_set_data</refname>
      <refpurpose>Manipulate per-thread data</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>cyg_ucount32 <function>cyg_thread_new_data_index</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_thread_free_data_index</function></funcdef>
          <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_addrword_t <function>cyg_thread_get_data</function></funcdef>
          <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_addrword_t* <function>cyg_thread_get_data_ptr</function></funcdef>
          <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_thread_set_data</function></funcdef>
          <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1><title id="kernel-thread-data-description">Description</title>
      <para>
In some applications and libraries it is useful to have some data that
is specific to each thread. For example, many of the functions in the
POSIX compatibility package return -1 to indicate an error and store
additional information in what appears to be a global variable
<varname>errno</varname>. However, if multiple threads make concurrent
calls into the POSIX library and if <varname>errno</varname> were
really a global variable then a thread would have no way of knowing
whether the current <varname>errno</varname> value really corresponded
to the last POSIX call it made, or whether some other thread had run
in the meantime and made a different POSIX call which updated the
variable. To avoid such confusion <varname>errno</varname> is instead
implemented as a per-thread variable, and each thread has its own
instance.
      </para>
      <para>
The support for per-thread data can be disabled via the configuration
option <varname>CYGVAR_KERNEL_THREADS_DATA</varname>. If enabled, each
<structname>cyg_thread</structname> data structure holds a small array
of words. The size of this array is determined by the configuration
option <varname>CYGNUM_KERNEL_THREADS_DATA_MAX</varname>. When a
thread is created the array is filled with zeroes.
      </para>
      <para>
If an application needs to use per-thread data then it needs an index
into this array which has not yet been allocated to other code. This
index can be obtained by calling
<function>cyg_thread_new_data_index</function>, and then used in
subsequent calls to <function>cyg_thread_get_data</function>.
Typically indices are allocated during system initialization and
stored in static variables. If for some reason a slot in the array is
no longer required and can be re-used then it can be released by calling
<function>cyg_thread_free_data_index</function>, 
      </para>
      <para>
The current per-thread data in a given slot can be obtained using
<function>cyg_thread_get_data</function>. This implicitly operates on
the current thread, and its single argument should be an index as
returned by <function>cyg_thread_new_data_index</function>. The
per-thread data can be updated using
<function>cyg_thread_set_data</function>. If a particular item of
per-thread data is needed repeatedly then
<function>cyg_thread_get_data_ptr</function> can be used to obtain the
address of the data, and indirecting through this pointer allows the
data to be examined and updated efficiently.
      </para>
      <para>
Some packages, for example the error and POSIX packages, have
pre-allocated slots in the array of per-thread data. These slots
should not normally be used by application code, and instead slots
should be allocated during initialization by a call to
<function>cyg_thread_new_data_index</function>. If it is known that,
for example, the configuration will never include the POSIX
compatibility package then application code may instead decide to
re-use the slot allocated to that package,
<varname>CYGNUM_KERNEL_THREADS_DATA_POSIX</varname>, but obviously
this does involve a risk of strange and subtle bugs if the
application's requirements ever change.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-data-context"><title>Valid contexts</title>
      <para>
Typically <function>cyg_thread_new_data_index</function> is only
called during initialization, but may also be called at any time in
thread context. <function>cyg_thread_free_data_index</function>, if
used at all, can also be called during initialization or from thread
context. <function>cyg_thread_get_data</function>,
<function>cyg_thread_get_data_ptr</function>, and
<function>cyg_thread_set_data</function> may only be called from
thread context because they implicitly operate on the current thread.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Thread destructors             -->

  <refentry id="kernel-thread-destructors">

    <refmeta>
    <refentrytitle>Thread destructors</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_thread_add_destructor</refname>
      <refname>cyg_thread_rem_destructor</refname>
      <refpurpose>Call functions on thread termination</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
typedef void (*<type>cyg_thread_destructor_fn</type>)(<type>cyg_addrword_t</type>);
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_thread_add_destructor</function></funcdef>
          <paramdef>cyg_thread_destructor_fn <parameter>fn</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_thread_rem_destructor</function></funcdef>
          <paramdef>cyg_thread_destructor_fn <parameter>fn</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1><title id="kernel-thread-destructors-description">Description</title>
      <para>
These functions are provided for cases when an application requires a
function to be automatically called when a thread exits. This is often
useful when, for example, freeing up resources allocated by the thread.
      </para>
      <para>
This support must be enabled with the configuration option
<varname>CYGPKG_KERNEL_THREADS_DESTRUCTORS</varname>. When enabled,
you may register a function of type
<type>cyg_thread_destructor_fn</type> to be called on thread
termination using <function>cyg_thread_add_destructor</function>. You
may also provide it with a piece of arbitrary information in the
<parameter>data</parameter> argument which will be passed to the
destructor function <parameter>fn</parameter> when the thread
terminates. If you no longer wish to call a function previous
registered with <function>cyg_thread_add_destructor</function>, you
may call <function>cyg_thread_rem_destructor</function> with the same
parameters used to register the destructor function. Both these
functions return <literal>true</literal> on success and
<literal>false</literal> on failure.
      </para>
      <para>
By default, thread destructors are per-thread, which means that registering
a destructor function only registers that function for the current thread.
In other words, each thread has its own list of destructors.
Alternatively you may disable the configuration option
<varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> in which
case any registered destructors will be run when <emphasis>any</emphasis>
threads exit. In other words, the thread destructor list is global and all
threads have the same destructors.
      </para>
      <para>
There is a limit to the number of destructors which may be registered,
which can be controlled with the
<varname>CYGNUM_KERNEL_THREADS_DESTRUCTORS</varname> configuration
option. Increasing this value will very slightly increase the amount
of memory in use, and when
<varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> is
enabled, the amount of memory used per thread will increase. When the
limit has been reached, <function>cyg_thread_add_destructor</function>
will return <literal>false</literal>.
      </para>
    </refsect1>

    <refsect1 id="kernel-thread-destructors-context"><title>Valid contexts</title>
      <para>
When <varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname>
is enabled, these functions must only be called from a thread context
as they implicitly operate on the current thread. When
<varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> is
disabled, these functions may be called from thread or DSR context,
or at initialization time.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Exceptions                     -->

  <refentry id="kernel-exceptions">

    <refmeta>
    <refentrytitle>Exception handling</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_exception_set_handler</refname>
      <refname>cyg_exception_clear_handler</refname>
      <refname>cyg_exception_call_handler</refname>
      <refpurpose>Handle processor exceptions</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_exception_set_handler</function></funcdef>
          <paramdef>cyg_code_t <parameter>exception_number</parameter></paramdef>
          <paramdef>cyg_exception_handler_t* <parameter>new_handler</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>new_data</parameter></paramdef>
          <paramdef>cyg_exception_handler_t** <parameter>old_handler</parameter></paramdef>
          <paramdef>cyg_addrword_t* <parameter>old_data</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_exception_clear_handler</function></funcdef>
          <paramdef>cyg_code_t <parameter>exception_number</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_exception_call_handler</function></funcdef>
          <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef>
          <paramdef>cyg_code_t <parameter>exception_number</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>exception_info</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1><title id="kernel-exceptions-description">Description</title>
      <para>
Sometimes code attempts operations that are not legal on the current
hardware, for example dividing by zero, or accessing data through a
pointer that is not properly aligned. When this happens the hardware
will raise an exception. This is very similar to an interrupt, but
happens synchronously with code execution rather than asynchronously
and hence can be tied to the thread that is currently running.
      </para>
      <para>
The exceptions that can be raised depend very much on the hardware,
especially the processor. The corresponding documentation should be
consulted for more details. Alternatively the architectural HAL header
file <filename class="headerfile">hal_intr.h</filename>, or one of the
variant or platform header files it includes, will contain appropriate
definitions. The details of how to handle exceptions, including
whether or not it is possible to recover from them, also depend on the
hardware. 
      </para>
      <para>
Exception handling is optional, and can be disabled through the
configuration option <varname>CYGPKG_KERNEL_EXCEPTIONS</varname>. If
an application has been exhaustively tested and is trusted never to
raise a hardware exception then this option can be disabled and code
and data sizes will be reduced somewhat. If exceptions are left
enabled then the system will provide default handlers for the various
exceptions, but these do nothing. Even the specific type of exception
is ignored, so there is no point in attempting to decode this and
distinguish between say a divide-by-zero and an unaligned access.
If the application installs its own handlers and wants details of the
specific exception being raised then the configuration option
<varname>CYGSEM_KERNEL_EXCEPTIONS_DECODE</varname> has to be enabled.
      </para>
      <para>
An alternative handler can be installed using
<function>cyg_exception_set_handler</function>. This requires a code
for the exception, a function pointer for the new exception handler,
and a parameter to be passed to this handler. Details of the
previously installed exception handler will be returned via the
remaining two arguments, allowing that handler to be reinstated, or
null pointers can be used if this information is of no interest. An
exception handling function should take the following form:
      </para>
      <programlisting width=72>
void
my_exception_handler(cyg_addrword_t data, cyg_code_t exception, cyg_addrword_t info)
{
    &hellip;
}
      </programlisting>
      <para>
The data argument corresponds to the <parameter class="function">new_data</parameter> 
parameter supplied to <function>cyg_exception_set_handler</function>.
The exception code is provided as well, in case a single handler is
expected to support multiple exceptions. The <parameter class="function">info</parameter> 
argument will depend on the hardware and on the specific exception.
      </para>
      <para>
<function>cyg_exception_clear_handler</function> can be used to
restore the default handler, if desired. It is also possible for
software to raise an exception and cause the current handler to be
invoked, but generally this is useful only for testing.
      </para>
      <para>
By default the system maintains a single set of global exception
handlers. However, since exceptions occur synchronously it is
sometimes useful to handle them on a per-thread basis, and have a
different set of handlers for each thread. This behaviour can be
obtained by disabling the configuration option
<varname>CYGSEM_KERNEL_EXCEPTIONS_GLOBAL</varname>. If per-thread
exception handlers are being used then
<function>cyg_exception_set_handler</function> and
<function>cyg_exception_clear_handler</function> apply to the current
thread. Otherwise they apply to the global set of handlers.
      </para>

      <caution><para>
In the current implementation
<function>cyg_exception_call_handler</function> can only be used on
the current thread. There is no support for delivering an exception to
another thread.
      </para></caution>
      <note><para>
Exceptions at the eCos kernel level refer specifically to
hardware-related events such as unaligned accesses to memory or
division by zero. There is no relation with other concepts that are
also known as exceptions, for example the <literal>throw</literal> and
<literal>catch</literal> facilities associated with C++.
      </para></note>

    </refsect1>

    <refsect1 id="kernel-exceptions-context"><title>Valid contexts</title>
      <para>
If the system is configured with a single set of global exception
handlers then
<function>cyg_exception_set_handler</function> and
<function>cyg_exception_clear_handler</function> may be called during
initialization or from thread context. If instead per-thread exception
handlers are being used then it is not possible to install new
handlers during initialization because the functions operate
implicitly on the current thread, so they can only be called from
thread context. <function>cyg_exception_call_handler</function> should
only be called from thread context.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Counters                       -->

  <refentry id="kernel-counters">

    <refmeta>
    <refentrytitle>Counters</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_counter_create</refname>
      <refname>cyg_counter_delete</refname>
      <refname>cyg_counter_current_value</refname>
      <refname>cyg_counter_set_value</refname>
      <refname>cyg_counter_tick</refname>
      <refpurpose>Count event occurrences</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_counter_create</function></funcdef>
          <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef>
          <paramdef>cyg_counter* <parameter>counter</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_counter_delete</function></funcdef>
          <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_tick_count_t <function>cyg_counter_current_value</function></funcdef>
          <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_counter_set_value</function></funcdef>
          <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>new_value</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_counter_tick</function></funcdef>
          <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-counters-description"><title>Description</title>
      <para>
Kernel counters can be used to keep track of how many times a
particular event has occurred. Usually this event is an external
signal of some sort. The most common use of counters is in the
implementation of clocks, but they can be useful with other event
sources as well. Application code can attach <link
linkend="kernel-alarms">alarms</link> to counters, causing a function
to be called when some number of events have occurred.
      </para>
      <para>
A new counter is initialized by a call to
<function>cyg_counter_create</function>. The first argument is used to
return a handle to the new counter which can be used for subsequent
operations. The second argument allows the application to provide the
memory needed for the object, thus eliminating any need for dynamic
memory allocation within the kernel. If a counter is no longer
required and does not have any alarms attached then
<function>cyg_counter_delete</function> can be used to release the
resources, allowing the <structname>cyg_counter</structname> data
structure to be re-used.
      </para>
      <para>
Initializing a counter does not automatically attach it to any source
of events. Instead some other code needs to call
<function>cyg_counter_tick</function> whenever a suitable event
occurs, which will cause the counter to be incremented and may cause
alarms to trigger. The current value associated with the counter can
be retrieved using <function>cyg_counter_current_value</function> and
modified with <function>cyg_counter_set_value</function>. Typically
the latter function is only used during initialization, for example to
set a clock to wallclock time, but it can be used to reset a counter
if necessary. However <function>cyg_counter_set_value</function> will
never trigger any alarms. A newly initialized counter has a starting
value of 0.
      </para>
      <para>
The kernel provides two different implementations of counters. The
default is <varname>CYGIMP_KERNEL_COUNTERS_SINGLE_LIST</varname> which
stores all alarms attached to the counter on a single list. This is
simple and usually efficient. However when a tick occurs the kernel
code has to traverse this list, typically at DSR level, so if there
are a significant number of alarms attached to a single counter this
will affect the system's dispatch latency. The alternative
implementation, <varname>CYGIMP_KERNEL_COUNTERS_MULTI_LIST</varname>,
stores each alarm in one of an array of lists such that at most one of
the lists needs to be searched per clock tick. This involves extra
code and data, but can improve real-time responsiveness in some
circumstances. Another configuration option that is relevant here
is <varname>CYGIMP_KERNEL_COUNTERS_SORT_LIST</varname>, which is
disabled by default. This provides a trade off between doing work
whenever a new alarm is added to a counter and doing work whenever a
tick occurs. It is application-dependent which of these is more
appropriate.
      </para>
    </refsect1>

    <refsect1 id="kernel-counters-context"><title>Valid contexts</title>
      <para>
<function>cyg_counter_create</function> is typically called during
system initialization but may also be called in thread context.
Similarly <function>cyg_counter_delete</function> may be called during
initialization or in thread context.
<function>cyg_counter_current_value</function>,
<function>cyg_counter_set_value</function> and
<function>cyg_counter_tick</function> may be called during
initialization or from thread or DSR context. In fact,
<function>cyg_counter_tick</function> is usually called from inside a
DSR in response to an external event of some sort.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Clocks                         -->

  <refentry id="kernel-clocks">

    <refmeta>
    <refentrytitle>Clocks</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_clock_create</refname>
      <refname>cyg_clock_delete</refname>
      <refname>cyg_clock_to_counter</refname>
      <refname>cyg_clock_set_resolution</refname>
      <refname>cyg_clock_get_resolution</refname>
      <refname>cyg_real_time_clock</refname>
      <refname>cyg_current_time</refname>
      <refpurpose>Provide system clocks</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_clock_create</function></funcdef>
          <paramdef>cyg_resolution_t <parameter>resolution</parameter></paramdef>
          <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef>
          <paramdef>cyg_clock* <parameter>clock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_clock_delete</function></funcdef>
          <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_clock_to_counter</function></funcdef>
          <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef>
          <paramdef>cyg_handle_t* <parameter>counter</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_clock_set_resolution</function></funcdef>
          <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef>
          <paramdef>cyg_resolution_t <parameter>resolution</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_resolution_t <function>cyg_clock_get_resolution</function></funcdef>
          <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_handle_t <function>cyg_real_time_clock</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_tick_count_t <function>cyg_current_time</function></funcdef>
          <void>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-clocks-description"><title>Description</title>
      <para>
In the eCos kernel clock objects are a special form of <link
linkend="kernel-counters">counter</link> objects. They are attached to
a specific type of hardware, clocks that generate ticks at very
specific time intervals, whereas counters can be used with any event
source.
      </para>
      <para>
In a default configuration the kernel provides a single clock
instance, the real-time clock. This gets used for timeslicing and for
operations that involve a timeout, for example
<function>cyg_semaphore_timed_wait</function>. If this functionality
is not required it can be removed from the system using the
configuration option <varname>CYGVAR_KERNEL_COUNTERS_CLOCK</varname>.
Otherwise the real-time clock can be accessed by a call to
<function>cyg_real_time_clock</function>, allowing applications to
attach alarms, and the current counter value can be obtained using
<function>cyg_current_time</function>.
      </para>
      <para>
Applications can create and destroy additional clocks if desired,
using <function>cyg_clock_create</function> and
<function>cyg_clock_delete</function>. The first argument to
<function>cyg_clock_create</function> specifies the
<link linkend="kernel-clocks-resolution">resolution</link> this clock
will run at. The second argument is used to return a handle for this
clock object, and the third argument provides the kernel with the
memory needed to hold this object. This clock will not actually tick
by itself. Instead it is the responsibility of application code to
initialize a suitable hardware timer to generate interrupts at the
appropriate frequency, install an interrupt handler for this, and
call <function>cyg_counter_tick</function> from inside the DSR.
Associated with each clock is a kernel counter, a handle for which can
be obtained using <function>cyg_clock_to_counter</function>.
      </para>
    </refsect1>

    <refsect1 id="kernel-clocks-resolution"><title>Clock Resolutions and Ticks</title>
      <para>
At the kernel level all clock-related operations including delays,
timeouts and alarms work in units of clock ticks, rather than in units
of seconds or milliseconds. If the calling code, whether the
application or some other package, needs to operate using units such
as milliseconds then it has to convert from these units to clock
ticks.
      </para>
      <para>
The main reason for this is that it accurately reflects the
hardware: calling something like <function>nanosleep</function> with a
delay of ten nanoseconds will not work as intended on any real
hardware because timer interrupts simply will not happen that
frequently; instead calling <function>cyg_thread_delay</function> with
the equivalent delay of 0 ticks gives a much clearer indication that
the application is attempting something inappropriate for the target
hardware. Similarly, passing a delay of five ticks to
<function>cyg_thread_delay</function> makes it fairly obvious that
the current thread will be suspended for somewhere between four and
five clock periods, as opposed to passing 50000000 to
<function>nanosleep</function> which suggests a granularity that is
not actually provided.
      </para>
      <para>
A secondary reason is that conversion between clock ticks and units
such as milliseconds can be somewhat expensive, and whenever possible
should be done at compile-time or by the application developer rather
than at run-time. This saves code size and cpu cycles.
      </para>
      <para>
The information needed to perform these conversions is the clock
resolution. This is a structure with two fields, a dividend and a
divisor, and specifies the number of nanoseconds between clock ticks.
For example a clock that runs at 100Hz will have 10 milliseconds
between clock ticks, or 10000000 nanoseconds. The ratio between the
resolution's dividend and divisor will therefore be 10000000 to 1, and
typical values for these might be 1000000000 and 100. If the clock
runs at a different frequency, say 60Hz, the numbers could be
1000000000 and 60 respectively. Given a delay in nanoseconds, this can
be converted to clock ticks by multiplying with the the divisor and
then dividing by the dividend. For example a delay of 50 milliseconds
corresponds to 50000000 nanoseconds, and with a clock frequency of
100Hz this can be converted to
((50000000&nbsp;*&nbsp;100)&nbsp;/&nbsp;1000000000)&nbsp;=&nbsp;5
clock ticks. Given the large numbers involved this arithmetic normally
has to be done using 64-bit precision and the
<type>long&nbsp;long</type> data type, but allows code to run on
hardware with unusual clock frequencies.
      </para>
      <para>
The default frequency for the real-time clock on any platform is
usually about 100Hz, but platform-specific documentation should be
consulted for this information. Usually it is possible to override
this default by configuration options, but again this depends on the
capabilities of the underlying hardware. The resolution for any clock
can be obtained using <function>cyg_clock_get_resolution</function>.
For clocks created by application code, there is also a function
<function>cyg_clock_set_resolution</function>. This does not affect
the underlying hardware timer in any way, it merely updates the
information that will be returned in subsequent calls to
<function>cyg_clock_get_resolution</function>: changing the actual
underlying clock frequency will require appropriate manipulation of
the timer hardware.
      </para>
    </refsect1>

    <refsect1 id="kernel-clocks-context"><title>Valid contexts</title>
      <para>
<function>cyg_clock_create</function> is usually only called during
system initialization (if at all), but may also be called from thread
context. The same applies to <function>cyg_clock_delete</function>.
The remaining functions may be called during initialization, from
thread context, or from DSR context, although it should be noted that
there is no locking between
<function>cyg_clock_get_resolution</function> and
<function>cyg_clock_set_resolution</function> so theoretically it is
possible that the former returns an inconsistent data structure.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Alarms                         -->

  <refentry id="kernel-alarms">

    <refmeta>
    <refentrytitle>Alarms</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_alarm_create</refname>
      <refname>cyg_alarm_delete</refname>
      <refname>cyg_alarm_initialize</refname>
      <refname>cyg_alarm_enable</refname>
      <refname>cyg_alarm_disable</refname>
      <refpurpose>Run an alarm function when a number of events have occurred</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_alarm_create</function></funcdef>
          <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef>
          <paramdef>cyg_alarm_t* <parameter>alarmfn</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef>
          <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef>
          <paramdef>cyg_alarm* <parameter>alarm</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_alarm_delete</function></funcdef>
          <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_alarm_initialize</function></funcdef>
          <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>trigger</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>interval</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_alarm_enable</function></funcdef>
          <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_alarm_disable</function></funcdef>
          <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-alarms-description"><title>Description</title>
      <para>
Kernel alarms are used together with counters and allow for action to
be taken when a certain number of events have occurred. If the counter
is associated with a clock then the alarm action happens when the
appropriate number of clock ticks have occurred, in other words after
a certain period of time.
      </para>
      <para>
Setting up an alarm involves a two-step process. First the alarm must
be created with a call to <function>cyg_alarm_create</function>. This
takes five arguments. The first identifies the counter to which the
alarm should be attached. If the alarm should be attached to the
system's real-time clock then <function>cyg_real_time_clock</function>
and <function>cyg_clock_to_counter</function> can be used to get hold
of the appropriate handle. The next two arguments specify the action
to be taken when the alarm is triggered, in the form of a function
pointer and some data. This function should take the form:
      </para>
      <programlisting width=72>
void
alarm_handler(cyg_handle_t alarm, cyg_addrword_t data)
{
    &hellip;
}
      </programlisting>
      <para>
The data argument passed to the alarm function corresponds to the
third argument passed to <function>cyg_alarm_create</function>.
The fourth argument to <function>cyg_alarm_create</function> is used
to return a handle to the newly-created alarm object, and the final
argument provides the memory needed for the alarm object and thus
avoids any need for dynamic memory allocation within the kernel.
      </para>
      <para>
Once an alarm has been created a further call to
<function>cyg_alarm_initialize</function> is needed to activate it.
The first argument specifies the alarm. The second argument indicates
the number of events, for example clock ticks, that need to occur
before the alarm triggers. If the third argument is 0 then the alarm
will only trigger once. A non-zero value specifies that the alarm
should trigger repeatedly, with an interval of the specified number of
events.
      </para>
      <para>
Alarms can be temporarily disabled and reenabled using
<function>cyg_alarm_disable</function> and
<function>cyg_alarm_enable</function>. Alternatively another call to
<function>cyg_alarm_initialize</function> can be used to modify the
behaviour of an existing alarm. If an alarm is no longer required then
the associated resources can be released using
<function>cyg_alarm_delete</function>. 
      </para>
      <para>
The alarm function is invoked when a counter tick occurs, in other
words when there is a call to <function>cyg_counter_tick</function>,
and will happen in the same context. If the alarm is associated with
the system's real-time clock then this will be DSR context, following
a clock interrupt. If the alarm is associated with some other
application-specific counter then the details will depend on how that
counter is updated.
      </para>
      <para>
If two or more alarms are registered for precisely the same counter tick,
the order of execution of the alarm functions is unspecified.
      </para>
    </refsect1>

    <refsect1 id="kernel-alarms-context"><title>Valid contexts</title>
      <para>
<function>cyg_alarm_create</function>
<function>cyg_alarm_initialize</function> is typically called during
system initialization but may also be called in thread context. The
same applies to <function>cyg_alarm_delete</function>.
<function>cyg_alarm_initialize</function>,
<function>cyg_alarm_disable</function> and
<function>cyg_alarm_enable</function> may be called during
initialization or from thread or DSR context, but
<function>cyg_alarm_enable</function> and
<function>cyg_alarm_initialize</function> may be expensive operations
and should only be called when necessary.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Mutexes                        -->

  <refentry id="kernel-mutexes">

    <refmeta>
    <refentrytitle>Mutexes</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_mutex_init</refname>
      <refname>cyg_mutex_destroy</refname>
      <refname>cyg_mutex_lock</refname>
      <refname>cyg_mutex_trylock</refname>
      <refname>cyg_mutex_unlock</refname>
      <refname>cyg_mutex_release</refname>
      <refname>cyg_mutex_set_ceiling</refname>
      <refname>cyg_mutex_set_protocol</refname>
      <refpurpose>Synchronization primitive</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_mutex_init</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_mutex_destroy</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mutex_lock</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mutex_trylock</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_mutex_unlock</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_mutex_release</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_mutex_set_ceiling</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
          <paramdef>cyg_priority_t <parameter>priority</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_mutex_set_protocol</function></funcdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
          <paramdef>enum cyg_mutex_protocol <parameter>protocol/</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-mutexes-description"><title>Description</title>
      <para>
The purpose of mutexes is to let threads share resources safely. If
two or more threads attempt to manipulate a data structure with no
locking between them then the system may run for quite some time
without apparent problems, but sooner or later the data structure will
become inconsistent and the application will start behaving strangely
and is quite likely to crash. The same can apply even when
manipulating a single variable or some other resource. For example,
consider:
      </para>
<programlisting width=72>
static volatile int counter = 0;

void
process_event(void)
{
    &hellip;

    counter++;
}
</programlisting>
      <para>
Assume that after a certain period of time <varname>counter</varname>
has a value of 42, and two threads A and B running at the same
priority call <function>process_event</function>. Typically thread A
will read the value of <varname>counter</varname> into a register,
increment this register to 43, and write this updated value back to
memory. Thread B will do the same, so usually
<varname>counter</varname> will end up with a value of 44. However if
thread A is timesliced after reading the old value 42 but before
writing back 43, thread B will still read back the old value and will
also write back 43. The net result is that the counter only gets
incremented once, not twice, which depending on the application may
prove disastrous.
      </para>
      <para>
Sections of code like the above which involve manipulating shared data
are generally known as critical regions. Code should claim a lock
before entering a critical region and release the lock when leaving.
Mutexes provide an appropriate synchronization primitive for this.
      </para>
      <programlisting width=72>
static volatile int counter = 0;
static cyg_mutex_t  lock;

void
process_event(void)
{
    &hellip;

    cyg_mutex_lock(&amp;lock);
    counter++;
    cyg_mutex_unlock(&amp;lock);
}
      </programlisting>
      <para>
A mutex must be initialized before it can be used, by calling
<function>cyg_mutex_init</function>. This takes a pointer to a
<structname>cyg_mutex_t</structname> data structure which is typically
statically allocated, and may be part of a larger data structure. If a
mutex is no longer required and there are no threads waiting on it
then <function>cyg_mutex_destroy</function> can be used.
      </para>
      <para>
The main functions for using a mutex are
<function>cyg_mutex_lock</function> and
<function>cyg_mutex_unlock</function>. In normal operation
<function>cyg_mutex_lock</function> will return success after claiming
the mutex lock, blocking if another thread currently owns the mutex.
However the lock operation may fail if other code calls
<function>cyg_mutex_release</function> or
<function>cyg_thread_release</function>, so if these functions may get
used then it is important to check the return value. The current owner
of a mutex should call <function>cyg_mutex_unlock</function> when a
lock is no longer required. This operation must be performed by the
owner, not by another thread.
      </para>
      <para>
<function>cyg_mutex_trylock</function> is a variant of
<function>cyg_mutex_lock</function> that will always return
immediately, returning success or failure as appropriate. This
function is rarely useful. Typical code locks a mutex just before
entering a critical region, so if the lock cannot be claimed then
there may be nothing else for the current thread to do. Use of this
function may also cause a form of priority inversion if the owner
owner runs at a lower priority, because the priority inheritance code
will not be triggered. Instead the current thread continues running,
preventing the owner from getting any cpu time, completing the
critical region, and releasing the mutex.
      </para>
      <para>
<function>cyg_mutex_release</function> can be used to wake up all
threads that are currently blocked inside a call to
<function>cyg_mutex_lock</function> for a specific mutex. These lock
calls will return failure. The current mutex owner is not affected.
      </para>
    </refsect1>

    <refsect1 id="kernel-mutexes-priority-inversion"><title>Priority Inversion</title>
      <para>
The use of mutexes gives rise to a problem known as priority
inversion. In a typical scenario this requires three threads A, B, and
C, running at high, medium and low priority respectively. Thread A and
thread B are temporarily blocked waiting for some event, so thread C
gets a chance to run, needs to enter a critical region, and locks
a mutex. At this point threads A and B are woken up - the exact order
does not matter. Thread A needs to claim the same mutex but has to
wait until C has left the critical region and can release the mutex.
Meanwhile thread B works on something completely different and can
continue running without problems. Because thread C is running a lower
priority than B it will not get a chance to run until B blocks for
some reason, and hence thread A cannot run either. The overall effect
is that a high-priority thread A cannot proceed because of a lower
priority thread B, and priority inversion has occurred.
      </para>
      <para>
In simple applications it may be possible to arrange the code such
that priority inversion cannot occur, for example by ensuring that a
given mutex is never shared by threads running at different priority
levels. However this may not always be possible even at the
application level. In addition mutexes may be used internally by
underlying code, for example the memory allocation package, so careful
analysis of the whole system would be needed to be sure that priority
inversion cannot occur. Instead it is common practice to use one of
two techniques: priority ceilings and priority inheritance.
      </para>
      <para>
Priority ceilings involve associating a priority with each mutex.
Usually this will match the highest priority thread that will ever
lock the mutex. When a thread running at a lower priority makes a
successful call to <function>cyg_mutex_lock</function> or
<function>cyg_mutex_trylock</function> its priority will be boosted to
that of the mutex. For example, given the previous example the
priority associated with the mutex would be that of thread A, so for
as long as it owns the mutex thread C will run in preference to thread
B. When C releases the mutex its priority drops to the normal value
again, allowing A to run and claim the mutex. Setting the
priority for a mutex involves a call to
<function>cyg_mutex_set_ceiling</function>, which is typically called
during initialization. It is possible to change the ceiling
dynamically but this will only affect subsequent lock operations, not
the current owner of the mutex.
      </para>
      <para>
Priority ceilings are very suitable for simple applications, where for
every thread in the system it is possible to work out which mutexes
will be accessed. For more complicated applications this may prove
difficult, especially if thread priorities change at run-time. An
additional problem occurs for any mutexes outside the application, for
example used internally within eCos packages. A typical eCos package
will be unaware of the details of the various threads in the system,
so it will have no way of setting suitable ceilings for its internal
mutexes. If those mutexes are not exported to application code then 
using priority ceilings may not be viable. The kernel does provide a
configuration option
<varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_DEFAULT_PRIORITY</varname>
that can be used to set the default priority ceiling for all mutexes,
which may prove sufficient.
      </para>
      <para>
The alternative approach is to use priority inheritance: if a thread
calls <function>cyg_mutex_lock</function> for a mutex that it
currently owned by a lower-priority thread, then the owner will have
its priority raised to that of the current thread. Often this is more
efficient than priority ceilings because priority boosting only
happens when necessary, not for every lock operation, and the required
priority is determined at run-time rather than by static analysis.
However there are complications when multiple threads running at
different priorities try to lock a single mutex, or when the current
owner of a mutex then tries to lock additional mutexes, and this makes
the implementation significantly more complicated than priority
ceilings. 
      </para>
      <para>
There are a number of configuration options associated with priority
inversion. First, if after careful analysis it is known that priority
inversion cannot arise then the component
<function>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL</function>
can be disabled. More commonly this component will be enabled, and one
of either
<varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_INHERIT</varname>
or
<varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_CEILING</varname>
will be selected, so that one of the two protocols is available for
all mutexes. It is possible to select multiple protocols, so that some
mutexes can have priority ceilings while others use priority
inheritance or no priority inversion protection at all. Obviously this
flexibility will add to the code size and to the cost of mutex
operations. The default for all mutexes will be controlled by
<varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_DEFAULT</varname>,
and can be changed at run-time using
<function>cyg_mutex_set_protocol</function>. 
      </para>
      <para>
Priority inversion problems can also occur with other synchronization
primitives such as semaphores. For example there could be a situation
where a high-priority thread A is waiting on a semaphore, a
low-priority thread C needs to do just a little bit more work before
posting the semaphore, but a medium priority thread B is running and
preventing C from making progress. However a semaphore does not have
the concept of an owner, so there is no way for the system to know
that it is thread C which would next post to the semaphore. Hence
there is no way for the system to boost the priority of C
automatically and prevent the priority inversion. Instead situations
like this have to be detected by application developers and
appropriate precautions have to be taken, for example making sure that
all the threads run at suitable priorities at all times.
      </para>
      <warning><para>
The current implementation of priority inheritance within the eCos
kernel does not handle certain exceptional circumstances completely
correctly. Problems will only arise if a thread owns one mutex,
then attempts to claim another mutex, and there are other threads
attempting to lock these same mutexes. Although the system will
continue running, the current owners of the various mutexes involved
may not run at the priority they should. This situation never arises
in typical code because a mutex will only be locked for a small
critical region, and there is no need to manipulate other shared resources
inside this region. A more complicated implementation of priority
inheritance is possible but would add significant overhead and certain
operations would no longer be deterministic.
      </para></warning>
      <warning><para>
Support for priority ceilings and priority inheritance is not
implemented for all schedulers. In particular neither priority
ceilings nor priority inheritance are currently available for the
bitmap scheduler.
      </para></warning>
    </refsect1>

    <refsect1 id="kernel-mutexes-alternatives"><title>Alternatives</title>
      <para>
In nearly all circumstances, if two or more threads need to share some
data then protecting this data with a mutex is the correct thing to
do. Mutexes are the only primitive that combine a locking mechanism
and protection against priority inversion problems. However this
functionality is achieved at a cost, and in exceptional circumstances
such as an application's most critical inner loop it may be desirable
to use some other means of locking.
      </para>
      <para>
When a critical region is very very small it is possible to lock the
scheduler, thus ensuring that no other thread can run until the
scheduler is unlocked again. This is achieved with calls to <link
linkend="kernel-schedcontrol"><function>cyg_scheduler_lock</function></link>
and <function>cyg_scheduler_unlock</function>. If the critical region
is sufficiently small then this can actually improve both performance
and dispatch latency because <function>cyg_mutex_lock</function> also
locks the scheduler for a brief period of time. This approach will not
work on SMP systems because another thread may already be running on a
different processor and accessing the critical region.
      </para>
      <para>
Another way of avoiding the use of mutexes is to make sure that all
threads that access a particular critical region run at the same
priority and configure the system with timeslicing disabled
(<varname>CYGSEM_KERNEL_SCHED_TIMESLICE</varname>). Without
timeslicing a thread can only be preempted by a higher-priority one,
or if it performs some operation that can block. This approach
requires that none of the operations in the critical region can block,
so for example it is not legal to call
<function>cyg_semaphore_wait</function>. It is also vulnerable to
any changes in the configuration or to the various thread priorities:
any such changes may now have unexpected side effects. It will not
work on SMP systems.
      </para>
    </refsect1>

    <refsect1 id="kernel-mutexes-recursive"><title>Recursive Mutexes</title>
      <para>
The implementation of mutexes within the eCos kernel does not support
recursive locks. If a thread has locked a mutex and then attempts to
lock the mutex again, typically as a result of some recursive call in
a complicated call graph, then either an assertion failure will be
reported or the thread will deadlock. This behaviour is deliberate.
When a thread has just locked a mutex associated with some data
structure, it can assume that that data structure is in a consistent
state. Before unlocking the mutex again it must ensure that the data
structure is again in a consistent state. Recursive mutexes allow a
thread to make arbitrary changes to a data structure, then in a
recursive call lock the mutex again while the data structure is still
inconsistent. The net result is that code can no longer make any
assumptions about data structure consistency, which defeats the
purpose of using mutexes.
      </para>
    </refsect1>

    <refsect1 id="kernel-mutexes-context"><title>Valid contexts</title>
      <para>
<function>cyg_mutex_init</function>,
<function>cyg_mutex_set_ceiling</function> and
<function>cyg_mutex_set_protocol</function> are normally called during
initialization but may also be called from thread context. The
remaining functions should only be called from thread context. Mutexes
serve as a mutual exclusion mechanism between threads, and cannot be
used to synchronize between threads and the interrupt handling
subsystem. If a critical region is shared between a thread and a DSR
then it must be protected using <link
linkend="kernel-schedcontrol"><function>cyg_scheduler_lock</function></link>
and <function>cyg_scheduler_unlock</function>. If a critical region is
shared between a thread and an ISR, it must be protected by disabling
or masking interrupts. Obviously these operations must be used with
care because they can affect dispatch and interrupt latencies.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Condition variables            -->

  <refentry id="kernel-condition-variables">

    <refmeta>
    <refentrytitle>Condition Variables</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_cond_init</refname>
      <refname>cyg_cond_destroy</refname>
      <refname>cyg_cond_wait</refname>
      <refname>cyg_cond_timed_wait</refname>
      <refname>cyg_cond_signal</refname>
      <refname>cyg_cond_broadcast</refname>
      <refpurpose>Synchronization primitive</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_cond_init</function></funcdef>
          <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef>
          <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_cond_destroy</function></funcdef>
          <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_cond_wait</function></funcdef>
          <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_cond_timed_wait</function></funcdef>
          <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_cond_signal</function></funcdef>
          <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_cond_broadcast</function></funcdef>
          <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-condition-variables-description"><title>Description</title>

      <para>
Condition variables are used in conjunction with mutexes to implement
long-term waits for some condition to become true. For example
consider a set of functions that control access to a pool of
resources:
      </para>

      <programlisting width=72>

cyg_mutex_t res_lock;
res_t res_pool[RES_MAX];
int res_count = RES_MAX;

void res_init(void)
{
    cyg_mutex_init(&amp;res_lock);
    &lt;fill pool with resources&gt;
}

res_t res_allocate(void)
{
    res_t res;

    cyg_mutex_lock(&amp;res_lock);               // lock the mutex

    if( res_count == 0 )                     // check for free resource
        res = RES_NONE;                      // return RES_NONE if none
    else
    {
        res_count--;                         // allocate a resources
        res = res_pool[res_count];
    }

    cyg_mutex_unlock(&amp;res_lock);             // unlock the mutex

    return res;
}

void res_free(res_t res)
{
    cyg_mutex_lock(&amp;res_lock);               // lock the mutex

    res_pool[res_count] = res;               // free the resource
    res_count++;

    cyg_mutex_unlock(&amp;res_lock);             // unlock the mutex
}
      </programlisting>

      <para>
These routines use the variable <varname>res_count</varname> to keep
track of the resources available. If there are none then
<function>res_allocate</function> returns <literal>RES_NONE</literal>,
which the caller must check for and take appropriate error handling
actions.
      </para>

      <para>
Now suppose that we do not want to return
<literal>RES_NONE</literal> when there are no resources, but want to
wait for one to become available. This is where a condition variable
can be used:
      </para>

      <programlisting width=72>

cyg_mutex_t res_lock;
cyg_cond_t res_wait;
res_t res_pool[RES_MAX];
int res_count = RES_MAX;

void res_init(void)
{
    cyg_mutex_init(&amp;res_lock);
    cyg_cond_init(&amp;res_wait, &amp;res_lock);
    &lt;fill pool with resources&gt;
}

res_t res_allocate(void)
{
    res_t res;

    cyg_mutex_lock(&amp;res_lock);               // lock the mutex

    while( res_count == 0 )                  // wait for a resources
        cyg_cond_wait(&amp;res_wait);

    res_count--;                             // allocate a resource
    res = res_pool[res_count];

    cyg_mutex_unlock(&amp;res_lock);             // unlock the mutex

    return res;
}

void res_free(res_t res)
{
    cyg_mutex_lock(&amp;res_lock);               // lock the mutex

    res_pool[res_count] = res;               // free the resource
    res_count++;

    cyg_cond_signal(&amp;res_wait);              // wake up any waiting allocators

    cyg_mutex_unlock(&amp;res_lock);             // unlock the mutex
}
      </programlisting>

      <para>
In this version of the code, when <function>res_allocate</function>
detects that there are no resources it calls
<function>cyg_cond_wait</function>. This does two things: it unlocks
the mutex, and puts the calling thread to sleep on the condition
variable. When <function>res_free</function> is eventually called, it
puts a resource back into the pool and calls
<function>cyg_cond_signal</function> to wake up any thread waiting on
the condition variable. When the waiting thread eventually gets to run again,
it will re-lock the mutex before returning from
<function>cyg_cond_wait</function>.
      </para>

      <para>
There are two important things to note about the way in which this
code works. The first is that the mutex unlock and wait in
<function>cyg_cond_wait</function> are atomic: no other thread can run
between the unlock and the wait. If this were not the case then a call
to <function>res_free</function> by that thread would release the
resource but the call to <function>cyg_cond_signal</function> would be
lost, and the first thread would end up waiting when there were
resources available.
      </para>

      <para>
The second feature is that the call to
<function>cyg_cond_wait</function> is in a <literal>while</literal>
loop and not a simple <literal>if</literal> statement. This is because
of the need to re-lock the mutex in <function>cyg_cond_wait</function>
when the signalled thread reawakens. If there are other threads
already queued to claim the lock then this thread must wait. Depending
on the scheduler and the queue order, many other threads may have
entered the critical section before this one gets to run. So the
condition that it was waiting for may have been rendered false. Using
a loop around all condition variable wait operations is the only way
to guarantee that the condition being waited for is still true after
waiting.
      </para>

      <para>
Before a condition variable can be used it must be initialized with a
call to <function>cyg_cond_init</function>. This requires two
arguments, memory for the data structure and a pointer to an existing
mutex. This mutex will not be initialized by
<function>cyg_cond_init</function>, instead a separate call to
<function>cyg_mutex_init</function> is required. If a condition
variable is no longer required and there are no threads waiting on it
then <function>cyg_cond_destroy</function> can be used.
      </para>
      <para>
When a thread needs to wait for a condition to be satisfied it can
call <function>cyg_cond_wait</function>. The thread must have already
locked the mutex that was specified in the
<function>cyg_cond_init</function> call. This mutex will be unlocked
and the current thread will be suspended in an atomic operation. When
some other thread performs a signal or broadcast operation the current
thread will be woken up and automatically reclaim ownership of the mutex
again, allowing it to examine global state and determine whether or
not the condition is now satisfied.
      </para>
      <para>
The kernel supplies a variant of this function,
<function>cyg_cond_timed_wait</function>, which can be used to wait on
the condition variable or until some number of clock ticks have
occurred. The number of ticks is specified as an absolute, not
relative tick count, and so in order to wait for a relative number of
ticks, the return value of the <function>cyg_current_time()</function>
function should be added to determine the absolute number of ticks.
The mutex will always be reclaimed before
<function>cyg_cond_timed_wait</function> returns, regardless of
whether it was a result of a signal operation or a timeout.
      </para>
      <para>
There is no <function>cyg_cond_trywait</function> function because
this would not serve any purpose. If a thread has locked the mutex and
determined that the condition is satisfied, it can just release the
mutex and return. There is no need to perform any operation on the
condition variable.
      </para>
      <para>
When a thread changes shared state that may affect some other thread
blocked on a condition variable, it should call either
<function>cyg_cond_signal</function> or
<function>cyg_cond_broadcast</function>. These calls do not require
ownership of the mutex, but usually the mutex will have been claimed
before updating the shared state. A signal operation only wakes up the
first thread that is waiting on the condition variable, while a
broadcast wakes up all the threads. If there are no threads waiting on
the condition variable at the time, then the signal or broadcast will
have no effect: past signals are not counted up or remembered in any
way. Typically a signal should be used when all threads will check the
same condition and at most one thread can continue running. A
broadcast should be used if threads check slightly different
conditions, or if the change to the global state might allow multiple
threads to proceed.
      </para>
    </refsect1>

    <refsect1 id="kernel-condition-variables-context"><title>Valid contexts</title>
      <para>
<function>cyg_cond_init</function> is typically called during system
initialization but may also be called in thread context. The same
applies to <function>cyg_cond_delete</function>.
<function>cyg_cond_wait</function> and
<function>cyg_cond_timedwait</function> may only be called from thread
context since they may block. <function>cyg_cond_signal</function> and
<function>cyg_cond_broadcast</function> may be called from thread or
DSR context.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Semaphores                     -->

  <refentry id="kernel-semaphores">

    <refmeta>
    <refentrytitle>Semaphores</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_semaphore_init</refname>
      <refname>cyg_semaphore_destroy</refname>
      <refname>cyg_semaphore_wait</refname>
      <refname>cyg_semaphore_timed_wait</refname>
      <refname>cyg_semaphore_post</refname>
      <refname>cyg_semaphore_peek</refname>
      <refpurpose>Synchronization primitive</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_semaphore_init</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
          <paramdef>cyg_count32 <parameter>val</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_semaphore_destroy</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_semaphore_wait</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_semaphore_timed_wait</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_semaphore_trywait</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_semaphore_post</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_semaphore_peek</function></funcdef>
          <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef>
          <paramdef>cyg_count32* <parameter>val</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-semaphores-description"><title>Description</title>
      <para>
Counting semaphores are a <link
linkend="kernel-overview-synch-primitives">synchronization
primitive</link> that allow threads to wait until an event has
occurred. The event may be generated by a producer thread, or by a DSR
in response to a hardware interrupt. Associated with each semaphore is
an integer counter that keeps track of the number of events that have
not yet been processed. If this counter is zero, an attempt by a
consumer thread to wait on the semaphore will block until some other
thread or a DSR posts a new event to the semaphore. If the counter is
greater than zero then an attempt to wait on the semaphore will
consume one event, in other words decrement the counter, and return
immediately. Posting to a semaphore will wake up the first thread that
is currently waiting, which will then resume inside the semaphore wait
operation and decrement the counter again.
      </para>
      <para>
Another use of semaphores is for certain forms of resource management.
The counter would correspond to how many of a certain type of resource
are currently available, with threads waiting on the semaphore to
claim a resource and posting to release the resource again. In
practice <link linkend="kernel-condition-variables">condition
variables</link> are usually much better suited for operations like
this.
      </para>
      <para>
<function>cyg_semaphore_init</function> is used to initialize a
semaphore. It takes two arguments, a pointer to a
<structname>cyg_sem_t</structname> structure and an initial value for
the counter. Note that semaphore operations, unlike some other parts
of the kernel API, use pointers to data structures rather than
handles. This makes it easier to embed semaphores in a larger data
structure. The initial counter value can be any number, zero, positive
or negative, but typically a value of zero is used to indicate that no
events have occurred yet.
      </para>
      <para>
<function>cyg_semaphore_wait</function> is used by a consumer thread
to wait for an event. If the current counter is greater than 0, in
other words if the event has already occurred in the past, then the
counter will be decremented and the call will return immediately.
Otherwise the current thread will be blocked until there is a
<function>cyg_semaphore_post</function> call.
      </para>
      <para>
<function>cyg_semaphore_post</function> is called when an event has
occurs. This increments the counter and wakes up the first thread
waiting on the semaphore (if any). Usually that thread will then
continue running inside <function>cyg_semaphore_wait</function> and
decrement the counter again. However other scenarioes are possible.
For example the thread calling <function>cyg_semaphore_post</function>
may be running at high priority, some other thread running at medium
priority may be about to call <function>cyg_semaphore_wait</function>
when it next gets a chance to run, and a low priority thread may be
waiting on the semaphore. What will happen is that the current high
priority thread continues running until it is descheduled for some
reason, then the medium priority thread runs and its call to
<function>cyg_semaphore_wait</function> succeeds immediately, and
later on the low priority thread runs again, discovers a counter value
of 0, and blocks until another event is posted. If there are multiple
threads blocked on a semaphore then the configuration option
<varname>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</varname> determines which
one will be woken up by a post operation.
      </para>
      <para>
<function>cyg_semaphore_wait</function> returns a boolean. Normally it
will block until it has successfully decremented the counter, retrying
as necessary, and return success. However the wait operation may be
aborted by a call to <link
linkend="kernel-thread-control"><function>cyg_thread_release</function></link>,
and <function>cyg_semaphore_wait</function> will then return false.
      </para>
      <para>
<function>cyg_semaphore_timed_wait</function> is a variant of
<function>cyg_semaphore_wait</function>. It can be used to wait until
either an event has occurred or a number of clock ticks have happened.
The number of ticks is specified as an absolute, not relative tick
count, and so in order to wait for a relative number of ticks, the
return value of the <function>cyg_current_time()</function> function
should be added to determine the absolute number of ticks. The
function returns success if the semaphore wait operation succeeded, or
false if the operation timed out or was aborted by
<function>cyg_thread_release</function>.
If support for the real-time
clock has been removed from the current configuration then this
function will not be available.
<function>cyg_semaphore_trywait</function> is another variant which
will always return immediately rather than block, again returning
success or failure. If <function>cyg_semaphore_timedwait</function>
is given a timeout in the past, it operates like
<function>cyg_semaphore_trywait</function>.
      </para>
      <para>
<function>cyg_semaphore_peek</function> can be used to get hold of the
current counter value. This function is rarely useful except for
debugging purposes since the counter value may change at any time if
some other thread or a DSR performs a semaphore operation.
      </para>
    </refsect1>

    <refsect1 id="kernel-semaphores-context"><title>Valid contexts</title>
      <para>
<function>cyg_semaphore_init</function> is normally called during
initialization but may also be called from thread context.
<function>cyg_semaphore_wait</function> and
<function>cyg_semaphore_timed_wait</function> may only be called from
thread context because these operations may block.
<function>cyg_semaphore_trywait</function>,
<function>cyg_semaphore_post</function> and
<function>cyg_semaphore_peek</function> may be called from thread or
DSR context.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Mail boxes                     -->

  <refentry id="kernel-mail-boxes">

    <refmeta>
    <refentrytitle>Mail boxes</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_mbox_create</refname>
      <refname>cyg_mbox_delete</refname>
      <refname>cyg_mbox_get</refname>
      <refname>cyg_mbox_timed_get</refname>
      <refname>cyg_mbox_tryget</refname>
      <refname>cyg_mbox_peek_item</refname>
      <refname>cyg_mbox_put</refname>
      <refname>cyg_mbox_timed_put</refname>
      <refname>cyg_mbox_tryput</refname>
      <refname>cyg_mbox_peek</refname>
      <refname>cyg_mbox_waiting_to_get</refname>
      <refname>cyg_mbox_waiting_to_put</refname>
      <refpurpose>Synchronization primitive</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_mbox_create</function></funcdef>
          <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef>
          <paramdef>cyg_mbox* <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_mbox_delete</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void* <function>cyg_mbox_get</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void* <function>cyg_mbox_timed_get</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void* <function>cyg_mbox_tryget</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_count32 <function>cyg_mbox_peek</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void* <function>cyg_mbox_peek_item</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mbox_put</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
          <paramdef>void* <parameter>item</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mbox_timed_put</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
          <paramdef>void* <parameter>item</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mbox_tryput</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
          <paramdef>void* <parameter>item</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mbox_waiting_to_get</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_mbox_waiting_to_put</function></funcdef>
          <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-mail-boxes-description"><title>Description</title>
      <para>
Mail boxes are a synchronization primitive. Like semaphores they
can be used by a consumer thread to wait until a certain event has
occurred, but the producer also has the ability to transmit some data
along with each event. This data, the message, is normally a pointer
to some data structure. It is stored in the mail box itself, so the
producer thread that generates the event and provides the data usually
does not have to block until some consumer thread is ready to receive
the event. However a mail box will only have a finite capacity,
typically ten slots. Even if the system is balanced and events are
typically consumed at least as fast as they are generated, a burst of
events can cause the mail box to fill up and the generating thread
will block until space is available again. This behaviour is very
different from semaphores, where it is only necessary to maintain a
counter and hence an overflow is unlikely.
      </para>
      <para>
Before a mail box can be used it must be created with a call to
<function>cyg_mbox_create</function>. Each mail box has a unique
handle which will be returned via the first argument and which should
be used for subsequent operations.
<function>cyg_mbox_create</function> also requires an area of memory
for the kernel structure, which is provided by the
<structname>cyg_mbox</structname> second argument. If a mail box is
no longer required then <function>cyg_mbox_delete</function> can be
used. This will simply discard any messages that remain posted.
      </para>
      <para>
The main function for waiting on a mail box is
<function>cyg_mbox_get</function>. If there is a pending message
because of a call to <function>cyg_mbox_put</function> then
<function>cyg_mbox_get</function> will return immediately with the
message that was put into the mail box. Otherwise this function
will block until there is a put operation. Exceptionally the thread
can instead be unblocked by a call to
<function>cyg_thread_release</function>, in which case
<function>cyg_mbox_get</function> will return a null pointer. It is
assumed that there will never be a call to
<function>cyg_mbox_put</function> with a null pointer, because it
would not be possible to distinguish between that and a release
operation. Messages are always retrieved in the order in which they
were put into the mail box, and there is no support for messages
with different priorities.
      </para>
      <para>
There are two variants of <function>cyg_mbox_get</function>. The
first, <function>cyg_mbox_timed_get</function> will wait until either
a message is available or until a number of clock ticks have occurred.
The number of ticks is specified as an absolute, not relative tick
count, and so in order to wait for a relative number of ticks, the
return value of the <function>cyg_current_time()</function> function
should be added to determine the absolute number of ticks.  If no
message is posted within the timeout then a null pointer will be
returned. <function>cyg_mbox_tryget</function> is a non-blocking
operation which will either return a message if one is available or a
null pointer.
      </para>
      <para>
New messages are placed in the mail box by calling
<function>cyg_mbox_put</function> or one of its variants. The main put
function takes two arguments, a handle to the mail box and a
pointer for the message itself. If there is a spare slot in the
mail box then the new message can be placed there immediately, and
if there is a waiting thread it will be woken up so that it can
receive the message. If the mail box is currently full then
<function>cyg_mbox_put</function> will block until there has been a
get operation and a slot is available. The
<function>cyg_mbox_timed_put</function> variant imposes a time limit
on the put operation, returning false if the operation cannot be
completed within the specified number of clock ticks and as for
<function>cyg_mbox_timed_get</function> this is an absolute tick
count. The <function>cyg_mbox_tryput</function> variant is
non-blocking, returning false if there are no free slots available and
the message cannot be posted without blocking.
      </para>
      <para>
There are a further four functions available for examining the current
state of a mailbox. The results of these functions must be used with
care because usually the state can change at any time as a result of
activity within other threads, but they may prove occasionally useful
during debugging or in special situations.
<function>cyg_mbox_peek</function> returns a count of the number of
messages currently stored in the mail box.
<function>cyg_mbox_peek_item</function> retrieves the first message,
but it remains in the mail box until a get operation is performed.
<function>cyg_mbox_waiting_to_get</function> and
<function>cyg_mbox_waiting_to_put</function> indicate whether or not
there are currently threads blocked in a get or a put operation on a
given mail box.
      </para>
      <para>
The number of slots in each mail box is controlled by a
configuration option
<varname>CYGNUM_KERNEL_SYNCH_MBOX_QUEUE_SIZE</varname>, with a default
value of 10. All mail boxes are the same size.
      </para>
    </refsect1>

    <refsect1 id="kernel-mail-boxes-context"><title>Valid contexts</title>
      <para>
<function>cyg_mbox_create</function> is typically called during
system initialization but may also be called in thread context.
The remaining functions are normally called only during thread
context. Of special note is <function>cyg_mbox_put</function> which
can be a blocking operation when the mail box is full, and which
therefore must never be called from DSR context. It is permitted to
call <function>cyg_mbox_tryput</function>,
<function>cyg_mbox_tryget</function>, and the information functions
from DSR context but this is rarely useful.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Flags                          -->

  <refentry id="kernel-flags">

    <refmeta>
    <refentrytitle>Event Flags</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_flag_init</refname>
      <refname>cyg_flag_destroy</refname>
      <refname>cyg_flag_setbits</refname>
      <refname>cyg_flag_maskbits</refname>
      <refname>cyg_flag_wait</refname>
      <refname>cyg_flag_timed_wait</refname>
      <refname>cyg_flag_poll</refname>
      <refname>cyg_flag_peek</refname>
      <refname>cyg_flag_waiting</refname>
      <refpurpose>Synchronization primitive</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_flag_init</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_flag_destroy</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_flag_setbits</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
          <paramdef>cyg_flag_value_t <parameter>value</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_flag_maskbits</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
          <paramdef>cyg_flag_value_t <parameter>value</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_flag_value_t <function>cyg_flag_wait</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
          <paramdef>cyg_flag_value_t <parameter>pattern</parameter></paramdef>
          <paramdef>cyg_flag_mode_t <parameter>mode</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_flag_value_t <function>cyg_flag_timed_wait</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
          <paramdef>cyg_flag_value_t <parameter>pattern</parameter></paramdef>
          <paramdef>cyg_flag_mode_t <parameter>mode</parameter></paramdef>
          <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_flag_value_t <function>cyg_flag_poll</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
          <paramdef>cyg_flag_value_t <parameter>pattern</parameter></paramdef>
          <paramdef>cyg_flag_mode_t <parameter>mode</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_flag_value_t <function>cyg_flag_peek</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_flag_waiting</function></funcdef>
          <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-flags-description"><title>Description</title>
      <para>
Event flags allow a consumer thread to wait for one of several
different types of event to occur. Alternatively it is possible to
wait for some combination of events. The implementation is relatively
straightforward. Each event flag contains a 32-bit integer.
Application code associates these bits with specific events, so for
example bit 0 could indicate that an I/O operation has completed and
data is available, while bit 1 could indicate that the user has
pressed a start button. A producer thread or a DSR can cause one or
more of the bits to be set, and a consumer thread currently waiting
for these bits will be woken up.
      </para>
      <para>
Unlike semaphores no attempt is made to keep track of event counts. It
does not matter whether a given event occurs once or multiple times
before being consumed, the corresponding bit in the event flag will
change only once. However semaphores cannot easily be used to handle
multiple event sources. Event flags can often be used as an
alternative to condition variables, although they cannot be used for
completely arbitrary conditions and they only support the equivalent
of condition variable broadcasts, not signals.
      </para>
      <para>
Before an event flag can be used it must be initialized by a call to
<function>cyg_flag_init</function>. This takes a pointer to a
<structname>cyg_flag_t</structname> data structure, which can be part of a
larger structure. All 32 bits in the event flag will be set to 0,
indicating that no events have yet occurred. If an event flag is no
longer required it can be cleaned up with a call to
<function>cyg_flag_destroy</function>, allowing the memory for the
<structfield>cyg_flag_t</structfield> structure to be re-used.
      </para>
      <para>
A consumer thread can wait for one or more events by calling
<function>cyg_flag_wait</function>. This takes three arguments. The
first identifies a particular event flag. The second is some
combination of bits, indicating which events are of interest. The
final argument should be one of the following:
      </para>
      <variablelist>
        <varlistentry>
          <term><literal>CYG_FLAG_WAITMODE_AND</literal></term>
          <listitem><para>
The call to <function>cyg_flag_wait</function> will block until all
the specified event bits are set. The event flag is not cleared when
the wait succeeds, in other words all the bits remain set.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term><literal>CYG_FLAG_WAITMODE_OR</literal></term>
          <listitem><para>
The call will block until at least one of the specified event bits is
set. The event flag is not cleared on return.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term><literal>CYG_FLAG_WAITMODE_AND | CYG_FLAG_WAITMODE_CLR</literal></term>
          <listitem><para>
The call will block until all the specified event bits are set, and
the entire event flag is cleared when the call succeeds. Note that
if this mode of operation is used then a single event flag cannot be
used to store disjoint sets of events, even though enough bits might
be available. Instead each disjoint set of events requires its own
event flag.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term><literal>CYG_FLAG_WAITMODE_OR | CYG_FLAG_WAITMODE_CLR</literal></term>
          <listitem><para>
The call will block until at least one of the specified event bits is
set, and the entire flag is cleared when the call succeeds.
          </para></listitem>
        </varlistentry>
      </variablelist>
      <para>
A call to <function>cyg_flag_wait</function> normally blocks until the
required condition is satisfied. It will return the value of the event
flag at the point that the operation succeeded, which may be a
superset of the requested events. If
<function>cyg_thread_release</function> is used to unblock a thread
that is currently in a wait operation, the
<function>cyg_flag_wait</function> call will instead return 0.
      </para>
      <para>
<function>cyg_flag_timed_wait</function> is a variant of
<function>cyg_flag_wait</function> which adds a timeout: the wait
operation must succeed within the specified number of ticks, or it
will fail with a return value of 0. The number of ticks is specified
as an absolute, not relative tick count, and so in order to wait for a
relative number of ticks, the return value of the
<function>cyg_current_time()</function> function should be added to
determine the absolute number of ticks.
<function>cyg_flag_poll</function> is a non-blocking variant: if the
wait operation can succeed immediately it acts like
<function>cyg_flag_wait</function>, otherwise it returns immediately
with a value of 0.
      </para>
      <para>
<function>cyg_flag_setbits</function> is called by a producer thread
or from inside a DSR when an event occurs. The specified bits are or'd
into the current event flag value. This may cause a waiting thread to
be woken up, if its condition is now satisfied.
      </para>
      <para>
<function>cyg_flag_maskbits</function> can be used to clear one or
more bits in the event flag. This can be called from a producer when a
particular condition is no longer satisfied, for example when the user
is no longer pressing a particular button. It can also be used by a
consumer thread if <literal>CYG_FLAG_WAITMODE_CLR</literal> was not
used as part of the wait operation, to indicate that some but not all
of the active events have been consumed. If there are multiple
consumer threads performing wait operations without using
<function>CYG_FLAG_WAITMODE_CLR</function> then typically some
additional synchronization such as a mutex is needed to prevent
multiple threads consuming the same event.
      </para>
      <para>
Two additional functions are provided to query the current state of an
event flag. <function>cyg_flag_peek</function> returns the current
value of the event flag, and <function>cyg_flag_waiting</function> can
be used to find out whether or not there are any threads currently
blocked on the event flag. Both of these functions must be used with
care because other threads may be operating on the event flag.
      </para>
    </refsect1>

    <refsect1 id="kernel-flags-context"><title>Valid contexts</title>
      <para>
<function>cyg_flag_init</function> is typically called during system
initialization but may also be called in thread context. The same
applies to <function>cyg_flag_destroy</function>.
<function>cyg_flag_wait</function> and
<function>cyg_flag_timed_wait</function> may only be called from
thread context. The remaining functions may be called from thread or
DSR context.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Spinlocks                      -->

  <refentry id="kernel-spinlocks">

    <refmeta>
    <refentrytitle>Spinlocks</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_spinlock_create</refname>
      <refname>cyg_spinlock_destroy</refname>
      <refname>cyg_spinlock_spin</refname>
      <refname>cyg_spinlock_clear</refname>
      <refname>cyg_spinlock_test</refname>
      <refname>cyg_spinlock_spin_intsave</refname>
      <refname>cyg_spinlock_clear_intsave</refname>
      <refpurpose>Low-level Synchronization Primitive</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_spinlock_init</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
          <paramdef>cyg_bool_t <parameter>locked</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_spinlock_destroy</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_spinlock_spin</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_spinlock_clear</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_spinlock_try</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_bool_t <function>cyg_spinlock_test</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_spinlock_spin_intsave</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
          <paramdef>cyg_addrword_t* <parameter>istate</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_spinlock_clear_intsave</function></funcdef>
          <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>istate</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-spinlocks-description"><title>Description</title>
      <para>
Spinlocks provide an additional synchronization primitive for
applications running on SMP systems. They operate at a lower level
than the other primitives such as mutexes, and for most purposes the
higher-level primitives should be preferred. However there are some
circumstances where a spinlock is appropriate, especially when
interrupt handlers and threads need to share access to hardware, and
on SMP systems the kernel implementation itself depends on spinlocks.
      </para>
      <para>
Essentially a spinlock is just a simple flag. When code tries to claim
a spinlock it checks whether or not the flag is already set. If not
then the flag is set and the operation succeeds immediately. The exact
implementation of this is hardware-specific, for example it may use a
test-and-set instruction to guarantee the desired behaviour even if
several processors try to access the spinlock at the exact same time.
If it is not possible to claim a spinlock then the current thead spins
in a tight loop, repeatedly checking the flag until it is clear. This
behaviour is very different from other synchronization primitives such
as mutexes, where contention would cause a thread to be suspended. The
assumption is that a spinlock will only be held for a very short time.
If claiming a spinlock could cause the current thread to be suspended
then spinlocks could not be used inside interrupt handlers, which is
not acceptable.
      </para>
      <para>
This does impose a constraint on any code which uses spinlocks.
Specifically it is important that spinlocks are held only for a short
period of time, typically just some dozens of instructions. Otherwise
another processor could be blocked on the spinlock for a long time,
unable to do any useful work. It is also important that a thread which
owns a spinlock does not get preempted because that might cause
another processor to spin for a whole timeslice period, or longer. One
way of achieving this is to disable interrupts on the current
processor, and the function
<function>cyg_spinlock_spin_intsave</function> is provided to
facilitate this.
      </para>
      <para>
Spinlocks should not be used on single-processor systems. Consider a
high priority thread which attempts to claim a spinlock already held
by a lower priority thread: it will just loop forever and the lower
priority thread will never get another chance to run and release the
spinlock. Even if the two threads were running at the same priority,
the one attempting to claim the spinlock would spin until it was
timesliced and a lot of cpu time would be wasted. If an interrupt
handler tried to claim a spinlock owned by a thread, the interrupt
handler would loop forever. Therefore spinlocks are only appropriate
for SMP systems where the current owner of a spinlock can continue
running on a different processor.
      </para>
      <para>
Before a spinlock can be used it must be initialized by a call to
<function>cyg_spinlock_init</function>. This takes two arguments, a
pointer to a <function>cyg_spinlock_t</function> data structure, and
a flag to specify whether the spinlock starts off locked or unlocked.
If a spinlock is no longer required then it can be destroyed by a call
to <function>cyg_spinlock_destroy</function>.
      </para>
      <para>
There are two routines for claiming a spinlock:
<function>cyg_spinlock_spin</function> and
<function>cyg_spinlock_spin_intsave</function>. The former can be used
when it is known the current code will not be preempted, for example
because it is running in an interrupt handler or because interrupts
are disabled. The latter will disable interrupts in addition to
claiming the spinlock, so is safe to use in all circumstances. The
previous interrupt state is returned via the second argument, and
should be used in a subsequent call to
<function>cyg_spinlock_clear_intsave</function>. 
      </para>
      <para>
Similarly there are two routines for releasing a spinlock:
<function>cyg_spinlock_clear</function> and
<function>cyg_spinlock_clear_intsave</function>. Typically
the former will be used if the spinlock was claimed by a call to
<function>cyg_spinlock_spin</function>, and the latter when
<function>cyg_spinlock_intsave</function> was used.
      </para>
      <para>
There are two additional routines.
<function>cyg_spinlock_try</function> is a non-blocking version of
<function>cyg_spinlock_spin</function>: if possible the lock will be
claimed and the function will return <literal>true</literal>; otherwise the function
will return immediately with failure.
<function>cyg_spinlock_test</function> can be used to find out whether
or not the spinlock is currently locked. This function must be used
with care because, especially on a multiprocessor system, the state of
the spinlock can change at any time.
      </para>
      <para>
Spinlocks should only be held for a short period of time, and
attempting to claim a spinlock will never cause a thread to be
suspended. This means that there is no need to worry about priority
inversion problems, and concepts such as priority ceilings and
inheritance do not apply.
      </para>
    </refsect1>

    <refsect1 id="kernel-spinlocks-context"><title>Valid contexts</title>
      <para>
All of the spinlock functions can be called from any context,
including ISR and DSR context. Typically
<function>cyg_spinlock_init</function> is only called during system
initialization. 
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Scheduler control              -->

  <refentry id="kernel-schedcontrol">

    <refmeta>
    <refentrytitle>Scheduler Control</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_scheduler_start</refname>
      <refname>cyg_scheduler_lock</refname>
      <refname>cyg_scheduler_unlock</refname>
      <refname>cyg_scheduler_safe_lock</refname>
      <refname>cyg_scheduler_read_lock</refname>
      <refpurpose>Control the state of the scheduler</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_scheduler_start</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_scheduler_lock</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_scheduler_unlock</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_ucount32 <function>cyg_scheduler_read_lock</function></funcdef>
          <void>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-schedcontrol-description"><title>Description</title>
      <para>
<function>cyg_scheduler_start</function> should only be called once,
to mark the end of system initialization. In typical configurations it
is called automatically by the system startup, but some applications
may bypass the standard startup in which case
<function>cyg_scheduler_start</function> will have to be called
explicitly. The call will enable system interrupts, allowing I/O
operations to commence. Then the scheduler will be invoked and control
will be transferred to the highest priority runnable thread. The call
will never return.
      </para>
      <para>
The various data structures inside the eCos kernel must be protected
against concurrent updates. Consider a call to
<function>cyg_semaphore_post</function> which causes a thread to be
woken up: the semaphore data structure must be updated to remove the
thread from its queue; the scheduler data structure must also be
updated to mark the thread as runnable; it is possible that the newly
runnable thread has a higher priority than the current one, in which
case preemption is required. If in the middle of the semaphore post
call an interrupt occurred and the interrupt handler tried to
manipulate the same data structures, for example by making another
thread runnable, then it is likely that the structures will be left in
an inconsistent state and the system will fail.
      </para>
      <para>
To prevent such problems the kernel contains a special lock known as
the scheduler lock. A typical kernel function such as
<function>cyg_semaphore_post</function> will claim the scheduler lock,
do all its manipulation of kernel data structures, and then release
the scheduler lock. The current thread cannot be preempted while it
holds the scheduler lock. If an interrupt occurs and a DSR is supposed
to run to signal that some event has occurred, that DSR is postponed
until the scheduler unlock operation. This prevents concurrent updates
of kernel data structures.
      </para>
      <para>
The kernel exports three routines for manipulating the scheduler lock.
<function>cyg_scheduler_lock</function> can be called to claim the
lock. On return it is guaranteed that the current thread will not be
preempted, and that no other code is manipulating any kernel data
structures. <function>cyg_scheduler_unlock</function> can be used to
release the lock, which may cause the current thread to be preempted.
<function>cyg_scheduler_read_lock</function> can be used to query the
current state of the scheduler lock. This function should never be
needed because well-written code should always know whether or not the
scheduler is currently locked, but may prove useful during debugging.
      </para>
      <para>
The implementation of the scheduler lock involves a simple counter.
Code can call <function>cyg_scheduler_lock</function> multiple times,
causing the counter to be incremented each time, as long as
<function>cyg_scheduler_unlock</function> is called the same number of
times. This behaviour is different from mutexes where an attempt by a
thread to lock a mutex multiple times will result in deadlock or an
assertion failure.
      </para>
      <para>
Typical application code should not use the scheduler lock. Instead
other synchronization primitives such as mutexes and semaphores should
be used. While the scheduler is locked the current thread cannot be
preempted, so any higher priority threads will not be able to run.
Also no DSRs can run, so device drivers may not be able to service
I/O requests. However there is one situation where locking the
scheduler is appropriate: if some data structure needs to be shared
between an application thread and a DSR associated with some interrupt
source, the thread can use the scheduler lock to prevent concurrent
invocations of the DSR and then safely manipulate the structure. It is
desirable that the scheduler lock is held for only a short period of
time, typically some tens of instructions. In exceptional cases there
may also be some performance-critical code where it is more
appropriate to use the scheduler lock rather than a mutex, because the
former is more efficient.
      </para>
    </refsect1>

    <refsect1 id="kernel-schedcontrol-context"><title>Valid contexts</title>
      <para>
<function>cyg_scheduler_start</function> can only be called during
system initialization, since it marks the end of that phase. The
remaining functions may be called from thread or DSR context. Locking
the scheduler from inside the DSR has no practical effect because the
lock is claimed automatically by the interrupt subsystem before
running DSRs, but allows functions to be shared between normal thread
code and DSRs.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ Interrupt handling             -->

  <refentry id="kernel-interrupts">

    <refmeta>
    <refentrytitle>Interrupt Handling</refentrytitle>
    </refmeta>

    <refnamediv>
      <refname>cyg_interrupt_create</refname>
      <refname>cyg_interrupt_delete</refname>
      <refname>cyg_interrupt_attach</refname>
      <refname>cyg_interrupt_detach</refname>
      <refname>cyg_interrupt_configure</refname>
      <refname>cyg_interrupt_acknowledge</refname>
      <refname>cyg_interrupt_enable</refname>
      <refname>cyg_interrupt_disable</refname>
      <refname>cyg_interrupt_mask</refname>
      <refname>cyg_interrupt_mask_intunsafe</refname>
      <refname>cyg_interrupt_unmask</refname>
      <refname>cyg_interrupt_unmask_intunsafe</refname>
      <refname>cyg_interrupt_set_cpu</refname>
      <refname>cyg_interrupt_get_cpu</refname>
      <refname>cyg_interrupt_get_vsr</refname>
      <refname>cyg_interrupt_set_vsr</refname>
      <refpurpose>Manage interrupt handlers</refpurpose>
    </refnamediv>

    <refsynopsisdiv>
      <funcsynopsis>
        <funcsynopsisinfo>
#include &lt;cyg/kernel/kapi.h&gt;
        </funcsynopsisinfo>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_create</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
          <paramdef>cyg_priority_t <parameter>priority</parameter></paramdef>
          <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef>
          <paramdef>cyg_ISR_t* <parameter>isr</parameter></paramdef>
          <paramdef>cyg_DSR_t* <parameter>dsr</parameter></paramdef>
          <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef>
          <paramdef>cyg_interrupt* <parameter>intr</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_delete</function></funcdef>
          <paramdef>cyg_handle_t <parameter>interrupt</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_attach</function></funcdef>
          <paramdef>cyg_handle_t <parameter>interrupt</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_detach</function></funcdef>
          <paramdef>cyg_handle_t <parameter>interrupt</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_configure</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
          <paramdef>cyg_bool_t <parameter>level</parameter></paramdef>
          <paramdef>cyg_bool_t <parameter>up</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_acknowledge</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_disable</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_enable</function></funcdef>
          <void>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_mask</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_mask_intunsafe</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_unmask</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_unmask_intunsafe</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_set_cpu</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
          <paramdef>cyg_cpu_t <parameter>cpu</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>cyg_cpu_t <function>cyg_interrupt_get_cpu</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_get_vsr</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
          <paramdef>cyg_VSR_t** <parameter>vsr</parameter></paramdef>
        </funcprototype>
        <funcprototype>
          <funcdef>void <function>cyg_interrupt_set_vsr</function></funcdef>
          <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef>
          <paramdef>cyg_VSR_t* <parameter>vsr</parameter></paramdef>
        </funcprototype>
      </funcsynopsis>
    </refsynopsisdiv>

    <refsect1 id="kernel-interrupts-description"><title>Description</title>
      <para>
The kernel provides an interface for installing interrupt handlers and
controlling when interrupts occur. This functionality is used
primarily by eCos device drivers and by any application code that
interacts directly with hardware. However in most cases it is better
to avoid using this kernel functionality directly, and instead the
device driver API provided by the common HAL package should be used.
Use of the kernel package is optional, and some applications such as
RedBoot work with no need for multiple threads or synchronization
primitives. Any code which calls the kernel directly rather than the
device driver API will not function in such a configuration. When the
kernel package is present the device driver API is implemented as
<literal>#define</literal>'s to the equivalent kernel calls, otherwise
it is implemented inside the common HAL package. The latter
implementation can be simpler than the kernel one because there is no
need to consider thread preemption and similar issues.
      </para>
      <para>
The exact details of interrupt handling vary widely between
architectures. The functionality provided by the kernel abstracts away
from many of the details of the underlying hardware, thus simplifying
application development. However this is not always successful. For
example, if some hardware does not provide any support at all for
masking specific interrupts then calling
<function>cyg_interrupt_mask</function> may not behave as intended:
instead of masking just the one interrupt source it might disable all
interrupts, because that is as close to the desired behaviour as is
possible given the hardware restrictions. Another possibility is that
masking a given interrupt source also affects all lower-priority
interrupts, but still allows higher-priority ones. The documentation
for the appropriate HAL packages should be consulted for more
information about exactly how interrupts are handled on any given
hardware. The HAL header files will also contain useful information. 
      </para>
    </refsect1>

    <refsect1 id="kernel-interrupts-handlers"><title>Interrupt Handlers</title>
      <para>
Interrupt handlers are created by a call to
<function>cyg_interrupt_create</function>. This takes the following
arguments: 
      </para>
      <variablelist>
        <varlistentry>
          <term>cyg_vector_t <parameter>vector</parameter></term>
          <listitem><para>
The interrupt vector, a small integer, identifies the specific
interrupt source. The appropriate hardware documentation or HAL header
files should be consulted for details of which vector corresponds to
which device.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term>cyg_priority_t <parameter>priority</parameter></term>
          <listitem><para>
Some hardware may support interrupt priorities, where a low priority
interrupt handler can in turn be interrupted by a higher priority one.
Again hardware-specific documentation should be consulted for details
about what the valid interrupt priority levels are.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term>cyg_addrword_t <parameter>data</parameter></term>
          <listitem><para>
When an interrupt occurs eCos will first call the associated
interrupt service routine or ISR, then optionally a deferred service
routine or DSR. The <parameter>data</parameter> argument to
<function>cyg_interrupt_create</function> will be passed to both these
functions. Typically it will be a pointer to some data structure.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term>cyg_ISR_t <parameter>isr</parameter></term>
          <listitem><para>
When an interrupt occurs the hardware will transfer control to the
appropriate vector service routine or VSR, which is usually provided
by eCos. This performs any appropriate processing, for example to work
out exactly which interrupt occurred, and then as quickly as possible
transfers control the installed ISR. An ISR is a C function which
takes the following form:
          </para>
          <programlisting width=72>
cyg_uint32
isr_function(cyg_vector_t vector, cyg_addrword_t data)
{
    cyg_bool_t dsr_required = 0;

    &hellip;

    return dsr_required ? CYG_ISR_CALL_DSR : CYG_ISR_HANDLED;
}
          </programlisting>
          <para>
The first argument identifies the particular interrupt source,
especially useful if there multiple instances of a given device and a
single ISR can be used for several different interrupt vectors. The
second argument is the <parameter>data</parameter> field passed to
<function>cyg_interrupt_create</function>, usually a pointer to some
data structure. The exact conditions under which an ISR runs will
depend partly on the hardware and partly on configuration options.
Interrupts may currently be disabled globally, especially if the
hardware does not support interrupt priorities. Alternatively
interrupts may be enabled such that higher priority interrupts are
allowed through. The ISR may be running on a separate interrupt stack,
or on the stack of whichever thread was running at the time the
interrupt happened.
          </para>
          <para>
A typical ISR will do as little work as possible, just enough to meet
the needs of the hardware and then acknowledge the interrupt by
calling <function>cyg_interrupt_acknowledge</function>. This ensures
that interrupts will be quickly reenabled, so higher priority devices
can be serviced. For some applications there may be one device which
is especially important and whose ISR can take much longer than
normal. However eCos device drivers usually will not assume that they
are especially important, so their ISRs will be as short as possible.
          </para>
          <para>
The return value of an ISR is normally one of
<literal>CYG_ISR_CALL_DSR</literal> or
<literal>CYG_ISR_HANDLED</literal>. The former indicates that further
processing is required at DSR level, and the interrupt handler's DSR
will be run as soon as possible. The latter indicates that the
interrupt has been fully handled and no further effort is required.
          </para>
          <para>
An ISR is allowed to make very few kernel calls. It can manipulate the
interrupt mask, and on SMP systems it can use spinlocks. However an
ISR must not make higher-level kernel calls such as posting to a
semaphore, instead any such calls must be made from the DSR. This
avoids having to disable interrupts throughout the kernel and thus
improves interrupt latency.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term>cyg_DSR_t <parameter>dsr</parameter></term>
          <listitem><para>
If an interrupt has occurred and the ISR has returned a value
<literal>CYG_ISR_CALL_DSR</literal>, the system will call the
deferred service routine or DSR associated with this interrupt
handler. If the scheduler is not currently locked then the DSR will
run immediately. However if the interrupted thread was in the middle
of a kernel call and had locked the scheduler, then the DSR will be
deferred until the scheduler is again unlocked. This allows the
DSR to make certain kernel calls safely, for example posting to a
semaphore or signalling a condition variable. A DSR is a C function
which takes the following form:
          </para>
          <programlisting width=72>
void
dsr_function(cyg_vector_t vector,
             cyg_ucount32 count,
             cyg_addrword_t data)
{
}
          </programlisting>
          <para>
The first argument identifies the specific interrupt that has caused
the DSR to run. The second argument indicates the number of these
interrupts that have occurred and for which the ISR requested a DSR.
Usually this will be <literal>1</literal>, unless the system is
suffering from a very heavy load. The third argument is the
<parameter>data</parameter> field passed to
<function>cyg_interrupt_create</function>. 
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term>cyg_handle_t* <parameter>handle</parameter></term>
          <listitem><para>
The kernel will return a handle to the newly created interrupt handler
via this argument. Subsequent operations on the interrupt handler such
as attaching it to the interrupt source will use this handle.
          </para></listitem>
        </varlistentry>
        <varlistentry>
          <term>cyg_interrupt* <parameter>intr</parameter></term>
          <listitem><para>
This provides the kernel with an area of memory for holding this
interrupt handler and associated data.
          </para></listitem>
        </varlistentry>
      </variablelist>
      <para>
The call to <function>cyg_interrupt_create</function> simply fills in
a kernel data structure. A typical next step is to call
<function>cyg_interrupt_attach</function> using the handle returned by
the create operation. This makes it possible to have several different
interrupt handlers for a given vector, attaching whichever one is
currently appropriate. Replacing an interrupt handler requires a call
to <function>cyg_interrupt_detach</function>, followed by another call
to <function>cyg_interrupt_attach</function> for the replacement
handler. <function>cyg_interrupt_delete</function> can be used if an
interrupt handler is no longer required.
      </para>
      <para>
Some hardware may allow for further control over specific interrupts,
for example whether an interrupt is level or edge triggered. Any such
hardware functionality can be accessed using
<function>cyg_interrupt_configure</function>: the
<parameter>level</parameter> argument selects between level versus
edge triggered; the <parameter>up</parameter> argument selects between
high and low level, or between rising and falling edges.
      </para>
      <para>
Usually interrupt handlers are created, attached and configured during
system initialization, while global interrupts are still disabled. On
most hardware it will also be necessary to call
<function>cyg_interrupt_unmask</function>, since the sensible default
for interrupt masking is to ignore any interrupts for which no handler
is installed.
      </para>
    </refsect1>

    <refsect1 id="kernel-interrupts-enable"><title>Controlling Interrupts</title>
      <para>
eCos provides two ways of controlling whether or not interrupts
happen. It is possible to disable and reenable all interrupts
globally, using <function>cyg_interrupt_disable</function> and
<function>cyg_interrupt_enable</function>. Typically this works by
manipulating state inside the cpu itself, for example setting a flag
in a status register or executing special instructions. Alternatively
it may be possible to mask a specific interrupt source by writing to
one or to several interrupt mask registers. Hardware-specific
documentation should be consulted for the exact details of how
interrupt masking works, because a full implementation is not possible
on all hardware.
      </para>
      <para>
The primary use for these functions is to allow data to be shared
between ISRs and other code such as DSRs or threads. If both a thread
and an ISR need to manipulate either a data structure or the hardware
itself, there is a possible conflict if an interrupt happens just when
the thread is doing such manipulation. Problems can be avoided by the
thread either disabling or masking interrupts during the critical
region. If this critical region requires only a few instructions then
usually it is more efficient to disable interrupts. For larger
critical regions it may be more appropriate to use interrupt masking,
allowing other interrupts to occur. There are other uses for interrupt
masking. For example if a device is not currently being used by the
application then it may be desirable to mask all interrupts generated
by that device.
      </para>
      <para>
There are two functions for masking a specific interrupt source,
<function>cyg_interrupt_mask</function> and
<function>cyg_interrupt_mask_intunsafe</function>. On typical hardware
masking an interrupt is not an atomic operation, so if two threads
were to perform interrupt masking operations at the same time there
could be problems. <function>cyg_interrupt_mask</function> disables
all interrupts while it manipulates the interrupt mask. In situations
where interrupts are already know to be disabled,
<function>cyg_interrupt_mask_intunsafe</function> can be used
instead. There are matching functions
<function>cyg_interrupt_unmask</function> and
<function>cyg_interrupt_unmask_intsafe</function>.
      </para>
    </refsect1>

    <refsect1 id="kernel-interrupts-smp"><title>SMP Support</title>
      <para>
On SMP systems the kernel provides an additional two functions related
to interrupt handling. <function>cyg_interrupt_set_cpu</function>
specifies that a particular hardware interrupt should always be
handled on one specific processor in the system. In other words when
the interrupt triggers it is only that processor which detects it, and
it is only on that processor that the VSR and ISR will run. If a DSR
is requested then it will also run on the same CPU. The
function <function>cyg_interrupt_get_cpu</function> can be used to
find out which interrupts are handled on which processor. 
      </para>
    </refsect1>

    <refsect1 id="kernel-interrupts-vsr"><title>VSR Support</title>
      <para>
When an interrupt occurs the hardware will transfer control to a piece
of code known as the VSR, or Vector Service Routine. By default this
code is provided by eCos. Usually it is written in assembler, but on
some architectures it may be possible to implement VSRs in C by
specifying an interrupt attribute. Compiler documentation should be
consulted for more information on this. The default eCos VSR will work
out which ISR function should process the interrupt, and set up a C
environment suitable for this ISR.
      </para>
      <para>
For some applications it may be desirable to replace the default eCos
VSR and handle some interrupts directly. This minimizes interrupt
latency, but it requires application developers to program at a lower
level. Usually the best way to write a custom VSR is to copy the
existing one supplied by eCos and then make appropriate modifications.
The function <function>cyg_interrupt_get_vsr</function> can be used to
get hold of the current VSR for a given interrupt vector, allowing it
to be restored if the custom VSR is no longer required.
<function>cyg_interrupt_set_vsr</function> can be used to install a
replacement VSR. Usually the <parameter>vsr</parameter> argument will
correspond to an exported label in an assembler source file.
      </para>
    </refsect1>

    <refsect1 id="kernel-interrupts-context"><title>Valid contexts</title>
      <para>
In a typical configuration interrupt handlers are created and attached
during system initialization, and never detached or deleted. However
it is possible to perform these operations at thread level, if
desired. Similarly <function>cyg_interrupt_configure</function>,
<function>cyg_interrupt_set_vsr</function>, and
<function>cyg_interrupt_set_cpu</function> are usually called only
during system initialization, but on typical hardware may be called at
any time. <function>cyg_interrupt_get_vsr</function> and
<function>cyg_interrupt_get_cpu</function> may be called at any time.
      </para>
      <para>
The functions for enabling, disabling, masking and unmasking
interrupts can be called in any context, when appropriate. It is the
responsibility of application developers to determine when the use of
these functions is appropriate.
      </para>
    </refsect1>

  </refentry>

<!-- }}} -->
<!-- {{{ tm_basic                       -->

  <refentry id="kernel-characterization">

    <refmeta>
    <refentrytitle>Kernel Real-time Characterization</refentrytitle>
    </refmeta>
    <refnamediv>
      <refname>tm_basic</refname>
      <refpurpose>Measure the performance of the eCos kernel</refpurpose>
    </refnamediv>

    <refsect1 id="kernel-characterization-description">
      <title>Description</title>
        <para>
When building a real-time system, care must be taken to ensure that
the system will be able to perform properly within the constraints of
that system. One of these constraints may be how fast certain
operations can be performed. Another might be how deterministic the
overall behavior of the system is. Lastly the memory footprint (size)
and unit cost may be important.
        </para>
        <para>
One of the major problems encountered while evaluating a system will
be how to compare it with possible alternatives. Most manufacturers of
real-time systems publish performance numbers, ostensibly so that
users can compare the different offerings. However, what these numbers
mean and how they were gathered is often not clear. The values are
typically measured on a particular piece of hardware, so in order to
truly compare, one must obtain measurements for exactly the same set
of hardware that were gathered in a similar fashion.
        </para>
        <para>
Two major items need to be present in any given set of measurements.
First, the raw values for the various operations; these are typically
quite easy to measure and will be available for most systems. Second,
the determinacy of the numbers; in other words how much the value
might change depending on other factors within the system. This value
is affected by a number of factors: how long interrupts might be
masked, whether or not the function can be interrupted, even very
hardware-specific effects such as cache locality and pipeline usage.
It is very difficult to measure the determinacy of any given
operation, but that determinacy is fundamentally important to proper
overall characterization of a system.
        </para>
        <para>
In the discussion and numbers that follow, three key measurements are
provided. The first measurement is an estimate of the interrupt
latency: this is the length of time from when a hardware interrupt
occurs until its Interrupt Service Routine (ISR) is called. The second
measurement is an estimate of overall interrupt overhead: this is the
length of time average interrupt processing takes, as measured by the
real-time clock interrupt (other interrupt sources will certainly take
a different amount of time, but this data cannot be easily gathered).
The third measurement consists of the timings for the various kernel
primitives.
          </para>
        </refsect1>
        <refsect1 id="kernel-characterization-methodology">
          <title>Methodology</title>
          <para>
Key operations in the kernel were measured by using a simple test
program which exercises the various kernel primitive operations. A
hardware timer, normally the one used to drive the real-time clock,
was used for these measurements. In most cases this timer can be read
with quite high resolution, typically in the range of a few
microseconds. For each measurement, the operation was repeated a
number of times. Time stamps were obtained directly before and after
the operation was performed. The data gathered for the entire set of
operations was then analyzed, generating average (mean), maximum and
minimum values. The sample variance (a measure of how close most
samples are to the mean) was also calculated. The cost of obtaining
the real-time clock timer values was also measured, and was subtracted
from all other times.
          </para>
          <para>
Most kernel functions can be measured separately. In each case, a
reasonable number of iterations are performed. Where the test case
involves a kernel object, for example creating a task, each iteration
is performed on a different object. There is also a set of tests which
measures the interactions between multiple tasks and certain kernel
primitives. Most functions are tested in such a way as to determine
the variations introduced by varying numbers of objects in the system.
For example, the mailbox tests measure the cost of a 'peek' operation
when the mailbox is empty, has a single item, and has multiple items
present. In this way, any effects of the state of the object or how
many items it contains can be determined.
          </para>
          <para>
There are a few things to consider about these measurements. Firstly,
they are quite micro in scale and only measure the operation in
question. These measurements do not adequately describe how the
timings would be perturbed in a real system with multiple interrupting
sources. Secondly, the possible aberration incurred by the real-time
clock (system heartbeat tick) is explicitly avoided. Virtually all
kernel functions have been designed to be interruptible. Thus the
times presented are typical, but best case, since any particular
function may be interrupted by the clock tick processing. This number
is explicitly calculated so that the value may be included in any
deadline calculations required by the end user. Lastly, the reported
measurements were obtained from a system built with all options at
their default values. Kernel instrumentation and asserts are also
disabled for these measurements. Any number of configuration options
can change the measured results, sometimes quite dramatically. For
example, mutexes are using priority inheritance in these measurements.
The numbers will change if the system is built with priority
inheritance on mutex variables turned off.
          </para>
          <para>
The final value that is measured is an estimate of interrupt latency.
This particular value is not explicitly calculated in the test program
used, but rather by instrumenting the kernel itself. The raw number of
timer ticks that elapse between the time the timer generates an
interrupt and the start of the timer ISR is kept in the kernel. These
values are printed by the test program after all other operations have
been tested. Thus this should be a reasonable estimate of the
interrupt latency over time.
          </para>
        </refsect1>

        <refsect1 id="kernel-characterization-using-measurements">
          <title>Using these Measurements</title>
          <para>
These measurements can be used in a number of ways. The most typical
use will be to compare different real-time kernel offerings on similar
hardware, another will be to estimate the cost of implementing a task
using eCos (applications can be examined to see what effect the kernel
operations will have on the total execution time). Another use would
be to observe how the tuning of the kernel affects overall operation.
          </para>
        </refsect1>

        <refsect1 id="kernel-characterization-influences">
          <title>Influences on Performance</title>
            <para>
A number of factors can affect real-time performance in a system. One
of the most common factors, yet most difficult to characterize, is the
effect of device drivers and interrupts on system timings. Different
device drivers will have differing requirements as to how long
interrupts are suppressed, for example. The eCos system has been
designed with this in mind, by separating the management of interrupts
(ISR handlers) and the processing required by the interrupt
(DSR&mdash;Deferred Service Routine&mdash; handlers). However, since
there is so much variability here, and indeed most device drivers will
come from the end users themselves, these effects cannot be reliably
measured. Attempts have been made to measure the overhead of the
single interrupt that eCos relies on, the real-time clock timer. This
should give you a reasonable idea of the cost of executing interrupt
handling for devices.
          </para>
        </refsect1>

       <refsect1 id="kernel-characterization-measured-items">
         <title>Measured Items</title>
         <para>
This section describes the various tests and the numbers presented.
All tests use the C kernel API (available by way of
<filename>cyg/kernel/kapi.h</filename>). There is a single main thread
in the system that performs the various tests. Additional threads may
be created as part of the testing, but these are short lived and are
destroyed between tests unless otherwise noted. The terminology
&ldquo;lower priority&rdquo; means a priority that is less important,
not necessarily lower in numerical value. A higher priority thread
will run in preference to a lower priority thread even though the
priority value of the higher priority thread may be numerically less
than that of the lower priority thread.
          </para>

          <refsect2 id="kernel-characterization-measure-threads">
            <title>Thread Primitives</title>
            <variablelist>
              <varlistentry>
                <term>Create thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_create()</function> call.
Each call creates a totally new thread. The set of threads created by
this test will be reused in the subsequent thread primitive tests.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Yield thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_yield()</function> call.
For this test, there are no other runnable threads, thus the test
should just measure the overhead of trying to give up the CPU.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Suspend &lsqb;suspended&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_suspend()</function> call.
A thread may be suspended multiple times; each thread is already
suspended from its initial creation, and is suspended again.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Resume thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_resume()</function> call.
All of the threads have a suspend count of 2, thus this call does not
make them runnable. This test just measures the overhead of resuming a
thread.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Set priority</term>
                <listitem><para>
This test measures the <function>cyg_thread_set_priority()</function>
call. Each thread, currently suspended, has its priority set to a new
value.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Get priority</term>
                <listitem><para>
This test measures the <function>cyg_thread_get_priority()</function>
call.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Kill &lsqb;suspended&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_kill()</function> call.
Each thread in the set is killed. All threads are known to be
suspended before being killed.
		</para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Yield &lsqb;no other&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_yield()</function> call
again. This is to demonstrate that the
<function>cyg_thread_yield()</function> call has a fixed overhead,
regardless of whether there are other threads in the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Resume &lsqb;suspended low priority&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_resume()</function> call
again. In this case, the thread being resumed is lower priority than
the main thread, thus it will simply become ready to run but not be
granted the CPU. This test measures the cost of making a thread ready
to run.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Resume &lsqb;runnable low priority&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_resume()</function> call
again. In this case, the thread being resumed is lower priority than
the main thread and has already been made runnable, so in fact the
resume call has no effect.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Suspend &lsqb;runnable&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_suspend()</function> call
again. In this case, each thread has already been made runnable (by
previous tests).
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Yield &lsqb;only low priority&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_yield()</function> call.
In this case, there are many other runnable threads, but they are all
lower priority than the main thread, thus no thread switches will take
place.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Suspend &lsqb;runnable-&gt;not runnable&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_suspend()</function> call
again. The thread being suspended will become non-runnable by this
action.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Kill &lsqb;runnable&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_kill()</function> call
again. In this case, the thread being killed is currently runnable,
but lower priority than the main thread.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Resume &lsqb;high priority&rsqb; thread</term>
                <listitem><para>
This test measures the <function>cyg_thread_resume()</function> call.
The thread being resumed is higher priority than the main thread, thus
a thread switch will take place on each call. In fact there will be
two thread switches; one to the new higher priority thread and a
second back to the test thread. The test thread exits immediately.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Thread switch</term>
                <listitem><para>
This test attempts to measure the cost of switching from one thread to
another. Two equal priority threads are started and they will each
yield to the other for a number of iterations. A time stamp is
gathered in one thread before the
<function>cyg_thread_yield()</function> call and after the call in the
other thread.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

          <refsect2 id="kernel-characterization-measure-scheduler"
            <title>Scheduler Primitives</title>
            <variablelist>
              <varlistentry>
                <term>Scheduler lock</term>
                <listitem><para>
This test measures the <function>cyg_scheduler_lock()</function> call.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Scheduler unlock &lsqb;0 threads&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_scheduler_unlock()</function>
call. There are no other threads in the system and the unlock happens
immediately after a lock so there will be no pending DSR&rsquo;s to
run.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Scheduler unlock &lsqb;1 suspended thread&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_scheduler_unlock()</function>
call. There is one other thread in the system which is currently
suspended.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Scheduler unlock &lsqb;many suspended threads&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_scheduler_unlock()</function>
call. There are many other threads in the system which are currently
suspended. The purpose of this test is to determine the cost of having
additional threads in the system when the scheduler is activated by
way of <function>cyg_scheduler_unlock()</function>.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Scheduler unlock &lsqb;many low priority threads&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_scheduler_unlock()</function>
call. There are many other threads in the system which are runnable
but are lower priority than the main thread. The purpose of this test
is to determine the cost of having additional threads in the system
when the scheduler is activated by way of
<function>cyg_scheduler_unlock()</function>.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

          <refsect2 id="kernel-characterization-measure-mutex">
            <title>Mutex Primitives</title>
            <variablelist>
              <varlistentry>
                <term>Init mutex</term>
                <listitem><para>
This test measures the <function>cyg_mutex_init()</function> call. A
number of separate mutex variables are created. The purpose of this
test is to measure the cost of creating a new mutex and introducing it
to the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Lock &lsqb;unlocked&rsqb; mutex</term>
                <listitem><para>
This test measures the <function>cyg_mutex_lock()</function> call. The
purpose of this test is to measure the cost of locking a mutex which
is currently unlocked. There are no other threads executing in the
system while this test runs.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Unlock &lsqb;locked&rsqb; mutex</term>
                <listitem><para>
This test measures the <function>cyg_mutex_unlock()</function> call.
The purpose of this test is to measure the cost of unlocking a mutex
which is currently locked. There are no other threads executing in the
system while this test runs.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Trylock &lsqb;unlocked&rsqb; mutex</term>
                <listitem><para>
This test measures the <function>cyg_mutex_trylock()</function> call.
The purpose of this test is to measure the cost of locking a mutex
which is currently unlocked. There are no other threads executing in
the system while this test runs.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Trylock &lsqb;locked&rsqb; mutex</term>
                <listitem><para>
This test measures the <function>cyg_mutex_trylock()</function> call.
The purpose of this test is to measure the cost of locking a mutex
which is currently locked. There are no other threads executing in the
system while this test runs.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Destroy mutex</term>
                <listitem><para>
This test measures the <function>cyg_mutex_destroy()</function> call.
The purpose of this test is to measure the cost of deleting a mutex
from the system. There are no other threads executing in the system
while this test runs.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Unlock/Lock mutex</term>
                <listitem><para>
This test attempts to measure the cost of unlocking a mutex for which
there is another higher priority thread waiting. When the mutex is
unlocked, the higher priority waiting thread will immediately take the
lock. The time from when the unlock is issued until after the lock
succeeds in the second thread is measured, thus giving the round-trip
or circuit time for this type of synchronizer.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

          <refsect2 id="kernel-characterization-measure-mailbox">
            <title>Mailbox Primitives</title>
            <variablelist>
              <varlistentry>
                <term>Create mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_create()</function> call. A
number of separate mailboxes is created. The purpose of this test is
to measure the cost of creating a new mailbox and introducing it to
the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Peek &lsqb;empty&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_peek()</function> call. An
attempt is made to peek the value in each mailbox, which is currently
empty. The purpose of this test is to measure the cost of checking a
mailbox for a value without blocking.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Put &lsqb;first&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_put()</function> call. One
item is added to a currently empty mailbox. The purpose of this test
is to measure the cost of adding an item to a mailbox. There are no
other threads currently waiting for mailbox items to arrive.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Peek &lsqb;1 msg&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_peek()</function> call. An
attempt is made to peek the value in each mailbox, which contains a
single item. The purpose of this test is to measure the cost of
checking a mailbox which has data to deliver.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Put &lsqb;second&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_put()</function> call. A
second item is added to a mailbox. The purpose of this test is to
measure the cost of adding an additional item to a mailbox. There are
no other threads currently waiting for mailbox items to arrive.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Peek &lsqb;2 msgs&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_peek()</function> call. An
attempt is made to peek the value in each mailbox, which contains two
items. The purpose of this test is to measure the cost of checking a
mailbox which has data to deliver.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Get &lsqb;first&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_get()</function> call. The
first item is removed from a mailbox that currently contains two
items. The purpose of this test is to measure the cost of obtaining an
item from a mailbox without blocking.
              </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Get &lsqb;second&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_get()</function> call. The
last item is removed from a mailbox that currently contains one item.
The purpose of this test is to measure the cost of obtaining an item
from a mailbox without blocking.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tryput &lsqb;first&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_tryput()</function> call. A
single item is added to a currently empty mailbox. The purpose of this
test is to measure the cost of adding an item to a mailbox.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Peek item &lsqb;non-empty&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_peek_item()</function> call.
A single item is fetched from a mailbox that contains a single item.
The purpose of this test is to measure the cost of obtaining an item
without disturbing the mailbox.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tryget &lsqb;non-empty&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_tryget()</function> call. A
single item is removed from a mailbox that contains exactly one item.
The purpose of this test is to measure the cost of obtaining one item
from a non-empty mailbox.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Peek item &lsqb;empty&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_peek_item()</function> call.
An attempt is made to fetch an item from a mailbox that is empty. The
purpose of this test is to measure the cost of trying to obtain an
item when the mailbox is empty.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tryget &lsqb;empty&rsqb; mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_tryget()</function> call. An
attempt is made to fetch an item from a mailbox that is empty. The
purpose of this test is to measure the cost of trying to obtain an
item when the mailbox is empty.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Waiting to get mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_waiting_to_get()</function>
call. The purpose of this test is to measure the cost of determining
how many threads are waiting to obtain a message from this mailbox.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Waiting to put mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_waiting_to_put()</function>
call. The purpose of this test is to measure the cost of determining
how many threads are waiting to put a message into this mailbox.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Delete mbox</term>
                <listitem><para>
This test measures the <function>cyg_mbox_delete()</function> call.
The purpose of this test is to measure the cost of destroying a
mailbox and removing it from the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Put/Get mbox</term>
                <listitem><para>
In this round-trip test, one thread is sending data to a mailbox that
is being consumed by another thread. The time from when the data is
put into the mailbox until it has been delivered to the waiting thread
is measured. Note that this time will contain a thread switch.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

          <refsect2 id="kernel-characterization-measure-semaphore">
            <title>Semaphore Primitives</title>
            <variablelist>
              <varlistentry>
                <term>Init semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_init()</function> call.
A number of separate semaphore objects are created and introduced to
the system. The purpose of this test is to measure the cost of
creating a new semaphore.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Post &lsqb;0&rsqb; semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_post()</function> call.
Each semaphore currently has a value of 0 and there are no other
threads in the system. The purpose of this test is to measure the
overhead cost of posting to a semaphore. This cost will differ if
there is a thread waiting for the semaphore.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Wait &lsqb;1&rsqb; semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_wait()</function> call.
The semaphore has a current value of 1 so the call is non-blocking.
The purpose of the test is to measure the overhead of
&ldquo;taking&rdquo; a semaphore.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Trywait &lsqb;0&rsqb; semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_trywait()</function>
call. The semaphore has a value of 0 when the call is made. The
purpose of this test is to measure the cost of seeing if a semaphore
can be &ldquo;taken&rdquo; without blocking. In this case, the answer
would be no.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Trywait &lsqb;1&rsqb; semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_trywait()</function>
call. The semaphore has a value of 1 when the call is made. The
purpose of this test is to measure the cost of seeing if a semaphore
can be &ldquo;taken&rdquo; without blocking. In this case, the answer
would be yes.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Peek semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_peek()</function> call.
The purpose of this test is to measure the cost of obtaining the
current semaphore count value.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Destroy semaphore</term>
                <listitem><para>
This test measures the <function>cyg_semaphore_destroy()</function>
call. The purpose of this test is to measure the cost of deleting a
semaphore from the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Post/Wait semaphore</term>
                <listitem><para>
In this round-trip test, two threads are passing control back and
forth by using a semaphore. The time from when one thread calls
<function>cyg_semaphore_post()</function> until the other thread
completes its <function>cyg_semaphore_wait()</function> is measured.
Note that each iteration of this test will involve a thread switch.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

          <refsect2 id="kernel-characterization-measure-counters">
            <title>Counters</title>
            <variablelist>
              <varlistentry>
                <term>Create counter</term>
                <listitem><para>
This test measures the <function>cyg_counter_create()</function> call.
A number of separate counters are created. The purpose of this test is
to measure the cost of creating a new counter and introducing it to
the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Get counter value</term>
                <listitem><para>
This test measures the
<function>cyg_counter_current_value()</function> call. The current
value of each counter is obtained.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Set counter value</term>
                <listitem><para>
This test measures the <function>cyg_counter_set_value()</function>
call. Each counter is set to a new value.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tick counter</term>
                <listitem><para>
This test measures the <function>cyg_counter_tick()</function> call.
Each counter is &ldquo;ticked&rdquo; once.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Delete counter</term>
                <listitem><para>
This test measures the <function>cyg_counter_delete()</function> call.
Each counter is deleted from the system. The purpose of this test is
to measure the cost of deleting a counter object.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

          <refsect2 id="kernel-characterization-measure-alarms">
            <title>Alarms</title>
            <variablelist>
              <varlistentry>
                <term>Create alarm</term>
                <listitem><para>
This test measures the <function>cyg_alarm_create()</function> call. A
number of separate alarms are created, all attached to the same
counter object. The purpose of this test is to measure the cost of
creating a new counter and introducing it to the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Initialize alarm</term>
                <listitem><para>
This test measures the <function>cyg_alarm_initialize()</function>
call. Each alarm is initialized to a small value.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Disable alarm</term>
                <listitem><para>
This test measures the <function>cyg_alarm_disable()</function> call.
Each alarm is explicitly disabled.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Enable alarm</term>
                <listitem><para>
This test measures the <function>cyg_alarm_enable()</function> call.
Each alarm is explicitly enabled.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Delete alarm</term>
                <listitem><para>
This test measures the <function>cyg_alarm_delete()</function> call.
Each alarm is destroyed. The purpose of this test is to measure the
cost of deleting an alarm and removing it from the system.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tick counter &lsqb;1 alarm&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_counter_tick()</function> call. A
counter is created that has a single alarm attached to it. The purpose
of this test is to measure the cost of &ldquo;ticking&rdquo; a counter
when it has a single attached alarm. In this test, the alarm is not
activated (fired).
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tick counter &lsqb;many alarms&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_counter_tick()</function> call. A
counter is created that has multiple alarms attached to it. The
purpose of this test is to measure the cost of &ldquo;ticking&rdquo; a
counter when it has many attached alarms. In this test, the alarms are
not activated (fired).
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tick &amp; fire counter &lsqb;1 alarm&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_counter_tick()</function> call. A
counter is created that has a single alarm attached to it. The purpose
of this test is to measure the cost of &ldquo;ticking&rdquo; a counter
when it has a single attached alarm. In this test, the alarm is
activated (fired). Thus the measured time will include the overhead of
calling the alarm callback function.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Tick &amp; fire counter &lsqb;many alarms&rsqb;</term>
                <listitem><para>
This test measures the <function>cyg_counter_tick()</function> call. A
counter is created that has multiple alarms attached to it. The
purpose of this test is to measure the cost of &ldquo;ticking&rdquo; a
counter when it has many attached alarms. In this test, the alarms are
activated (fired). Thus the measured time will include the overhead of
calling the alarm callback function.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Alarm latency &lsqb;0 threads&rsqb;</term>
                <listitem><para>
This test attempts to measure the latency in calling an alarm callback
function. The time from the clock interrupt until the alarm function
is called is measured. In this test, there are no threads that can be
run, other than the system idle thread, when the clock interrupt
occurs (all threads are suspended).
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Alarm latency &lsqb;2 threads&rsqb;</term>
                <listitem><para>
This test attempts to measure the latency in calling an alarm callback
function. The time from the clock interrupt until the alarm function
is called is measured. In this test, there are exactly two threads
which are running when the clock interrupt occurs. They are simply
passing back and forth by way of the
<function>cyg_thread_yield()</function> call. The purpose of this test
is to measure the variations in the latency when there are executing
threads.
                </para></listitem>
              </varlistentry>
              <varlistentry>
                <term>Alarm latency &lsqb;many threads&rsqb;</term>
                <listitem><para>
This test attempts to measure the latency in calling an alarm callback
function. The time from the clock interrupt until the alarm function
is called is measured. In this test, there are a number of threads
which are running when the clock interrupt occurs. They are simply
passing back and forth by way of the
<function>cyg_thread_yield()</function> call. The purpose of this test
is to measure the variations in the latency when there are many
executing threads.
                </para></listitem>
              </varlistentry>
            </variablelist>
          </refsect2>

    </refsect1>

  </refentry>

<!-- }}} -->

</part>
