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NAME

       pthread_cleanup_push,   pthread_cleanup_pop   -  push  and  pop  thread
       cancellation clean-up handlers

SYNOPSIS

       #include <pthread.h>

       void pthread_cleanup_push(void (*routine)(void *),
                                 void *arg);
       void pthread_cleanup_pop(int execute);

       Compile and link with -pthread.

DESCRIPTION

       These functions  manipulate  the  calling  thread's  stack  of  thread-
       cancellation  clean-up handlers.  A clean-up handler is a function that
       is automatically executed when a thread  is  canceled  (or  in  various
       other  circumstances  described below); it might, for example, unlock a
       mutex so that it becomes available to other threads in the process.

       The pthread_cleanup_push() function pushes routine onto the top of  the
       stack  of clean-up handlers.  When routine is later invoked, it will be
       given arg as its argument.

       The pthread_cleanup_pop() function removes the routine at  the  top  of
       the  stack  of clean-up handlers, and optionally executes it if execute
       is nonzero.

       A cancellation clean-up handler is popped from the stack  and  executed
       in the following circumstances:

       1. When  a thread is canceled, all of the stacked clean-up handlers are
          popped and executed in the reverse of the order in which  they  were
          pushed onto the stack.

       2. When  a  thread  terminates by calling pthread_exit(3), all clean-up
          handlers are executed as described in the preceding point.   (Clean-
          up  handlers are not called if the thread terminates by performing a
          return from the thread start function.)

       3. When a thread calls pthread_cleanup_pop()  with  a  nonzero  execute
          argument, the top-most clean-up handler is popped and executed.

       POSIX.1  permits pthread_cleanup_push() and pthread_cleanup_pop() to be
       implemented as macros that expand  to  text  containing  '{'  and  '}',
       respectively.   For  this  reason, the caller must ensure that calls to
       these functions are paired within the same function, and  at  the  same
       lexical  nesting  level.   (In  other words, a clean-up handler is only
       established during the execution of a specified section of code.)

       Calling longjmp(3) (siglongjmp(3)) produces undefined  results  if  any
       call  has  been made to pthread_cleanup_push() or pthread_cleanup_pop()
       without the matching call of the pair since the jump buffer was  filled
       by    setjmp(3)    (sigsetjmp(3)).     Likewise,   calling   longjmp(3)
       (siglongjmp(3)) from  inside  a  clean-up  handler  produces  undefined
       results   unless   the   jump  buffer  was  also  filled  by  setjmp(3)
       (sigsetjmp(3)) inside the handler.

RETURN VALUE

       These functions do not return a value.

ERRORS

       There are no errors.

CONFORMING TO

       POSIX.1-2001.

NOTES

       On  Linux,   the   pthread_cleanup_push()   and   pthread_cleanup_pop()
       functions  are implemented as macros that expand to text containing '{'
       and '}', respectively.  This means that variables declared  within  the
       scope  of  paired  calls to these functions will only be visible within
       that scope.

       POSIX.1 says that the effect of using return, break, continue, or  goto
       to  prematurely  leave  a  block  bracketed  pthread_cleanup_push() and
       pthread_cleanup_pop() is undefined.  Portable applications should avoid
       doing this.

EXAMPLE

       The program below provides a simple example of the use of the functions
       described in this page.  The program creates a thread that  executes  a
       loop  bracketed  by  pthread_cleanup_push()  and pthread_cleanup_pop().
       This  loop  increments  a  global  variable,  cnt,  once  each  second.
       Depending  on what command-line arguments are supplied, the main thread
       sends the other  thread  a  cancellation  request,  or  sets  a  global
       variable  that  causes  the other thread to exit its loop and terminate
       normally (by doing a return).

       In the following shell session, the main thread  sends  a  cancellation
       request to the other thread:

           $ ./a.out
           New thread started
           cnt = 0
           cnt = 1
           Canceling thread
           Called clean-up handler
           Thread was canceled; cnt = 0

       From  the  above,  we  see  that  the thread was canceled, and that the
       cancellation clean-up handler was called and it reset the value of  the
       global variable cnt to 0.

       In  the  next  run, the main program sets a global variable that causes
       other thread to terminate normally:

           $ ./a.out x
           New thread started
           cnt = 0
           cnt = 1
           Thread terminated normally; cnt = 2

       From the above, we see that  the  clean-up  handler  was  not  executed
       (because cleanup_pop_arg was 0), and therefore the value of cnt was not
       reset.

       In the next run, the main program sets a global  variable  that  causes
       the  other  thread  to terminate normally, and supplies a nonzero value
       for cleanup_pop_arg:

           $ ./a.out x 1
           New thread started
           cnt = 0
           cnt = 1
           Called clean-up handler
           Thread terminated normally; cnt = 0

       In the above, we see that although the thread  was  not  canceled,  the
       clean-up   handler   was   executed,  because  the  argument  given  to
       pthread_cleanup_pop() was nonzero.

   Program source

       #include <pthread.h>
       #include <sys/types.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <errno.h>

       #define handle_error_en(en, msg) \
               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

       static int done = 0;
       static int cleanup_pop_arg = 0;
       static int cnt = 0;

       static void
       cleanup_handler(void *arg)
       {
           printf("Called clean-up handler\n");
           cnt = 0;
       }

       static void *
       thread_start(void *arg)
       {
           time_t start, curr;

           printf("New thread started\n");

           pthread_cleanup_push(cleanup_handler, NULL);

           curr = start = time(NULL);

           while (!done) {
               pthread_testcancel();           /* A cancellation point */
               if (curr < time(NULL)) {
                   curr = time(NULL);
                   printf("cnt = %d\n", cnt);  /* A cancellation point */
                   cnt++;
               }
           }

           pthread_cleanup_pop(cleanup_pop_arg);
           return NULL;
       }

       int
       main(int argc, char *argv[])
       {
           pthread_t thr;
           int s;
           void *res;

           s = pthread_create(&thr, NULL, thread_start, NULL);
           if (s != 0)
               handle_error_en(s, "pthread_create");

           sleep(2);           /* Allow new thread to run a while */

           if (argc > 1) {
               if (argc > 2)
                   cleanup_pop_arg = atoi(argv[2]);
               done = 1;

           } else {
               printf("Canceling thread\n");
               s = pthread_cancel(thr);
               if (s != 0)
                   handle_error_en(s, "pthread_cancel");
           }

           s = pthread_join(thr, &res);
           if (s != 0)
               handle_error_en(s, "pthread_join");

           if (res == PTHREAD_CANCELED)
               printf("Thread was canceled; cnt = %d\n", cnt);
           else
               printf("Thread terminated normally; cnt = %d\n", cnt);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       pthread_cancel(3),                    pthread_cleanup_push_defer_np(3),
       pthread_setcancelstate(3), pthread_testcancel(3), pthreads(7)

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.