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<HTML
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>Semaphores</TITLE
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HREF="kernel.html"><LINK
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>eCos Reference Manual</TH
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="KERNEL-SEMAPHORES">Semaphores</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN1330"
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></A
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><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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CLASS="REFSYNOPSISDIV"
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NAME="AEN1338"><H2
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>Synopsis</H2
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CLASS="FUNCSYNOPSIS"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/kernel/kapi.h>
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</PRE
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_semaphore_init</CODE
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>(cyg_sem_t* sem, cyg_count32 val);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_semaphore_destroy</CODE
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>(cyg_sem_t* sem);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_semaphore_wait</CODE
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>(cyg_sem_t* sem);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_semaphore_timed_wait</CODE
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>(cyg_sem_t* sem, cyg_tick_count_t abstime);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_semaphore_trywait</CODE
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>(cyg_sem_t* sem);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_semaphore_post</CODE
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>(cyg_sem_t* sem);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_semaphore_peek</CODE
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>(cyg_sem_t* sem, cyg_count32* val);</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-SEMAPHORES-DESCRIPTION"
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></A
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><H2
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>Description</H2
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><P
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>Counting semaphores are a <A
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HREF="kernel-overview.html#KERNEL-OVERVIEW-SYNCH-PRIMITIVES"
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>synchronization
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primitive</A
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> that allow threads to wait until an event has
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occurred. The event may be generated by a producer thread, or by a DSR
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in response to a hardware interrupt. Associated with each semaphore is
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an integer counter that keeps track of the number of events that have
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not yet been processed. If this counter is zero, an attempt by a
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consumer thread to wait on the semaphore will block until some other
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thread or a DSR posts a new event to the semaphore. If the counter is
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greater than zero then an attempt to wait on the semaphore will
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consume one event, in other words decrement the counter, and return
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immediately. Posting to a semaphore will wake up the first thread that
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is currently waiting, which will then resume inside the semaphore wait
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operation and decrement the counter again.
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</P
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><P
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>Another use of semaphores is for certain forms of resource management.
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The counter would correspond to how many of a certain type of resource
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are currently available, with threads waiting on the semaphore to
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claim a resource and posting to release the resource again. In
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practice <A
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HREF="kernel-condition-variables.html"
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>condition
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variables</A
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> are usually much better suited for operations like
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this.
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</P
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_semaphore_init</TT
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> is used to initialize a
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semaphore. It takes two arguments, a pointer to a
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<SPAN
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CLASS="STRUCTNAME"
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>cyg_sem_t</SPAN
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> structure and an initial value for
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the counter. Note that semaphore operations, unlike some other parts
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of the kernel API, use pointers to data structures rather than
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handles. This makes it easier to embed semaphores in a larger data
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structure. The initial counter value can be any number, zero, positive
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or negative, but typically a value of zero is used to indicate that no
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events have occurred yet.
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</P
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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> is used by a consumer thread
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to wait for an event. If the current counter is greater than 0, in
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other words if the event has already occurred in the past, then the
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counter will be decremented and the call will return immediately.
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Otherwise the current thread will be blocked until there is a
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<TT
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CLASS="FUNCTION"
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>cyg_semaphore_post</TT
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> call.
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</P
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_semaphore_post</TT
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> is called when an event has
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occurs. This increments the counter and wakes up the first thread
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waiting on the semaphore (if any). Usually that thread will then
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continue running inside <TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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> and
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decrement the counter again. However other scenarioes are possible.
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For example the thread calling <TT
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CLASS="FUNCTION"
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>cyg_semaphore_post</TT
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>
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may be running at high priority, some other thread running at medium
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priority may be about to call <TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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>
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when it next gets a chance to run, and a low priority thread may be
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waiting on the semaphore. What will happen is that the current high
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priority thread continues running until it is descheduled for some
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reason, then the medium priority thread runs and its call to
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<TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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> succeeds immediately, and
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later on the low priority thread runs again, discovers a counter value
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of 0, and blocks until another event is posted. If there are multiple
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threads blocked on a semaphore then the configuration option
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<TT
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CLASS="VARNAME"
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>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</TT
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> determines which
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one will be woken up by a post operation.
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</P
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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> returns a boolean. Normally it
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will block until it has successfully decremented the counter, retrying
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as necessary, and return success. However the wait operation may be
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aborted by a call to <A
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HREF="kernel-thread-control.html"
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><TT
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CLASS="FUNCTION"
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>cyg_thread_release</TT
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></A
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>,
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and <TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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> will then return false.
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</P
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_semaphore_timed_wait</TT
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> is a variant of
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<TT
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CLASS="FUNCTION"
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>cyg_semaphore_wait</TT
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>. It can be used to wait until
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either an event has occurred or a number of clock ticks have happened.
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The function returns success if the semaphore wait operation
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succeeded, or false if the operation timed out or was aborted by
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<TT
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CLASS="FUNCTION"
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>cyg_thread_release</TT
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>. If support for the real-time
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clock has been removed from the current configuration then this
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function will not be available.
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<TT
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CLASS="FUNCTION"
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>cyg_semaphore_trywait</TT
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| 313 |
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> is another variant which
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will always return immediately rather than block, again returning
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success or failure.
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</P
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><P
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><TT
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CLASS="FUNCTION"
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| 320 |
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>cyg_semaphore_peek</TT
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| 321 |
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> can be used to get hold of the
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| 322 |
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current counter value. This function is rarely useful except for
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| 323 |
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debugging purposes since the counter value may change at any time if
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| 324 |
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some other thread or a DSR performs a semaphore operation.
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</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-SEMAPHORES-CONTEXT"
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></A
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><H2
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>Valid contexts</H2
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| 334 |
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><P
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| 335 |
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><TT
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| 336 |
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CLASS="FUNCTION"
|
| 337 |
|
|
>cyg_semaphore_init</TT
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| 338 |
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> is normally called during
|
| 339 |
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initialization but may also be called from thread context.
|
| 340 |
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<TT
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| 341 |
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CLASS="FUNCTION"
|
| 342 |
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|
>cyg_semaphore_wait</TT
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| 343 |
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> and
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| 344 |
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<TT
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| 345 |
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CLASS="FUNCTION"
|
| 346 |
|
|
>cyg_semaphore_timed_wait</TT
|
| 347 |
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> may only be called from
|
| 348 |
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thread context because these operations may block.
|
| 349 |
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<TT
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| 350 |
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CLASS="FUNCTION"
|
| 351 |
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>cyg_semaphore_trywait</TT
|
| 352 |
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>,
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| 353 |
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<TT
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| 354 |
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CLASS="FUNCTION"
|
| 355 |
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>cyg_semaphore_post</TT
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| 356 |
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> and
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| 357 |
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<TT
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| 358 |
|
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CLASS="FUNCTION"
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>cyg_semaphore_peek</TT
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> may be called from thread or
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DSR context.
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</P
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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CELLPADDING="0"
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CELLSPACING="0"
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WIDTH="33%"
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VALIGN="top"
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><A
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HREF="kernel-condition-variables.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="ecos-ref.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><A
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HREF="kernel-mail-boxes.html"
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ACCESSKEY="N"
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>Next</A
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>Condition Variables</TD
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HREF="kernel.html"
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>Up</A
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WIDTH="33%"
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>Mail boxes</TD
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></BODY
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></HTML
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>
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