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PTHREAD_TESTCANCEL(3)                       DragonFly Library Functions Manual


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 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
     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: clock_nanosleep(), close(), creat(), fcntl(), fdatasync(),
     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
     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 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 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).

DragonFly 6.1-DEVELOPMENT     September 14, 2021     DragonFly 6.1-DEVELOPMENT

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