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
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
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.
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
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
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