Question

I am studying thread in APUE book 2e

I thought pthread_cleanup_pop functions is for setting the pushed function by pthread_cleanup_push() to be executed or not. So if the argument is zero, it’s not executed and non-zero, executed.

But I looked at the code Figure 11.5 in APUE. That is…

#include "apue.h"
#include <pthread.h>

void
cleanup(void *arg)
{
    printf("cleanup: %s\n", (char *)arg);
}

void *
thr_fn1(void *arg)
{
    printf("thread 1 start\n");
    pthread_cleanup_push(cleanup, "thread 1 first handler");
    pthread_cleanup_push(cleanup, "thread 1 second handler");
    printf("thread 1 push complete\n");
    if (arg)
        return((void *)1);
    pthread_cleanup_pop(0);
    pthread_cleanup_pop(0);
    return((void *)1);
}

void *
thr_fn2(void *arg)
{
    printf("thread 2 start\n");
    pthread_cleanup_push(cleanup, "thread 2 first handler");
    pthread_cleanup_push(cleanup, "thread 2 second handler");
    printf("thread 2 push complete\n");
    if (arg)
        pthread_exit((void *)2);
    pthread_cleanup_pop(0);
    pthread_cleanup_pop(0);
    pthread_exit((void *)2);
}

int
main(void)
{
    int         err;
    pthread_t   tid1, tid2;
    void        *tret;

    err = pthread_create(&tid1, NULL, thr_fn1, (void *)1);
    if (err != 0)
        err_quit("can't create thread 1: %s\n", strerror(err));
    err = pthread_create(&tid2, NULL, thr_fn2, (void *)1);
    if (err != 0)
        err_quit("can't create thread 2: %s\n", strerror(err));
    err = pthread_join(tid1, &tret);
    if (err != 0)
        err_quit("can't join with thread 1: %s\n", strerror(err));
    printf("thread 1 exit code %d\n", (int)tret);
    err = pthread_join(tid2, &tret);
    if (err != 0)
        err_quit("can't join with thread 2: %s\n", strerror(err));
    printf("thread 2 exit code %d\n", (int)tret);
    exit(0);
}

in that program, although pop functions has zero as an argument, the pushed functions are excuted( “thread 2 first handler”, “thread 2 second handler”, are printed by cleanup function)

why pop works even though put 0 for argument?

did I get pthread_cleanup_pop wrong??

Answer 1

did I get pthread_cleanup_pop wrong?

No, you didn’t.

The reason your code executed cleanup handlers, is that your control never reaches pthread_cleanup_pop(0). Instead, you always execute return in thr_fn1, and pthread_exit in thr_fn2.

Try creating the threads with pthread_create(&tid1, NULL, thr_fn1, (void *)0); instead of 1. When I do that, I get (expected):

thread 1 start
thread 1 push complete
thread 2 start
thread 2 push complete
thread 1 exit code 1
thread 2 exit code 2