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

       pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cancellation clean-up han‐

       #include <pthread.h>

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

       Compile and link with -pthread.

       These functions manipulate the calling thread's stack of thread-cancellation clean-up han‐
       dlers.   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

       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.

       These functions do not return a value.

       There are no errors.


       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 vari‐
       ables 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.

       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

           $ ./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  ter‐
       minate 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);

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

       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2008-11-24                    PTHREAD_CLEANUP_PUSH(3)

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