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PTHREAD_TESTCANCEL(3)	     Library Functions Manual	   PTHREAD_TESTCANCEL(3)

NAME
     pthread_setcancelstate,  pthread_setcanceltype,  pthread_testcancel  -- set
     cancelability state

LIBRARY
     POSIX Threads Library (libpthread, -lpthread)

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  atomically	both  sets  the  calling
     thread's cancelability state to the indicated state and, if oldstate is not
     NULL,  returns  the previous cancelability state at the location referenced
     by  oldstate.   Legal  values  for  state	are  PTHREAD_CANCEL_ENABLE   and
     PTHREAD_CANCEL_DISABLE.  The function is async-signal-safe.

     The  pthread_setcanceltype()  function  atomically  both  sets  the calling
     thread's cancelability type to the indicated type and, if	oldtype  is  not
     NULL, returns 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, are PTHREAD_CANCEL_ENABLE and
     PTHREAD_CANCEL_DEFERRED respectively.

     The pthread_testcancel() function creates a cancellation point in the call-
     ing thread.  The pthread_testcancel() function has no effect if cancelabil-
     ity is disabled.

   Cancelability States
     The  cancelability  state	of a thread determines the action taken upon re-
     ceipt of a cancellation request.  The thread may control cancellation in  a
     number of ways.

     Each thread maintains its own "cancelability state" which may be encoded in
     two bits:

     Cancelability  Enable When cancelability is PTHREAD_CANCEL_DISABLE, cancel-
	     lation requests against the target thread are held pending.

     Cancelability Type When cancelability is enabled and the cancelability type
	     is PTHREAD_CANCEL_ASYNCHRONOUS, new  or  pending  cancellation  re-
	     quests  may  be  acted upon at any time.  When cancelability is en-
	     abled and the cancelability type is  PTHREAD_CANCEL_DEFERRED,  can-
	     cellation requests are held pending until a cancellation point (see
	     below)  is  reached.   If cancelability is disabled, the setting of
	     the cancelability type has no immediate effect as all  cancellation
	     requests  are  held pending; however, once cancelability is enabled
	     again the new type will be in effect.

   Cancellation Points
     Cancellation points will occur when a thread  is  executing  the  following
     functions:
     accept()
     accept4()
     aio_suspend()
     connect()
     clock_nanosleep()
     close()
     creat()
     fcntl()			The  fcntl() function is a cancellation point if
				cmd is F_SETLKW.
     fdatasync()
     fsync()
     kevent()			The kevent() function is a cancellation point if
				it is potentially blocking,  such  as  when  the
				nevents argument is non-zero.
     mq_receive()
     mq_send()
     mq_timedreceive()
     mq_timedsend()
     msync()
     nanosleep()
     open()
     openat()
     pause()
     poll()
     ppoll()
     pselect()
     pthread_cond_timedwait()
     pthread_cond_wait()
     pthread_join()
     pthread_testcancel()
     read()
     readv()
     recv()
     recvfrom()
     recvmsg()
     select()
     sem_timedwait()
     sem_clockwait_np()
     sem_wait()
     send()
     sendmsg()
     sendto()
     sigsuspend()
     sigtimedwait()
     sigwaitinfo()
     sigwait()
     sleep()
     system()
     tcdrain()
     usleep()
     wait()
     wait3()
     wait4()
     wait6()
     waitid()
     waitpid()
     write()
     writev()

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

     For  purposes of this discussion, consider an object 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 have to worry about how to clean  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 cancel-
     lation request is pending for that thread, then the  thread  will	be  can-
     celed, 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 that 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.

RETURN VALUES
     If  successful,  the  pthread_setcancelstate()  and pthread_setcanceltype()
     functions will return zero.  Otherwise, an error number shall  be	returned
     to indicate the error.

ERRORS
     The function pthread_setcancelstate() may fail with:

     [EINVAL]		The  specified	state  is  not	PTHREAD_CANCEL_ENABLE or
			PTHREAD_CANCEL_DISABLE.

     The function pthread_setcanceltype() may fail with:

     [EINVAL]		The specified state is	not  PTHREAD_CANCEL_DEFERRED  or
			PTHREAD_CANCEL_ASYNCHRONOUS.

SEE ALSO
     pthread_cancel(3)

STANDARDS
     The   pthread_testcancel()   function   conforms	to  ISO/IEC  9945-1:1996
     ("POSIX.1").  The standard allows implementations to make many  more  func-
     tions cancellation points.

     The  pthread_setcancelstate()  function is async-signal-safe as required by
     IEEE Std 1003.1-2024 ("POSIX.1").

AUTHORS
     This manual page was written  by  David  Leonard  <d@openbsd.org>	for  the
     OpenBSD implementation of pthread_cancel(3).

FreeBSD 15.1			 March 18, 2017 	   PTHREAD_TESTCANCEL(3)
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