pthread_cleanup_pop

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

NAME
       pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cancel-
       lation 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-cancel-
       lation clean-up handlers.  A clean-up handler is a function that is au-
       tomatically 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 ar-
          gument, 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  '}',  re-
       spectively.   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 estab-
       lished 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)  (sig-
       longjmp(3)) from inside a clean-up handler produces  undefined  results
       unless  the jump buffer was also filled by setjmp(3) (sigsetjmp(3)) in-
       side 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  at-
       tributes(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()  func-
       tions  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.  Depend-
       ing  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 do-
       ing 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 can-
       cellation 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  (be-
       cause  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_setcancel-
       state(3), pthread_testcancel(3), pthreads(7)

COLOPHON
       This  page  is  part of release 5.05 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)