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     pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel -- set cancelability state

     #include <pthread.h>

     pthread_setcancelstate(int state, int *oldstate);

     pthread_setcanceltype(int type, int *oldtype);


     The pthread_setcancelstate() function atomically both sets the calling thread's cancelability state to
     the indicated state and returns 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 atomically both sets the calling thread's cancelability type to
     the indicated type and returns the previous cancelability type at the location referenced by oldtype.

     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 cancelability 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, cancellation 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 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(),
     aio_suspend(), close(), connect(), creat(), fcntl(), fsync(), lockf(), msgrcv(), msgsnd(), msync(),
     nanosleep(), open(), pause(), poll(), pread(), pselect(), pthread_cond_timedwait(),
     pthread_cond_wait(), pthread_join(), pthread_testcancel(), pwrite(), read(), readv(), recv(),
     recvfrom(), recvmsg(), select(), sem_wait(), send(), sendmsg(), sendto(), sigpause(), sigsuspend(),
     sigwait(), sleep(), system(), tcdrain(), usleep(), wait(), waitpid(), write(), writev().

     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 canceled.  For cancellation control to be usable in modular fash-ion, fashion,
     ion, 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 doesn't 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 cancellation request is pending for that
     thread, then the thread will be 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 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 cance-lable. cancelable.

     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.


     pthread_testcancel() conforms to ISO/IEC 9945-1:1996 (``POSIX.1'').

     This man page was written by David Leonard <> for the OpenBSD implementation of

BSD                            January 17, 1999                            BSD

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