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WIDTH="100%"></DIV
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><H1
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><H1
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><A
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><A
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NAME="KERNEL-SEMAPHORES">Semaphores</H1
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NAME="KERNEL-SEMAPHORES">Semaphores</H1
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><DIV
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><DIV
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CLASS="REFNAMEDIV"
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CLASS="REFNAMEDIV"
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><A
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><A
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NAME="AEN1330"
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NAME="AEN1330"
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></A
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></A
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><H2
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><H2
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>Name</H2
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>Name</H2
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>cyg_semaphore_init, cyg_semaphore_destroy, cyg_semaphore_wait, cyg_semaphore_timed_wait, cyg_semaphore_post, cyg_semaphore_peek -- Synchronization primitive</DIV
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>cyg_semaphore_init, cyg_semaphore_destroy, cyg_semaphore_wait, cyg_semaphore_timed_wait, cyg_semaphore_post, cyg_semaphore_peek -- Synchronization primitive</DIV
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><DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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CLASS="REFSYNOPSISDIV"
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><A
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><A
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NAME="AEN1338"><H2
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NAME="AEN1338"><H2
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>Synopsis</H2
|
>Synopsis</H2
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><DIV
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><DIV
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CLASS="FUNCSYNOPSIS"
|
CLASS="FUNCSYNOPSIS"
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><A
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><A
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NAME="AEN1339"><P
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NAME="AEN1339"><P
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></P
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></P
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><TABLE
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><TABLE
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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WIDTH="70%"
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><TR
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><TR
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><TD
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><TD
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><PRE
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><PRE
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CLASS="FUNCSYNOPSISINFO"
|
CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/kernel/kapi.h>
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>#include <cyg/kernel/kapi.h>
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</PRE
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</PRE
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></TD
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></TD
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></TR
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></TR
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></TABLE
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></TABLE
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><P
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><P
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><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>void cyg_semaphore_init</CODE
|
>void cyg_semaphore_init</CODE
|
>(cyg_sem_t* sem, cyg_count32 val);</CODE
|
>(cyg_sem_t* sem, cyg_count32 val);</CODE
|
></P
|
></P
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><P
|
><P
|
><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>void cyg_semaphore_destroy</CODE
|
>void cyg_semaphore_destroy</CODE
|
>(cyg_sem_t* sem);</CODE
|
>(cyg_sem_t* sem);</CODE
|
></P
|
></P
|
><P
|
><P
|
><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>cyg_bool_t cyg_semaphore_wait</CODE
|
>cyg_bool_t cyg_semaphore_wait</CODE
|
>(cyg_sem_t* sem);</CODE
|
>(cyg_sem_t* sem);</CODE
|
></P
|
></P
|
><P
|
><P
|
><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>cyg_bool_t cyg_semaphore_timed_wait</CODE
|
>cyg_bool_t cyg_semaphore_timed_wait</CODE
|
>(cyg_sem_t* sem, cyg_tick_count_t abstime);</CODE
|
>(cyg_sem_t* sem, cyg_tick_count_t abstime);</CODE
|
></P
|
></P
|
><P
|
><P
|
><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>cyg_bool_t cyg_semaphore_trywait</CODE
|
>cyg_bool_t cyg_semaphore_trywait</CODE
|
>(cyg_sem_t* sem);</CODE
|
>(cyg_sem_t* sem);</CODE
|
></P
|
></P
|
><P
|
><P
|
><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>void cyg_semaphore_post</CODE
|
>void cyg_semaphore_post</CODE
|
>(cyg_sem_t* sem);</CODE
|
>(cyg_sem_t* sem);</CODE
|
></P
|
></P
|
><P
|
><P
|
><CODE
|
><CODE
|
><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>void cyg_semaphore_peek</CODE
|
>void cyg_semaphore_peek</CODE
|
>(cyg_sem_t* sem, cyg_count32* val);</CODE
|
>(cyg_sem_t* sem, cyg_count32* val);</CODE
|
></P
|
></P
|
><P
|
><P
|
></P
|
></P
|
></DIV
|
></DIV
|
></DIV
|
></DIV
|
><DIV
|
><DIV
|
CLASS="REFSECT1"
|
CLASS="REFSECT1"
|
><A
|
><A
|
NAME="KERNEL-SEMAPHORES-DESCRIPTION"
|
NAME="KERNEL-SEMAPHORES-DESCRIPTION"
|
></A
|
></A
|
><H2
|
><H2
|
>Description</H2
|
>Description</H2
|
><P
|
><P
|
>Counting semaphores are a <A
|
>Counting semaphores are a <A
|
HREF="kernel-overview.html#KERNEL-OVERVIEW-SYNCH-PRIMITIVES"
|
HREF="kernel-overview.html#KERNEL-OVERVIEW-SYNCH-PRIMITIVES"
|
>synchronization
|
>synchronization
|
primitive</A
|
primitive</A
|
> that allow threads to wait until an event has
|
> that allow threads to wait until an event has
|
occurred. The event may be generated by a producer thread, or by a DSR
|
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
|
in response to a hardware interrupt. Associated with each semaphore is
|
an integer counter that keeps track of the number of events that have
|
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
|
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
|
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
|
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
|
greater than zero then an attempt to wait on the semaphore will
|
consume one event, in other words decrement the counter, and return
|
consume one event, in other words decrement the counter, and return
|
immediately. Posting to a semaphore will wake up the first thread that
|
immediately. Posting to a semaphore will wake up the first thread that
|
is currently waiting, which will then resume inside the semaphore wait
|
is currently waiting, which will then resume inside the semaphore wait
|
operation and decrement the counter again.
|
operation and decrement the counter again.
|
</P
|
</P
|
><P
|
><P
|
>Another use of semaphores is for certain forms of resource management.
|
>Another use of semaphores is for certain forms of resource management.
|
The counter would correspond to how many of a certain type of resource
|
The counter would correspond to how many of a certain type of resource
|
are currently available, with threads waiting on the semaphore to
|
are currently available, with threads waiting on the semaphore to
|
claim a resource and posting to release the resource again. In
|
claim a resource and posting to release the resource again. In
|
practice <A
|
practice <A
|
HREF="kernel-condition-variables.html"
|
HREF="kernel-condition-variables.html"
|
>condition
|
>condition
|
variables</A
|
variables</A
|
> are usually much better suited for operations like
|
> are usually much better suited for operations like
|
this.
|
this.
|
</P
|
</P
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_init</TT
|
>cyg_semaphore_init</TT
|
> is used to initialize a
|
> is used to initialize a
|
semaphore. It takes two arguments, a pointer to a
|
semaphore. It takes two arguments, a pointer to a
|
<SPAN
|
<SPAN
|
CLASS="STRUCTNAME"
|
CLASS="STRUCTNAME"
|
>cyg_sem_t</SPAN
|
>cyg_sem_t</SPAN
|
> structure and an initial value for
|
> structure and an initial value for
|
the counter. Note that semaphore operations, unlike some other parts
|
the counter. Note that semaphore operations, unlike some other parts
|
of the kernel API, use pointers to data structures rather than
|
of the kernel API, use pointers to data structures rather than
|
handles. This makes it easier to embed semaphores in a larger data
|
handles. This makes it easier to embed semaphores in a larger data
|
structure. The initial counter value can be any number, zero, positive
|
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
|
or negative, but typically a value of zero is used to indicate that no
|
events have occurred yet.
|
events have occurred yet.
|
</P
|
</P
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
> is used by a consumer thread
|
> is used by a consumer thread
|
to wait for an event. If the current counter is greater than 0, in
|
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
|
other words if the event has already occurred in the past, then the
|
counter will be decremented and the call will return immediately.
|
counter will be decremented and the call will return immediately.
|
Otherwise the current thread will be blocked until there is a
|
Otherwise the current thread will be blocked until there is a
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_post</TT
|
>cyg_semaphore_post</TT
|
> call.
|
> call.
|
</P
|
</P
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_post</TT
|
>cyg_semaphore_post</TT
|
> is called when an event has
|
> is called when an event has
|
occurs. This increments the counter and wakes up the first thread
|
occurs. This increments the counter and wakes up the first thread
|
waiting on the semaphore (if any). Usually that thread will then
|
waiting on the semaphore (if any). Usually that thread will then
|
continue running inside <TT
|
continue running inside <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
> and
|
> and
|
decrement the counter again. However other scenarioes are possible.
|
decrement the counter again. However other scenarioes are possible.
|
For example the thread calling <TT
|
For example the thread calling <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_post</TT
|
>cyg_semaphore_post</TT
|
>
|
>
|
may be running at high priority, some other thread running at medium
|
may be running at high priority, some other thread running at medium
|
priority may be about to call <TT
|
priority may be about to call <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
>
|
>
|
when it next gets a chance to run, and a low priority thread may be
|
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
|
waiting on the semaphore. What will happen is that the current high
|
priority thread continues running until it is descheduled for some
|
priority thread continues running until it is descheduled for some
|
reason, then the medium priority thread runs and its call to
|
reason, then the medium priority thread runs and its call to
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
> succeeds immediately, and
|
> succeeds immediately, and
|
later on the low priority thread runs again, discovers a counter value
|
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
|
of 0, and blocks until another event is posted. If there are multiple
|
threads blocked on a semaphore then the configuration option
|
threads blocked on a semaphore then the configuration option
|
<TT
|
<TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</TT
|
>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</TT
|
> determines which
|
> determines which
|
one will be woken up by a post operation.
|
one will be woken up by a post operation.
|
</P
|
</P
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
> returns a boolean. Normally it
|
> returns a boolean. Normally it
|
will block until it has successfully decremented the counter, retrying
|
will block until it has successfully decremented the counter, retrying
|
as necessary, and return success. However the wait operation may be
|
as necessary, and return success. However the wait operation may be
|
aborted by a call to <A
|
aborted by a call to <A
|
HREF="kernel-thread-control.html"
|
HREF="kernel-thread-control.html"
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_thread_release</TT
|
>cyg_thread_release</TT
|
></A
|
></A
|
>,
|
>,
|
and <TT
|
and <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
> will then return false.
|
> will then return false.
|
</P
|
</P
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_timed_wait</TT
|
>cyg_semaphore_timed_wait</TT
|
> is a variant of
|
> is a variant of
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
>. It can be used to wait until
|
>. It can be used to wait until
|
either an event has occurred or a number of clock ticks have happened.
|
either an event has occurred or a number of clock ticks have happened.
|
The function returns success if the semaphore wait operation
|
The function returns success if the semaphore wait operation
|
succeeded, or false if the operation timed out or was aborted by
|
succeeded, or false if the operation timed out or was aborted by
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_thread_release</TT
|
>cyg_thread_release</TT
|
>. If support for the real-time
|
>. If support for the real-time
|
clock has been removed from the current configuration then this
|
clock has been removed from the current configuration then this
|
function will not be available.
|
function will not be available.
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_trywait</TT
|
>cyg_semaphore_trywait</TT
|
> is another variant which
|
> is another variant which
|
will always return immediately rather than block, again returning
|
will always return immediately rather than block, again returning
|
success or failure.
|
success or failure.
|
</P
|
</P
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_peek</TT
|
>cyg_semaphore_peek</TT
|
> can be used to get hold of the
|
> can be used to get hold of the
|
current counter value. This function is rarely useful except for
|
current counter value. This function is rarely useful except for
|
debugging purposes since the counter value may change at any time if
|
debugging purposes since the counter value may change at any time if
|
some other thread or a DSR performs a semaphore operation.
|
some other thread or a DSR performs a semaphore operation.
|
</P
|
</P
|
></DIV
|
></DIV
|
><DIV
|
><DIV
|
CLASS="REFSECT1"
|
CLASS="REFSECT1"
|
><A
|
><A
|
NAME="KERNEL-SEMAPHORES-CONTEXT"
|
NAME="KERNEL-SEMAPHORES-CONTEXT"
|
></A
|
></A
|
><H2
|
><H2
|
>Valid contexts</H2
|
>Valid contexts</H2
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_init</TT
|
>cyg_semaphore_init</TT
|
> is normally called during
|
> is normally called during
|
initialization but may also be called from thread context.
|
initialization but may also be called from thread context.
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_wait</TT
|
>cyg_semaphore_wait</TT
|
> and
|
> and
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_timed_wait</TT
|
>cyg_semaphore_timed_wait</TT
|
> may only be called from
|
> may only be called from
|
thread context because these operations may block.
|
thread context because these operations may block.
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_trywait</TT
|
>cyg_semaphore_trywait</TT
|
>,
|
>,
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_post</TT
|
>cyg_semaphore_post</TT
|
> and
|
> and
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_peek</TT
|
>cyg_semaphore_peek</TT
|
> may be called from thread or
|
> may be called from thread or
|
DSR context.
|
DSR context.
|
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