<!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>