<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8"> <TITLE>PTHREAD_CANCEL(3) manual page</TITLE> <META NAME="GENERATOR" CONTENT="OpenOffice.org 1.1.3 (Linux)"> <META NAME="CREATED" CONTENT="20050504;12090500"> <META NAME="CHANGED" CONTENT="20050505;18220000"> <!-- 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>pthread_cancel, pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel - thread cancellation </P> <H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2> <P><B>#include <pthread.h></B> </P> <P><B>int pthread_cancel(pthread_t </B><I>thread</I><B>);</B> </P> <P><B>int pthread_setcancelstate(int </B><I>state</I><B>, int *</B><I>oldstate</I><B>);</B> </P> <P><B>int pthread_setcanceltype(int </B><I>type</I><B>, int *</B><I>oldtype</I><B>);</B> </P> <P><B>void pthread_testcancel(void);</B> </P> <H2><A HREF="#toc2" NAME="sect2">Description</A></H2> <P>Cancellation is the mechanism by which a thread can terminate the execution of another thread. More precisely, a thread can send a cancellation request to another thread. Depending on its settings, the target thread can then either ignore the request, honor it immediately, or defer it until it reaches a cancellation point. </P> <P>When a thread eventually honors a cancellation request, it performs as if <B>pthread_exit(PTHREAD_CANCELED)</B> has been called at that point: all cleanup handlers are executed in reverse order, destructor functions for thread-specific data are called, and finally the thread stops executing with the return value <B>PTHREAD_CANCELED</B>. See <A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> for more information. </P> <P><B>pthread_cancel</B> sends a cancellation request to the thread denoted by the <I>thread</I> argument. </P> <P><B>pthread_setcancelstate</B> changes the cancellation state for the calling thread -- that is, whether cancellation requests are ignored or not. The <I>state</I> argument is the new cancellation state: either <B>PTHREAD_CANCEL_ENABLE</B> to enable cancellation, or <B>PTHREAD_CANCEL_DISABLE</B> to disable cancellation (cancellation requests are ignored). If <I>oldstate</I> is not <B>NULL</B>, the previous cancellation state is stored in the location pointed to by <I>oldstate</I>, and can thus be restored later by another call to <B>pthread_setcancelstate</B>. </P> <P><B>pthread_setcanceltype</B> changes the type of responses to cancellation requests for the calling thread: asynchronous (immediate) or deferred. The <I>type</I> argument is the new cancellation type: either <B>PTHREAD_CANCEL_ASYNCHRONOUS</B> to cancel the calling thread as soon as the cancellation request is received, or <B>PTHREAD_CANCEL_DEFERRED</B> to keep the cancellation request pending until the next cancellation point. If <I>oldtype</I> is not <B>NULL</B>, the previous cancellation state is stored in the location pointed to by <I>oldtype</I>, and can thus be restored later by another call to <B>pthread_setcanceltype</B>. </P> <P><B>Pthreads-w32</B> provides two levels of support for <B>PTHREAD_CANCEL_ASYNCHRONOUS</B>: full and partial. Full support requires an additional DLL and driver be installed on the Windows system (see the See Also section below) that allows blocked threads to be cancelled immediately. Partial support means that the target thread will not cancel until it resumes execution naturally. Partial support is provided if either the DLL or the driver are not automatically detected by the pthreads-w32 library at run-time.</P> <P>Threads are always created by <A HREF="pthread_create.html"><B>pthread_create</B>(3)</A> with cancellation enabled and deferred. That is, the initial cancellation state is <B>PTHREAD_CANCEL_ENABLE</B> and the initial type is <B>PTHREAD_CANCEL_DEFERRED</B>. </P> <P>Cancellation points are those points in the program execution where a test for pending cancellation requests is performed and cancellation is executed if positive. The following POSIX threads functions are cancellation points: </P> <P><A HREF="pthread_join.html"><B>pthread_join</B>(3)</A> <BR><A HREF="pthread_cond_init.html"><B>pthread_cond_wait</B>(3)</A> <BR><A HREF="pthread_cond_init.html"><B>pthread_cond_timedwait</B>(3)</A> <BR><A HREF=""><B>pthread_testcancel</B>(3)</A> <BR><A HREF="sem_init.html"><B>sem_wait</B>(3)</A> <BR><A HREF="sem_init.html"><B>sem_timedwait</B>(3)</A> <BR><A HREF="pthread_kill.html"><B>sigwait</B>(3)</A></P> <P><B>Pthreads-w32</B> provides two functions to enable additional cancellation points to be created in user functions that block on Win32 HANDLEs:</P> <P><A HREF="pthreadCancelableWait.html">pthreadCancelableWait()</A> <BR><A HREF="pthreadCancelableTimedWait.html">pthreadCancelableTimedWait()</A></P> <P>All other POSIX threads functions are guaranteed not to be cancellation points. That is, they never perform cancellation in deferred cancellation mode. </P> <P><B>pthread_testcancel</B> does nothing except testing for pending cancellation and executing it. Its purpose is to introduce explicit checks for cancellation in long sequences of code that do not call cancellation point functions otherwise. </P> <H2><A HREF="#toc3" NAME="sect3">Return Value</A></H2> <P><B>pthread_cancel</B>, <B>pthread_setcancelstate</B> and <B>pthread_setcanceltype</B> return 0 on success and a non-zero error code on error. </P> <H2><A HREF="#toc4" NAME="sect4">Errors</A></H2> <P><B>pthread_cancel</B> returns the following error code on error: </P> <DL> <DL> <DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ESRCH</B> </DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm"> no thread could be found corresponding to that specified by the <I>thread</I> ID. </DD></DL> </DL> <P> <B>pthread_setcancelstate</B> returns the following error code 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"> the <I>state</I> argument is not </DD></DL> </DL> <BLOCKQUOTE> <B>PTHREAD_CANCEL_ENABLE</B> nor <B>PTHREAD_CANCEL_DISABLE</B> </BLOCKQUOTE> <P><B>pthread_setcanceltype</B> returns the following error code 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"> the <I>type</I> argument is not </DD></DL> </DL> <BLOCKQUOTE> <B>PTHREAD_CANCEL_DEFERRED</B> nor <B>PTHREAD_CANCEL_ASYNCHRONOUS</B> </BLOCKQUOTE> <H2><A HREF="#toc5" NAME="sect5">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="#toc6" NAME="sect6">See Also</A></H2> <P><A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> , <A HREF="pthread_cleanup_push.html"><B>pthread_cleanup_push</B>(3)</A> , <A HREF="pthread_cleanup_pop.html"><B>pthread_cleanup_pop</B>(3)</A> , Pthreads-w32 package README file 'Prerequisites' section. </P> <H2><A HREF="#toc7" NAME="sect7">Bugs</A></H2> <P>POSIX specifies that a number of system calls (basically, all system calls that may block, such as <A HREF="read.html"><B>read</B>(2)</A> , <A HREF="write.html"><B>write</B>(2)</A> , <A HREF="wait.html"><B>wait</B>(2)</A> , etc.) and library functions that may call these system calls (e.g. <A HREF="fprintf.html"><B>fprintf</B>(3)</A> ) are cancellation points. <B>Pthreads-win32</B> is not integrated enough with the C library to implement this, and thus none of the C library functions is a cancellation point. </P> <P>A workaround for these calls is to temporarily switch to asynchronous cancellation (assuming full asynchronous cancellation support is installed). So, checking for cancellation during a <B>read</B> system call, for instance, can be achieved as follows: </P> <BLOCKQUOTE><BR><BR> </BLOCKQUOTE> <PRE STYLE="margin-left: 1cm; margin-right: 1cm">pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, &oldCancelType); read(fd, buffer, length); pthread_setcanceltype(oldCancelType, NULL);</PRE> <HR> <BLOCKQUOTE><A NAME="toc"></A><B>Table of Contents</B></BLOCKQUOTE> <UL> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A> </BLOCKQUOTE> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A> </BLOCKQUOTE> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A> </BLOCKQUOTE> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return Value</A> </BLOCKQUOTE> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A> </BLOCKQUOTE> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Author</A> </BLOCKQUOTE> <LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">See Also</A> </BLOCKQUOTE> <LI><BLOCKQUOTE><A HREF="#sect7" NAME="toc7">Bugs</A> </BLOCKQUOTE> </UL> </BODY> </HTML>