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<META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8">
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<TITLE>PTHREAD_CANCEL(3) manual page</TITLE>
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</HEAD>
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<BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR">
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<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
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<P><A HREF="index.html">Reference Index</A></P>
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<P><A HREF="#toc">Table of Contents</A></P>
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<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
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<P>pthread_cancel, pthread_setcancelstate, pthread_setcanceltype,
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pthread_testcancel - thread cancellation
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</P>
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<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
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<P><B>#include <pthread.h></B>
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</P>
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<P><B>int pthread_cancel(pthread_t </B><I>thread</I><B>);</B>
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</P>
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<P><B>int pthread_setcancelstate(int </B><I>state</I><B>, int
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*</B><I>oldstate</I><B>);</B>
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</P>
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<P><B>int pthread_setcanceltype(int </B><I>type</I><B>, int
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*</B><I>oldtype</I><B>);</B>
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</P>
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<P><B>void pthread_testcancel(void);</B>
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</P>
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<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
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<P>Cancellation is the mechanism by which a thread can terminate the
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execution of another thread. More precisely, a thread can send a
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cancellation request to another thread. Depending on its settings,
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the target thread can then either ignore the request, honor it
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immediately, or defer it until it reaches a cancellation point.
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</P>
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<P>When a thread eventually honors a cancellation request, it
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performs as if <B>pthread_exit(PTHREAD_CANCELED)</B> has been called
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at that point: all cleanup handlers are executed in reverse order,
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destructor functions for thread-specific data are called, and finally
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the thread stops executing with the return value <B>PTHREAD_CANCELED</B>.
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See <A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> for more
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information.
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</P>
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<P><B>pthread_cancel</B> sends a cancellation request to the thread
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denoted by the <I>thread</I> argument.
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</P>
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<P><B>pthread_setcancelstate</B> changes the cancellation state for
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the calling thread -- that is, whether cancellation requests are
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ignored or not. The <I>state</I> argument is the new cancellation
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state: either <B>PTHREAD_CANCEL_ENABLE</B> to enable cancellation, or
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<B>PTHREAD_CANCEL_DISABLE</B> to disable cancellation (cancellation
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requests are ignored). If <I>oldstate</I> is not <B>NULL</B>, the
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previous cancellation state is stored in the location pointed to by
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<I>oldstate</I>, and can thus be restored later by another call to
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<B>pthread_setcancelstate</B>.
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</P>
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<P><B>pthread_setcanceltype</B> changes the type of responses to
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cancellation requests for the calling thread: asynchronous
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(immediate) or deferred. The <I>type</I> argument is the new
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cancellation type: either <B>PTHREAD_CANCEL_ASYNCHRONOUS</B> to
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cancel the calling thread as soon as the cancellation request is
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received, or <B>PTHREAD_CANCEL_DEFERRED</B> to keep the cancellation
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request pending until the next cancellation point. If <I>oldtype</I>
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is not <B>NULL</B>, the previous cancellation state is stored in the
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location pointed to by <I>oldtype</I>, and can thus be restored later
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by another call to <B>pthread_setcanceltype</B>.
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</P>
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<P><B>Pthreads-w32</B> provides two levels of support for
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<B>PTHREAD_CANCEL_ASYNCHRONOUS</B>: full and partial. Full support
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requires an additional DLL and driver be installed on the Windows
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system (see the See Also section below) that allows blocked threads
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to be cancelled immediately. Partial support means that the target
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thread will not cancel until it resumes execution naturally. Partial
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support is provided if either the DLL or the driver are not
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automatically detected by the pthreads-w32 library at run-time.</P>
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<P>Threads are always created by <A HREF="pthread_create.html"><B>pthread_create</B>(3)</A>
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with cancellation enabled and deferred. That is, the initial
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cancellation state is <B>PTHREAD_CANCEL_ENABLE</B> and the initial
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type is <B>PTHREAD_CANCEL_DEFERRED</B>.
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</P>
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<P>Cancellation points are those points in the program execution
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where a test for pending cancellation requests is performed and
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cancellation is executed if positive. The following POSIX threads
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functions are cancellation points:
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</P>
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<P><A HREF="pthread_join.html"><B>pthread_join</B>(3)</A>
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<BR><A HREF="pthread_cond_init.html"><B>pthread_cond_wait</B>(3)</A>
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<BR><A HREF="pthread_cond_init.html"><B>pthread_cond_timedwait</B>(3)</A>
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<BR><A HREF=""><B>pthread_testcancel</B>(3)</A> <BR><A HREF="sem_init.html"><B>sem_wait</B>(3)</A>
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<BR><A HREF="sem_init.html"><B>sem_timedwait</B>(3)</A> <BR><A HREF="pthread_kill.html"><B>sigwait</B>(3)</A></P>
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<P><B>Pthreads-w32</B> provides two functions to enable additional
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cancellation points to be created in user functions that block on
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Win32 HANDLEs:</P>
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<P><A HREF="pthreadCancelableWait.html">pthreadCancelableWait()</A>
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<BR><A HREF="pthreadCancelableTimedWait.html">pthreadCancelableTimedWait()</A></P>
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<P>All other POSIX threads functions are guaranteed not to be
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cancellation points. That is, they never perform cancellation in
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deferred cancellation mode.
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</P>
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<P><B>pthread_testcancel</B> does nothing except testing for pending
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cancellation and executing it. Its purpose is to introduce explicit
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checks for cancellation in long sequences of code that do not call
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cancellation point functions otherwise.
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</P>
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<H2><A HREF="#toc3" NAME="sect3">Return Value</A></H2>
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<P><B>pthread_cancel</B>, <B>pthread_setcancelstate</B> and
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<B>pthread_setcanceltype</B> return 0 on success and a non-zero error
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code on error.
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</P>
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<H2><A HREF="#toc4" NAME="sect4">Errors</A></H2>
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<P><B>pthread_cancel</B> returns the following error code on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ESRCH</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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no thread could be found corresponding to that specified by the
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<I>thread</I> ID.
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</DD></DL>
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</DL>
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<P>
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<B>pthread_setcancelstate</B> returns the following error code on
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error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EINVAL</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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the <I>state</I> argument is not
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</DD></DL>
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</DL>
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<BLOCKQUOTE>
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<B>PTHREAD_CANCEL_ENABLE</B> nor <B>PTHREAD_CANCEL_DISABLE</B>
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</BLOCKQUOTE>
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<P><B>pthread_setcanceltype</B> returns the following error code on
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error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EINVAL</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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the <I>type</I> argument is not
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</DD></DL>
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</DL>
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<BLOCKQUOTE>
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<B>PTHREAD_CANCEL_DEFERRED</B> nor <B>PTHREAD_CANCEL_ASYNCHRONOUS</B>
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</BLOCKQUOTE>
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<H2><A HREF="#toc5" NAME="sect5">Author</A></H2>
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<P>Xavier Leroy <Xavier.Leroy@inria.fr>
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</P>
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<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
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<H2><A HREF="#toc6" NAME="sect6">See Also</A></H2>
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<P><A HREF="pthread_exit.html"><B>pthread_exit</B>(3)</A> ,
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<A HREF="pthread_cleanup_push.html"><B>pthread_cleanup_push</B>(3)</A>
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, <A HREF="pthread_cleanup_pop.html"><B>pthread_cleanup_pop</B>(3)</A>
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, Pthreads-w32 package README file 'Prerequisites' section.
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</P>
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<H2><A HREF="#toc7" NAME="sect7">Bugs</A></H2>
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<P>POSIX specifies that a number of system calls (basically, all
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system calls that may block, such as <A HREF="read.html"><B>read</B>(2)</A>
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, <A HREF="write.html"><B>write</B>(2)</A> , <A HREF="wait.html"><B>wait</B>(2)</A>
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, etc.) and library functions that may call these system calls (e.g.
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<A HREF="fprintf.html"><B>fprintf</B>(3)</A> ) are cancellation
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points. <B>Pthreads-win32</B> is not integrated enough with the C
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library to implement this, and thus none of the C library functions
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is a cancellation point.
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</P>
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<P>A workaround for these calls is to temporarily switch to
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asynchronous cancellation (assuming full asynchronous cancellation
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support is installed). So, checking for cancellation during a <B>read</B>
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system call, for instance, can be achieved as follows:
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</P>
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<BLOCKQUOTE><BR><BR>
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</BLOCKQUOTE>
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<PRE STYLE="margin-left: 1cm; margin-right: 1cm">pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, &oldCancelType);
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read(fd, buffer, length);
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pthread_setcanceltype(oldCancelType, NULL);</PRE>
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<HR>
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<BLOCKQUOTE><A NAME="toc"></A><B>Table of Contents</B></BLOCKQUOTE>
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<UL>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return
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Value</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Author</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">See
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Also</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE><A HREF="#sect7" NAME="toc7">Bugs</A>
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</BLOCKQUOTE>
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</UL>
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