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.

EXAMPLES

None.

APPLICATION USAGE

None.

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

None.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .