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
       calling 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
              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:

              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
              effect 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

       │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

       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
       operations 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)),
       acquire 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
       cancellation occur before the resources were reserved, while they were
       reserved, or after they were released?  Furthermore, some internal data
       structures (e.g., the linked list of free blocks managed by the
       malloc(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_setcanceltype() 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

       See pthread_cancel(3).

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

       This page is part of release 5.07 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                             2020-06-09         PTHREAD_SETCANCELSTATE(3)