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author: rpj <rpj> 2005-05-05 14:18:27 +0000
committer: rpj <rpj> 2005-05-05 14:18:27 +0000
commit: 41f88a82b33cdb357c83b582381232733ed2d039 (patch)
tree: b3690e999b4d934fce7aab79b2693ab086a4ad46 /manual/sem_init.html
parent: b75686484d99c90317b64184ce5080093ee7d2a9 (diff)
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+<!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">
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+<BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR">
+<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 &lt;semaphore.h&gt;</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><SPAN STYLE="font-style: normal">,
+<B>const struct timespec *</B></SPAN><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><SPAN STYLE="font-style: normal"><B>int</B>
+</SPAN><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><SPAN STYLE="font-weight: medium">
+or </SPAN><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><SPAN STYLE="font-weight: medium"> 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></SPAN>. <B>sem_trywait</B><SPAN STYLE="font-weight: medium">
+never blocks.</SPAN></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><SPAN STYLE="font-style: normal"> and/or
+it </SPAN>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>. <SPAN STYLE="font-weight: medium">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 </SPAN><B>sem_getvalue.  </B><SPAN STYLE="font-weight: medium">POSIX
+does not require an implementation of </SPAN><B>sem_getvalue</B><SPAN STYLE="font-weight: medium">
+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.</SPAN></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>_POSIX_SEM_VALUE_MAX</B>
+				</DD><DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
+		<B>ENOSYS</B> 
+		</DT></DL>
+</DL>
+<BLOCKQUOTE STYLE="margin-left: 2cm">
+<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: 2cm">
+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: 2cm">
+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>_POSIX_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 &lt;Xavier.Leroy@inria.fr&gt; 
+</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>
author	rpj <rpj>	2005-05-05 14:18:27 +0000
committer	rpj <rpj>	2005-05-05 14:18:27 +0000
commit	41f88a82b33cdb357c83b582381232733ed2d039 (patch)
tree	b3690e999b4d934fce7aab79b2693ab086a4ad46 /manual/sem_init.html
parent	b75686484d99c90317b64184ce5080093ee7d2a9 (diff)