PTHREAD_SETCANCELSTATE(3)  POSIX Programmer's Manual PTHREAD_SETCANCELSTATE(3)



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

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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017            PTHREAD_SETCANCELSTATE(3)
