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156 lines
4.9 KiB
Groff
156 lines
4.9 KiB
Groff
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.TH PTHREAD_CANCEL 3 LinuxThreads
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.XREF pthread_setcancelstate
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.XREF pthread_setcanceltype
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.XREF pthread_testcancel
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.SH NAME
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pthread_cancel, pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel \- thread cancellation
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.SH SYNOPSIS
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#include <pthread.h>
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int pthread_cancel(pthread_t thread);
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int pthread_setcancelstate(int state, int *oldstate);
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int pthread_setcanceltype(int type, int *oldtype);
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void pthread_testcancel(void);
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.SH DESCRIPTION
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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, the
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target thread can then either ignore the request, honor it
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immediately, or defer it till it reaches a cancellation point.
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When a thread eventually honors a cancellation request, it performs as
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if !pthread_exit(PTHREAD_CANCELED)! has been called at that point:
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all cleanup handlers are executed in reverse order, finalization
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functions for thread-specific data are called, and finally the thread
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stops executing with the return value !PTHREAD_CANCELED!. See
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!pthread_exit!(3) for more information.
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!pthread_cancel! sends a cancellation request to the thread denoted
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by the |thread| argument.
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!pthread_setcancelstate! changes the cancellation state for the
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calling thread -- that is, whether cancellation requests are ignored
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or not. The |state| argument is the new cancellation state: either
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!PTHREAD_CANCEL_ENABLE! to enable cancellation, or
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!PTHREAD_CANCEL_DISABLE! to disable cancellation (cancellation
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requests are ignored). If |oldstate| is not !NULL!, the previous
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cancellation state is stored in the location pointed to by |oldstate|,
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and can thus be restored later by another call to
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!pthread_setcancelstate!.
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!pthread_setcanceltype! changes the type of responses to cancellation
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requests for the calling thread: asynchronous (immediate) or deferred.
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The |type| argument is the new cancellation type: either
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!PTHREAD_CANCEL_ASYNCHRONOUS! to cancel the calling thread as soon as
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the cancellation request is received, or !PTHREAD_CANCEL_DEFERRED! to
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keep the cancellation request pending until the next cancellation
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point. If |oldtype| is not !NULL!, the previous
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cancellation state is stored in the location pointed to by |oldtype|,
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and can thus be restored later by another call to
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!pthread_setcanceltype!.
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Threads are always created by !pthread_create!(3) with cancellation
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enabled and deferred. That is, the initial cancellation state is
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!PTHREAD_CANCEL_ENABLE! and the initial type is
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!PTHREAD_CANCEL_DEFERRED!.
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Cancellation points are those points in the program execution where a
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test for pending cancellation requests is performed and cancellation
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is executed if positive. The following POSIX threads functions
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are cancellation points:
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!pthread_join!(3)
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.br
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!pthread_cond_wait!(3)
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.br
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!pthread_cond_timedwait!(3)
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.br
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!pthread_testcancel!(3)
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.br
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!sem_wait!(3)
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.br
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!sigwait!(3)
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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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!pthread_testcancel! 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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.SH "RETURN VALUE"
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!pthread_cancel!, !pthread_setcancelstate! and
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!pthread_setcanceltype! return 0 on success and a non-zero error code
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on error.
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.SH ERRORS
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!pthread_cancel! returns the following error code on error:
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.RS
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.TP
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!ESRCH!
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no thread could be found corresponding to that specified by the |thread| ID.
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.RE
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!pthread_setcancelstate! returns the following error code on error:
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.RS
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.TP
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!EINVAL!
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the |state| argument is not !PTHREAD_CANCEL_ENABLE! nor
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!PTHREAD_CANCEL_DISABLE!
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.RE
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!pthread_setcanceltype! returns the following error code on error:
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.RS
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.TP
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!EINVAL!
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the |type| argument is not !PTHREAD_CANCEL_DEFERRED! nor
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!PTHREAD_CANCEL_ASYNCHRONOUS!
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.RE
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.SH AUTHOR
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Xavier Leroy <Xavier.Leroy@inria.fr>
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.SH "SEE ALSO"
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!pthread_exit!(3),
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!pthread_cleanup_push!(3),
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!pthread_cleanup_pop!(3).
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.SH BUGS
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POSIX specifies that a number of system calls (basically, all
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system calls that may block, such as !read!(2), !write!(2), !wait!(2),
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etc.) and library functions that may call these system calls (e.g.
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!fprintf!(3)) are cancellation points. LinuxThreads is not yet
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integrated enough with the C library to implement this, and thus none
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of the C library functions is a cancellation point.
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For system calls at least, there is a workaround. Cancellation
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requests are transmitted to the target thread by sending it a
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signal. That signal will interrupt all blocking system calls, causing
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them to return immediately with the !EINTR! error. So, checking for
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cancellation during a !read! system call, for instance, can be
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achieved as follows:
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.RS
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.ft 3
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.nf
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.sp
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pthread_testcancel();
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retcode = read(fd, buffer, length);
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pthread_testcancel();
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.ft
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.LP
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.RE
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.fi
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