200 lines
8.2 KiB
HTML
200 lines
8.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
|
||
<HTML>
|
||
<HEAD>
|
||
<META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8">
|
||
<TITLE>SEMAPHORES(3) manual page</TITLE>
|
||
<META NAME="GENERATOR" CONTENT="OpenOffice.org 1.1.3 (Linux)">
|
||
<META NAME="CREATED" CONTENT="20050505;14061900">
|
||
<META NAME="CHANGED" CONTENT="20050514;195200">
|
||
<!-- manual page source format generated by PolyglotMan v3.2, -->
|
||
<!-- available at http://polyglotman.sourceforge.net/ -->
|
||
</HEAD>
|
||
<BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR">
|
||
<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
|
||
<P><A HREF="index.html">Reference Index</A></P>
|
||
<P><A HREF="#toc">Table of Contents</A></P>
|
||
<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
|
||
<P>sem_init, sem_wait, sem_trywait, sem_post, sem_getvalue,
|
||
sem_destroy - operations on semaphores
|
||
</P>
|
||
<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
|
||
<P><B>#include <semaphore.h></B>
|
||
</P>
|
||
<P><B>int sem_init(sem_t *</B><I>sem</I><B>, int </B><I>pshared</I><B>,
|
||
unsigned int </B><I>value</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_wait(sem_t * </B><I>sem</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_timedwait(sem_t * </B><I>sem</I>, <B>const struct
|
||
timespec *</B><I>abstime</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_trywait(sem_t * </B><I>sem</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_post(sem_t * </B><I>sem</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_post_multiple(sem_t * </B><I>sem, </I><B>int</B>
|
||
<I>number</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_getvalue(sem_t * </B><I>sem</I><B>, int * </B><I>sval</I><B>);</B>
|
||
</P>
|
||
<P><B>int sem_destroy(sem_t * </B><I>sem</I><B>);</B>
|
||
</P>
|
||
<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
|
||
<P>Semaphores are counters for resources shared between threads. The
|
||
basic operations on semaphores are: increment the counter atomically,
|
||
and wait until the counter is non-null and decrement it atomically.
|
||
</P>
|
||
<P><B>sem_init</B> initializes the semaphore object pointed to by
|
||
<I>sem</I>. The count associated with the semaphore is set initially
|
||
to <I>value</I>. The <I>pshared</I> argument indicates whether the
|
||
semaphore is local to the current process ( <I>pshared</I> is zero)
|
||
or is to be shared between several processes ( <I>pshared</I> is not
|
||
zero).</P>
|
||
<P><B>Pthreads-w32</B> currently does not support process-shared
|
||
semaphores, thus <B>sem_init</B> always returns with error <B>EPERM</B>
|
||
if <I>pshared</I> is not zero.
|
||
</P>
|
||
<P><B>sem_wait</B> atomically decrements <I>sem</I>'s count if it is
|
||
greater than 0 and returns immediately or it suspends the calling
|
||
thread until it can resume following a call to <B>sem_post</B> or
|
||
<B>sem_post_multiple</B>.</P>
|
||
<P><B>sem_timedwait</B> atomically decrements <I>sem</I>'s count if
|
||
it is greater than 0 and returns immediately, or it suspends the
|
||
calling thread. If <I>abstime</I> time arrives before the thread can
|
||
resume following a call to <B>sem_post</B> or <B>sem_post_multiple</B>,
|
||
then <B>sem_timedwait</B> returns with a return code of -1 after
|
||
having set <B>errno</B> to <B>ETIMEDOUT</B>. If the call can return
|
||
without suspending then <I>abstime</I> is not checked.</P>
|
||
<P><B>sem_trywait</B> atomically decrements <I>sem</I>'s count if it
|
||
is greater than 0 and returns immediately, or it returns immediately
|
||
with a return code of -1 after having set <B>errno</B> to <B>EAGAIN</B>.
|
||
<B>sem_trywait</B> never blocks.</P>
|
||
<P><B>sem_post</B> either releases one thread if there are any
|
||
waiting on <I>sem</I>, or it atomically increments <I>sem</I>'s
|
||
count.</P>
|
||
<P><B>sem_post_multiple</B> either releases multiple threads if there
|
||
are any waiting on <I>sem</I> and/or it atomically increases <I>sem</I>'s
|
||
count. If there are currently <I>n</I> waiters, where <I>n</I> the
|
||
largest number less than or equal to <I>number</I>, then <I>n</I>
|
||
waiters are released and <I>sem</I>'s count is incremented by <I>number</I>
|
||
minus <I>n</I>.</P>
|
||
<P><B>sem_getvalue</B> stores in the location pointed to by <I>sval</I>
|
||
the current count of the semaphore <I>sem</I>. In the <B>Pthreads-w32</B>
|
||
implementation: if the value returned in <I>sval</I> is greater than
|
||
or equal to 0 it was the <I>sem</I>'s count at some point during the
|
||
call to <B>sem_getvalue</B>. If the value returned in <I>sval</I> is
|
||
less than 0 then it's absolute value represents the number of threads
|
||
waiting on <I>sem</I> at some point during the call to <B>sem_getvalue.
|
||
</B>POSIX does not require an implementation of <B>sem_getvalue</B>
|
||
to return a value in <I>sval</I> that is less than 0, but if it does
|
||
then it's absolute value must represent the number of waiters.</P>
|
||
<P><B>sem_destroy</B> destroys a semaphore object, freeing the
|
||
resources it might hold. No threads should be waiting on the
|
||
semaphore at the time <B>sem_destroy</B> is called.</P>
|
||
<H2><A HREF="#toc3" NAME="sect3">Cancellation</A></H2>
|
||
<P><B>sem_wait</B> and <B>sem_timedwait</B> are cancellation points.
|
||
</P>
|
||
<H2><A HREF="#toc4" NAME="sect4">Async-signal Safety</A></H2>
|
||
<P>These routines are not async-cancel safe.</P>
|
||
<H2><A HREF="#toc5" NAME="sect5">Return Value</A></H2>
|
||
<P>All semaphore functions return 0 on success, or -1 on error in
|
||
which case they write an error code in <B>errno</B>.
|
||
</P>
|
||
<H2><A HREF="#toc6" NAME="sect6">Errors</A></H2>
|
||
<P>The <B>sem_init</B> function sets <B>errno</B> to the following
|
||
codes on error:
|
||
</P>
|
||
<DL>
|
||
<DL>
|
||
<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EINVAL</B>
|
||
</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
|
||
<I>value</I> exceeds the maximal counter value <B>SEM_VALUE_MAX</B>
|
||
</DD><DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
|
||
<B>ENOSYS</B>
|
||
</DT></DL>
|
||
</DL>
|
||
<BLOCKQUOTE STYLE="margin-left: 3cm">
|
||
<I>pshared</I> is not zero
|
||
</BLOCKQUOTE>
|
||
<P>The <B>sem_timedwait</B> function sets <B>errno</B> to the
|
||
following error code on error:
|
||
</P>
|
||
<DL>
|
||
<DL>
|
||
<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ETIMEDOUT</B>
|
||
</DT></DL>
|
||
</DL>
|
||
<BLOCKQUOTE STYLE="margin-left: 3cm">
|
||
if <I>abstime</I> arrives before the waiting thread can resume
|
||
following a call to <B>sem_post</B> or <B>sem_post_multiple</B>.
|
||
</BLOCKQUOTE>
|
||
<P>The <B>sem_trywait</B> function sets <B>errno</B> to the following
|
||
error code on error:
|
||
</P>
|
||
<DL>
|
||
<DL>
|
||
<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EAGAIN</B>
|
||
</DT></DL>
|
||
</DL>
|
||
<BLOCKQUOTE STYLE="margin-left: 3cm">
|
||
if the semaphore count is currently 0
|
||
</BLOCKQUOTE>
|
||
<P>The <B>sem_post</B> and <B>sem_post_multiple</B> functions set
|
||
<B>errno</B> to the following error code on error:
|
||
</P>
|
||
<DL>
|
||
<DL>
|
||
<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ERANGE</B>
|
||
</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
|
||
if after incrementing, the semaphore count would exceed
|
||
<B>SEM_VALUE_MAX</B> (the semaphore count is left unchanged in this
|
||
case)
|
||
</DD></DL>
|
||
</DL>
|
||
<P>
|
||
The <B>sem_destroy</B> function sets <B>errno</B> to the following
|
||
error code on error:
|
||
</P>
|
||
<DL>
|
||
<DL>
|
||
<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EBUSY</B>
|
||
</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
|
||
if some threads are currently blocked waiting on the semaphore.
|
||
</DD></DL>
|
||
</DL>
|
||
<H2>
|
||
<A HREF="#toc7" NAME="sect7">Author</A></H2>
|
||
<P>Xavier Leroy <Xavier.Leroy@inria.fr>
|
||
</P>
|
||
<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
|
||
<H2><A HREF="#toc8" NAME="sect8">See Also</A></H2>
|
||
<P><A HREF="pthread_mutex_init.html"><B>pthread_mutex_init</B>(3)</A>
|
||
, <A HREF="pthread_cond_init.html"><B>pthread_cond_init</B>(3)</A> ,
|
||
<A HREF="pthread_cancel.html"><B>pthread_cancel</B>(3)</A> .
|
||
</P>
|
||
<HR>
|
||
<P><A NAME="toc"></A><B>Table of Contents</B></P>
|
||
<UL>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Cancellation</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Async-signal
|
||
Safety</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Return
|
||
Value</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">Errors</A>
|
||
</P>
|
||
<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect7" NAME="toc7">Author</A>
|
||
</P>
|
||
<LI><P><A HREF="#sect8" NAME="toc8">See Also</A>
|
||
</P>
|
||
</UL>
|
||
</BODY>
|
||
</HTML> |