CODEWITHSUNDEEP
csignalsforkkill
Aage Torleif
Aage Torleif

Reputation: 2013

Unexpected termination in simple C child/parent program

So i've been reading posts on StackOverflow and the fork man-page and I just don't get the behavior I see, probably because I'm only looking for what I expect.

Its a simple program that takes a file name, spawns a child, runs stat() on the file, then returns SIGUSR1 or 2 back to the parent in a loop. The parent just wants to know if its 1 or 2... What I get, is "User defined signal 2" and then the program exits. But the parent is in a while loop right?

Anyway I was really hoping someone could explain why I'm not getting the expected output, specifically, the loop to continually ask for a file-name and spawn a child each time and the parent to know what signal the child returns with kill().

pid_t pid;

while(1) {
    if(pid == 0) // i am the child
    {
        FILE *fp = fopen(fname, "r");
        if(fp == NULL){
            printf("%d] Child cannot find [%s].", msg++, fname);
            fclose(fp);
        }
        else {
            stat(fname, &st);
            n = st.st_size;
            printf("%d] Child read %d chars.\n", msg++, n);
            if(n%2) kill(pid, SIGUSR1);
                else kill(pid, SIGUSR2);
        }

    }
    else // im the parent
    {
        if( signal(SIGUSR1,NULL) ) // Code never gets here because it ends
            printf("%s is odd\n", fname );

        if( signal(SIGUSR2, NULL) )
            printf("$s is even\n", fname );

        printf("%d] Enter filenames until you're happy. 'die' to end.\n", msg++);
        scanf("%s", fname);
        pid = fork();
    }

}
return 0;

Upvotes: 3

Views: 403

Answers (1)

Sakthi Kumar
Sakthi Kumar

Reputation: 3045

In the child the pid is zero, fork returns child pid to parent, but child gets zero, and u r trying to kill a process with pid 0, but pid of init is 1, u can use getpid() to get the current pid of the child. and then try to signal it.

Edit: And also, pid is un initialized when the loop enters first time, which may also result in undefined behavior. Thanks @Giresh

Upvotes: 4

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