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.TH PTHREAD_SETCANCELSTATE 3 2008-11-24 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setcancelstate, pthread_setcanceltype \-
set cancelability state and type
.SH SYNOPSIS
.nf
.B #include <pthread.h>

.BI "int pthread_setcancelstate(int " state ", int *" oldstate );
.BI "int pthread_setcanceltype(int " type ", int *" oldtype );
.sp
Compile and link with \fI\-pthread\fP.
.fi
.SH DESCRIPTION
The
.BR pthread_setcancelstate ()
sets the cancelability state of the calling thread to the value
given in
.IR state .
The previous cancelability state of the thread is returned
in the buffer pointed to by
.IR oldstate .
The
.I state
argument must have one of the following values:
.TP
.B PTHREAD_CANCEL_ENABLE
The thread is cancelable.
This is the default cancelability state in all new threads,
including the initial thread.
The thread's cancelability type determines when a cancelable thread
will respond to a cancellation request.
.TP
.B PTHREAD_CANCEL_DISABLE
The thread is not cancelable.
If a cancellation request is received,
it is blocked until cancelability is enabled.
.PP
The
.BR pthread_setcanceltype ()
sets the cancelability type of the calling thread to the value
given in
.IR type .
The previous cancelability type of the thread is returned
in the buffer pointed to by
.IR oldtype .
The
.I type
argument must have one of the following values:
.TP
.B PTHREAD_CANCEL_DEFERRED
A cancellation request is deferred until the thread next calls
a function that is a cancellation point (see
.BR pthreads (7)).
This is the default cancelability type in all new threads,
including the initial thread.
.TP
.B PTHREAD_CANCEL_ASYNCHRONOUS
The thread can be canceled at any time.
(Typically,
it will be canceled immediately upon receiving a cancellation request,
but the system doesn't guarantee this.)
.PP
The set-and-get operation performed by each of these functions
is atomic with respect to other threads in the process
calling the same function.
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a nonzero error number.
.SH ERRORS
The
.BR pthread_setcancelstate ()
can fail with the following error:
.TP
.B EINVAL
Invalid value for
.IR state .
.PP
The
.BR pthread_setcanceltype ()
can fail with the following error:
.TP
.B EINVAL
Invalid value for
.IR type .
.\" .SH VERSIONS
.\" Available since glibc 2.0
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
For details of what happens when a thread is canceled, see
.BR pthread_cancel (3).

Briefly disabling cancelability is useful
if a thread performs some critical action
that must not be interrupted by a cancellation request.
Beware of disabling cancelability for long periods,
or around operations that may block for long periods,
since that will render the thread unresponsive to cancellation requests.

Setting the cancelability type to
.B PTHREAD_CANCEL_ASYNCHRONOUS
is rarely useful.
Since the thread could be canceled at
.I any
time, it cannot safely reserve resources (e.g., allocating memory with
.BR malloc (3)),
acquire mutexes, semaphores, or locks, and so on.
Reserving resources is unsafe because the application has no way of
knowing what the state of these resources is when the thread is canceled;
that is, did cancellation occur before the resources were reserved,
while they were reserved, or after they were released?
Furthermore, some internal data structures
(e.g., the linked list of free blocks managed by the
.BR malloc (3)
family of functions) may be left in an inconsistent state
if cancellation occurs in the middle of the function call.
Consequently, clean-up handlers cease to be useful.
Functions that can be safely asynchronously canceled are called
.IR "async-cancel-safe functions" .
POSIX.1-2001 only requires that
.BR pthread_cancel (3),
.BR pthread_setcancelstate (),
and
.BR pthread_setcanceltype ()
be async-cancel-safe.
In general, other library functions
can't be safely called from an asynchronously cancelable thread.
One of the few circumstances in which asynchronous cancelability is useful
is for cancellation of a thread that is in a pure compute-bound loop.

The Linux threading implementations permit the
.I oldstate
argument of
.BR pthread_setcancelstate ()
to be NULL, in which case the information about the previous
cancelability state is not returned to the caller.
Many other implementations also permit a NULL
.I oldstat
argument,
.\" It looks like at least Solaris, FreeBSD and Tru64 support this.
but POSIX.1-2001 does not specify this point,
so portable applications should always specify a non-NULL value in
.IR oldstate .
A precisely analogous set of statements applies for the
.I oldtype
argument of
.BR pthread_setcanceltype ().
.SH EXAMPLE
See
.BR pthread_cancel (3).
.SH SEE ALSO
.BR pthread_cancel (3),
.BR pthread_cleanup_push (3),
.BR pthread_testcancel (3),
.BR pthreads (7)