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)  (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.

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).

       +------------------------+---------------+---------+
       |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() 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.

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
");
	   cnt = 0;
       }

       static void *
       thread_start(void *arg)
       {
	   time_t start, curr;

	   printf("New thread started
");

	   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
", 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
");
	       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
", cnt);
	   else
	       printf("Thread terminated normally; cnt = %d
", cnt);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

       This  page is part of release 4.15 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								    2017-09-15						   PTHREAD_CLEANUP_PUSH(3)