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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  pro‐
       duces 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).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚pthread_cleanup_push(), pthread_cleanup_pop()                                                                                         β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

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.

EXAMPLES
       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 non‐
       zero.

   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)

Linux                                                                          2021-03-22                                                        PTHREAD_CLEANUP_PUSH(3)