pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cancelation clean-up handlers

#include <pthread.h>
void pthread_cleanup_push(void (*routine)(void *), void *arg);
void pthread_cleanup_pop(int execute);

These functions manipulate the calling thread's stack of thread-cancelation 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 cancelation clean-up handler is popped from the stack and executed in the following circumstances:

• 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.

• 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.)

• 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) (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.

These functions do not return a value.

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 cancelation 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 cancelation 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 cancelation 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 <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.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 curr;
    printf("New thread started\n");
    pthread_cleanup_push(cleanup_handler, NULL);
    curr = time(NULL);
    while (!done) {
        pthread_testcancel();           /* A cancelation point */
        if (curr < time(NULL)) {
            curr = time(NULL);
            printf("cnt = %d\n", cnt);  /* A cancelation 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);
}

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