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