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
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.
SEE ALSO
pthread_cancel() , the Base Definitions volume of IEEE Std 1003.1-2001,
<pthread.h>
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 .