pthread_cleanup_push

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



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
       established only 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.

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

       ┌────────────────────────┬───────────────┬─────────┐
       │Interface               Attribute     Value   │
       ├────────────────────────┼───────────────┼─────────┤
       │pthread_cleanup_push(), │ Thread safety │ MT-Safe │
       │pthread_cleanup_pop()   │               │         │
       └────────────────────────┴───────────────┴─────────┘

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

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 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
       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
       This page is part of release 5.03 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06           PTHREAD_CLEANUP_PUSH(3)