NAME    [Toc]    [Back]

     pthread_setcancelstate,               pthread_setcanceltype,
pthread_testcancel - set
     cancelability state

SYNOPSIS    [Toc]    [Back]

     #include <pthread.h>

     int
     pthread_setcancelstate(int state, int *oldstate);

     int
     pthread_setcanceltype(int type, int *oldtype);

     void
     pthread_testcancel(void);

DESCRIPTION    [Toc]    [Back]

     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    [Toc]    [Back]
     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    [Toc]    [Back]
     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    [Toc]    [Back]

     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 cancelled.
  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 doesn't want to have to worry
about how to
     clean up if the thread is cancelled 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 cancelled, 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    [Toc]    [Back]

     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    [Toc]    [Back]

     pthread_cancel(3)

STANDARDS    [Toc]    [Back]

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

OpenBSD      3.6                         January     17,     1999