PTHREAD_SETCANCELSTATE(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel — set cancelability state

SYNOPSIS

#include <pthread.h>

int pthread_setcancelstate(int state, int *oldstate);
int pthread_setcanceltype(int type, int *oldtype);
void pthread_testcancel(void);

DESCRIPTION
The pthread_setcancelstate() function shall atomically both set the calling thread's cancelability state to the indicated state and return the previous cancelability state at the location referenced by oldstate. Legal values for state are PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DISABLE.

The pthread_setcanceltype() function shall atomically both set the calling thread's cancelability type to the indicated type and return the previous cancelability type at the location referenced by oldtype. Legal values for type are PTHREAD_CANCEL_DEFERRED and PTHREAD_CANCEL_ASYNCHRONOUS.

The cancelability state and type of any newly created threads, including the thread in which main() was first invoked, shall be PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DEFERRED respectively.

The pthread_testcancel() function shall create a cancellation point in the calling thread. The pthread_testcancel() function shall have no effect if cancelability is disabled.

RETURN VALUE
If successful, the pthread_setcancelstate() and pthread_setcanceltype() functions shall return zero; otherwise, an error number shall be returned to indicate the error.

ERRORS
The pthread_setcancelstate() function may fail if:

EINVAL
    The specified state is not PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE.

The pthread_setcanceltype() function may fail if:

EINVAL
    The specified type is not PTHREAD_CANCEL_DEFERRED or PTHREAD_CANCEL_ASYNCHRONOUS.

These functions shall not return an error code of [EINTR].

The following sections are informative.

EXAMPLES
None.

APPLICATION USAGE
In order to write a signal handler for an asynchronous signal which can run safely in a cancellable thread, pthread_setcancelstate() must be used to disable cancellation for the duration of any calls that the signal handler makes which are cancellation points. However, the standard does not permit strictly conforming applications to call pthread_setcancelstate() from a signal handler since it is not currently required to be async-signal-safe. On implementations where pthread_setcancelstate() is not async-signal-safe, alternatives are to ensure either that the corresponding signals are blocked during execution of functions that are not async-cancel-safe or that cancellation is disabled during times when those signals could be delivered. Implementations are strongly encouraged to make pthread_setcancelstate() async-signal-safe.

RATIONALE
The pthread_setcancelstate() and pthread_setcanceltype() functions control the points at which a thread may be asynchronously canceled. For cancellation control to be usable in modular fashion, some rules need to be followed.

An object can be considered to be a generalization of a procedure. It is a set of procedures and global variables written as a unit and called by clients not known by the object. Objects may depend on other objects.

First, cancelability should only be disabled on entry to an object, never explicitly enabled. On exit from an object, the cancelability state should always be restored to its value on entry to the object.

This follows from a modularity argument: if the client of an object (or the client of an object that uses that object) has disabled cancelability, it is because the client does not want to be concerned about cleaning up if the thread is canceled while executing some sequence of actions. If an object is called in such a state and it enables cancelability and a cancellation request is pending for that thread, then the thread is canceled, contrary to the wish of the client that disabled.

Second, the cancelability type may be explicitly set to either deferred or asynchronous upon entry to an object. But as with the cancelability state, on exit from an object the cancelability type should always be restored to its value on entry to the object.

Finally, only functions that are cancel-safe may be called from a thread that is asynchronously cancelable.

FUTURE DIRECTIONS
The pthread_setcancelstate() function may be added to the table of async-signal-safe functions in Section 2.4.3, Signal Actions.

SEE ALSO
pthread_cancel()

The Base Definitions volume of POSIX.1-2017, <pthread.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

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