CODEWITHSUNDEEP
c
S.Tian
S.Tian

Reputation: 29

c: how does function waitpid work?

In studying CSAPP, I encounter the practice: list all of the possible output sequences for the following program:

int main()
{
        if(Fork()==0)   {
                printf("a");
        }
        else {
                printf("b");
                waitpid(-1,NULL,0);
        }
        printf("c");
        exit(0);
}

the answer is :acbc abcc bcac bacc;

Why is bcac correct? The function waitpid() suspends execution of the calling process until the child process in its wait set terminates. So the parent can't not print c, until the child process terminate, which means the child prints both a and c.
I'm really confused about it. I don't know why bcac is correct. The parent process should hold or suspend until child terminates.

Upvotes: 2

Views: 731

Answers (1)

jdarthenay
jdarthenay

Reputation: 3157

As Joachim Pileborg suggests, this is a flushing output problem. Just use the following:

int main()
{
    if(Fork()==0)
    {
        printf("a\n");
    }
    else
    {
        printf("b\n");
        waitpid(-1,NULL,0);
    }
    printf("c\n");
    exit(0);
}

'\n' characters should automatically flush stdout or stderr. You can also use fflush(stdout);

Upvotes: 1

Related Questions