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PTHREAD_SETCANCELSTATE(3) Linux Programmer's Manual PTHREAD_SETCANCELSTATE(3)

NAME

     pthread_setcancelstate, pthread_setcanceltype - set cancelability state
     and type

SYNOPSIS

     #include <pthread.h>

     int pthread_setcancelstate(int state, int *oldstate);
     int pthread_setcanceltype(int type, int *oldtype);

     Compile and link with -pthread.

DESCRIPTION

     The pthread_setcancelstate() sets the cancelability state of the  call-
     ing  thread  to  the  value given in state.  The previous cancelability
     state of the thread is returned in the buffer pointed to  by  oldstate.
     The state argument must have one of the following values:

     PTHREAD_CANCEL_ENABLE
            The  thread  is  cancelable.   This is the default cancelability
            state in all new threads, including  the  initial  thread.   The
            thread's  cancelability type determines when a cancelable thread
            will respond to a cancellation request.

     PTHREAD_CANCEL_DISABLE
            The thread is not cancelable.   If  a  cancellation  request  is
            received, it is blocked until cancelability is enabled.

     The  pthread_setcanceltype() sets the cancelability type of the calling
     thread to the value given in type.  The previous cancelability type  of
     the  thread  is returned in the buffer pointed to by oldtype.  The type
     argument must have one of the following values:

     PTHREAD_CANCEL_DEFERRED
            A cancellation request is deferred until the thread next calls a
            function  that  is a cancellation point (see pthreads(7)).  This
            is the default cancelability type in all new threads,  including
            the initial thread.

     PTHREAD_CANCEL_ASYNCHRONOUS
            The  thread can be canceled at any time.  (Typically, it will be
            canceled immediately upon receiving a cancellation request,  but
            the system doesn't guarantee this.)

     The  set-and-get  operation  performed  by  each  of these functions is
     atomic with respect to other threads in the process  calling  the  same
     function.

RETURN VALUE

     On  success,  these functions return 0; on error, they return a nonzero
     error number.

ERRORS

     The pthread_setcancelstate() can fail with the following error:

     EINVAL Invalid value for state.

     The pthread_setcanceltype() can fail with the following error:

     EINVAL Invalid value for type.

ATTRIBUTES

     For  an  explanation  of  the  terms  used   in   this   section,   see
     attributes(7).

     +--------------------------+---------------------+---------+
     |Interface                 | Attribute           | Value   |
     +--------------------------+---------------------+---------+
     |pthread_setcancelstate(), | Thread safety       | MT-Safe |
     |pthread_setcanceltype()   |                     |         |
     +--------------------------+---------------------+---------+
     |pthread_setcancelstate(), | Async-cancel-safety | AC-Safe |
     |pthread_setcanceltype()   |                     |         |
     +--------------------------+---------------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

NOTES

     For details of what happens when a thread is canceled, see pthread_can-
     cel(3).

     Briefly disabling cancelability is useful if  a  thread  performs  some
     critical action that must not be interrupted by a cancellation request.
     Beware of disabling cancelability for long periods,  or  around  opera-
     tions  that  may  block  for  long  periods, since that will render the
     thread unresponsive to cancellation requests.

 Asynchronous cancelability
     Setting the cancelability type to PTHREAD_CANCEL_ASYNCHRONOUS is rarely
     useful.   Since  the  thread  could  be canceled at any time, it cannot
     safely reserve resources (e.g., allocating memory with malloc(3)),  ac-
     quire mutexes, semaphores, or locks, and so on.  Reserving resources is
     unsafe because the application has no way of knowing what the state  of
     these  resources is when the thread is canceled; that is, did cancella-
     tion occur before the resources were  reserved,  while  they  were  re-
     served,  or  after they were released?  Furthermore, some internal data
     structures (e.g., the linked list of free blocks managed  by  the  mal-
     loc(3)  family  of  functions)  may be left in an inconsistent state if
     cancellation occurs in the middle of the function call.   Consequently,
     clean-up handlers cease to be useful.

     Functions  that can be safely asynchronously canceled are called async-
     cancel-safe functions.  POSIX.1-2001 and POSIX.1-2008 require only that
     pthread_cancel(3),   pthread_setcancelstate(),  and  pthread_setcancel-
     type() be async-cancel-safe.  In general, other library functions can't
     be safely called from an asynchronously cancelable thread.

     One  of  the  few  circumstances in which asynchronous cancelability is
     useful is for cancellation of a thread that is in a pure  compute-bound
     loop.

 Portability notes
     The  Linux  threading  implementations  permit the oldstate argument of
     pthread_setcancelstate() to be NULL,  in  which  case  the  information
     about  the  previous cancelability state is not returned to the caller.
     Many other implementations also permit a  NULL  oldstat  argument,  but
     POSIX.1  does  not  specify this point, so portable applications should
     always specify a non-NULL value in oldstate.  A precisely analogous set
     of  statements  applies  for the oldtype argument of pthread_setcancel-
     type().

EXAMPLE

     See pthread_cancel(3).