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pthreadspthread-join

Predict output of concurrent code

This is a question related to coursework that I got wrong earlier, and I cannot figure out why.

The following code is given:

int i = 0;

void *doit(void *vargp) {
    i = i + 5;
}

int main() { 
    pthread_t tid; 
    ptr = &i;
    pthread_create(&tid, NULL, doit, NULL);
    i = i + 3; 
    pthread_join(tid, NULL); 
    printf("%d",i);
}

The possible outputs are 8, 3, and 5.

My understanding of the code is that the main thread calls pthread_join(), which waits on the doit thread before proceeding, so both operations (i += 5 and i += 3) must be executed before the print statement is reached. So, I don't understand how 3 and 5 are possible outputs.

Am I understanding pthread_join() incorrectly? Or might this be a mistake on the professor's part? I can definitely see how 8 is a possible output value, but I'm not so sure about 3 and 5.

Solution

  • 3 is possible if i = i + 3 starts to run first. i + 3 will evaluate to 3. This might be written back to i after all the i = i + 5 statements have been ran.

    8 and 5 are yielded similarly.