Why is the thread not waiting for the cancellation point in deferred mode?-CodePudding

Why is the thread in my program cancelling before reaching of the testcancel function? I exepected thread will be cancelled when testcancel called, but it cancelling immediately with a changing cancelstate to enable.

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

int i = 0;

void proc1()
{
    pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
    for (i = 0; i < 7; i  )
    {    
        if (i == 3) {
            pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, NULL);
            pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
        }
        if (i == 5) {
            pthread_testcancel();
        }
        printf("I'm still running! %d\n", i);
    sleep(1);
    }
}

int main(void)
{
    pthread_t thread;
    pthread_create(&thread, NULL, (void*)proc1, NULL);
    sleep(1);
    printf("Requested to cancel the thread\n");
    pthread_cancel(thread);
    pthread_join(thread, NULL);
    printf("The thread is stopped\n");
    return 0;
}

Result:

enter image description here I tried to run it without printf (due to printf is cancellation point too) but it didn't solve the problem.

CodePudding user response：

I exepected thread will be cancelled when testcancel called,

This expectation is not correct.

From the phread_cancel spec

Deferred cancelability means that cancellation will be delayed until the thread next calls a function that is a cancellation point.

There is also a link included to check what a cancellation point is:

The following functions are required to be cancellation points by POSIX.1-2001 and/or POSIX.1-2008:

 ...
 pthread_testcancel()
 ...
 sleep()
 ...

Each of them will make your thread respond to cancellation.

This means, also this assumption is not fully correct:

but it cancelling immediately with a changing cancelstate to enable.

Instead your thread is cancelled as soon as it calls sleep in the same iteration when it sets cancel state to enabled. (BTW: Cancel type is deferred by default)

You seem to expect that the thread only checks whether it is cancelled, when it actively queries for cancel state. I don't think this can be done using pthread_cancel. Instead you need to introduce some communication mechanism (maybe via sockets) to tell the thread that it shall terminate itself.