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PTHREAD_TESTCANCEL(3)	 BSD 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 refer‐
     enced by oldstate.	 Legal values for state are PTHREAD_CANCEL_ENABLE and
     PTHREAD_CANCEL_DISABLE.

     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
     calling thread.  The pthread_testcancel() function has no effect if can‐
     celability is disabled.

   Cancelability States
     The cancelability state of a thread determines the action taken upon
     receipt 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, can‐
	     cellation 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
	     requests may be acted upon at any time.  When cancelability is
	     enabled and the cancelability type is PTHREAD_CANCEL_DEFERRED,
	     cancellation requests are held pending until a cancellation point
	     (see below) is reached.  If cancelability is disabled, the set‐
	     ting of the cancelability type has no immediate effect as all
	     cancellation requests are held pending; however, once cancelabil‐
	     ity is enabled again the new type will be in effect.

   Cancellation Points
     Cancellation points will occur when a thread is executing the following
     functions: close(), creat(), fcntl(), fsync(), msync(), nanosleep(),
     open(), pause(), pthread_cond_timedwait(), pthread_cond_wait(),
     pthread_join(), pthread_testcancel(), read(), sigwaitinfo(),
     sigsuspend(), sigwait(), sleep(), system(), tcdrain(), wait(), waitpid(),
     write().

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.

     The pthread_setcancelstate() and pthread_setcanceltype() functions are
     used to control the points at which a thread may be asynchronously can‐
     celed.  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 generaliza‐
     tion 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 cancelabil‐
     ity, 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 cancela‐
     bility and a cancellation request is pending for that thread, then the
     thread will be canceled, contrary to the wish of the client that dis‐
     abled.

     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.

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”).

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

BSD			       January 17, 1999				   BSD
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