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pthread_setcancelstate.3: New page for pthread_setcancelstate(3) and pthread_setcanceltype(3)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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.\" <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH PTHREAD_SETCANCELSTATE 3 2008-11-14 "Linux" "Linux Programmer's Manual"
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.SH NAME
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pthread_setcancelstate, pthread_setcanceltype \-
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set cancelability state and type
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.SH SYNOPSIS
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.nf
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.B #include <pthread.h>
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.BI "int pthread_setcancelstate(int " state ", int *" oldstate );
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.BI "int pthread_setcanceltype(int " type ", int *" oldtype );
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.sp
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Compile and link with \fI\-pthread\fP.
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.SH DESCRIPTION
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The
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.BR pthread_setcancelstate ()
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sets the cancelability state of the calling thread to the value
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given in
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.IR state .
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The previous cancelability state of the thread is returned
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in the buffer pointed to by
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.IR oldstate .
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The
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.I state
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argument must have one of the following values:
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.TP
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.B PTHREAD_CANCEL_ENABLE
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The thread is cancelable.
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This is the default cancelability state in all new threads,
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including the initial thread.
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The thread's cancelability type determines when a cancelable thread
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will respond to a cancellation request.
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.TP
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.B PTHREAD_CANCEL_DISABLE
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The thread is not cancelable.
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If a cancellation request is received,
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it is blocked until cancelability is enabled.
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.PP
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The
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.BR pthread_setcanceltype ()
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sets the cancelability type of the calling thread to the value
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given in
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.IR type .
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The previous cancelability type of the thread is returned
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in the buffer pointed to by
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.IR oldtype .
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The
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.I type
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argument must have one of the following values:
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.TP
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.B PTHREAD_CANCEL_DEFERRED
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A cancellation request is deferred until the thread next calls
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a function that is a cancellation point (see
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.BR pthreads (7)).
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This is the default cancelability type in all new threads,
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including the initial thread.
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.TP
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.B PTHREAD_CANCEL_ASYNCHRONOUS
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The thread can be canceled at any time.
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(Typically,
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it will be canceled immediately upon receiving a cancellation request,
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but the system doesn't guarantee this.)
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.PP
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The set-and-get operation performed by each of these functions
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is atomic with respect to other threads in the process
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calling the same function.
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.SH RETURN VALUE
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On success, these functions return 0;
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on error, they return a non-zero error number.
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.SH ERRORS
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The
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.BR pthread_setcancelstate ()
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can fail with the following error:
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.TP
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.B EINVAL
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Invalid value for
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.IR state .
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.PP
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The
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.BR pthread_setcanceltype ()
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can fail with the following error:
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.TP
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.B EINVAL
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Invalid value for
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.IR type .
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.\" .SH VERSIONS
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.\" Available since glibc 2.0
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH NOTES
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For details of what happens when a thread is canceled, see
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.BR pthread_cancel (3).
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Briefly disabling cancelability is useful
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if a thread performs some critical action
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that must not be interrupted by a cancellation request.
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Beware of disabling cancelability for long periods,
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or around operations that may block for long periods,
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since that will render the thread unresponsive to cancellation requests.
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Setting the cancelability type to
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.B PTHREAD_CANCEL_ASYNCHRONOUS
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is rarely useful.
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Since the thread could be canceled at
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.I any
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time, it cannot reserve resources (e.g., allocating memory),
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acquire mutexes, semaphores, or locks, and so on,
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since, when the thread is canceled,
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the application has no way of knowing what the state of these resources is;
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that is, did the canceled thread manage to release the resources or not?
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(Among other things, this means that clean-up handlers cease to be useful,
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since they can't determine the state of resources that
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they are intended to clean up.)
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In general, most library functions, including most pthreads functions,
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can't be safely called from an asynchronously cancelable thread.
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(POSIX.1-2001 only requires that
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.BR pthread_cancel (3),
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.BR pthread_setcancelstate (),
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and
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.BR pthread_setcanceltype ()
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be safe to call from an asynchronously cancelable thread.)
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One of the few circumstances in which asynchronous cancelability is useful
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is for cancellation of a thread that is in a pure compute-bound loop.
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The Linux threading implementations permit the
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.I oldstate
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argument of
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.BR pthread_setcancelstate ()
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to be NULL, in which case the information about the previous
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cancelability state is not returned to the caller.
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Many other implementations also permit a NULL
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.I oldstat
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argument,
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.\" It looks like at least Solaris, FreeBSD and Tru64 support this.
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but POSIX.1-2001 does not specify this point,
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so portable applications should always specify a non-NULL value in
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.IR oldstate .
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A precisely analogous set of statements applies for the
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.I oldtype
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argument of
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.BR pthread_setcanceltype ().
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.SH EXAMPLE
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See
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.BR pthread_cancel (3).
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.SH SEE ALSO
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.BR pthread_cleanup_push (3),
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.BR pthread_cancel (3),
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.BR pthread_testcancel (3),
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.BR pthreads (7)
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