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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 cancel-
       lation clean-up handlers

       #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-cancel-
       lation  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 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))
       inside the handler.

       These functions do not return a value.

       There are no errors.

       For  an	explanation  of	 the  terms  used   in	 this	section,   see

       |Interface		| Attribute	| Value	  |
       |pthread_cleanup_push(), | Thread safety | MT-Safe |
       |pthread_cleanup_pop()	|		|	  |

       POSIX.1-2001, POSIX.1-2008.

       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

       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.  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
       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 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
       (because cleanup_pop_arg was 0), and therefore the value of cnt was not

       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_setcancel-
       state(3), pthread_testcancel(3), pthreads(7)

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Linux				  2015-07-23	       PTHREAD_CLEANUP_PUSH(3)