NAME         top

       pthread_cleanup_push,  pthread_cleanup_pop - push and pop thread can‐
       cellation clean-up handlers

SYNOPSIS         top

       #include <pthread.h>

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

       Compile and link with -pthread.

DESCRIPTION         top

       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

       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
       established only during the execution of a specified section of

       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         top

       These functions do not return a value.

ERRORS         top

       There are no errors.

ATTRIBUTES         top

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

       │Interface               Attribute     Value   │
       │pthread_cleanup_push(), │ Thread safety │ MT-Safe │
       │pthread_cleanup_pop()   │               │         │

CONFORMING TO         top

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

NOTES         top

       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 be visible within
       only 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         top

       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 */

           return NULL;

       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);
               printf("Thread terminated normally; cnt = %d\n", cnt);

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.08 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                            2015-07-23          PTHREAD_CLEANUP_PUSH(3)