pthread_setcancelstate, pthread_setcanceltype - set cancelability state
       and type

       #include <pthread.h>

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

       Compile and link with -pthread.

       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:

              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.

              The thread is not cancelable.  If a cancellation request is  re-
              ceived, 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:

              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.

              Even  with  deferred  cancellation,  a  cancellation point in an
              asynchronous signal handler may still be acted upon and the  ef-
              fect is as if it was an asynchronous cancellation.

              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

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

       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.

       For an  explanation  of  the  terms  used  in  this  section,  see  at-

       |Interface                 | Attribute           | Value   |
       |pthread_setcancelstate(), | Thread safety       | MT-Safe |
       |pthread_setcanceltype()   |                     |         |
       |pthread_setcancelstate(), | Async-cancel-safety | AC-Safe |
       |pthread_setcanceltype()   |                     |         |
       POSIX.1-2001, POSIX.1-2008.

       For details of what happens when a thread is canceled, see pthread_can-

       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

   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-

       See pthread_cancel(3).

       pthread_cancel(3),   pthread_cleanup_push(3),    pthread_testcancel(3),

       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2019-10-10         PTHREAD_SETCANCELSTATE(3)
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