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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
     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.

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).

     allbox;   lbw23   lb   lb   l   l   l.    Interface Attribute Value  T{
     pthread_cleanup_push(), pthread_cleanup_pop()  T}   Thread  safety  MT-
     Safe

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
     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
     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); }