PTHREAD_CLEANUP_PUSH

NAME

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

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 non-zero.

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 non-zero 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 aq{aq and aq}aq, 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 only established 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.  

RETURN VALUE

ERRORS

CONFORMING TO

NOTES

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

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 non-zero 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 non-zero.  

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

COLOPHON