How to run two child processes simultaneously in C?

Question

So I'm getting into Concurrent Programming, but for some reason I can't even get the basics to work. I have a file called fork.c, which contains a method main. In this method main I fork twice, into child processes 1 and 2.

In child 1, I print the character 'A' 50 times.

In child 2, I print the character 'B' 50 times.

When I run my code, I get the output AAAAA...AAAABBBBBB....BBBBBB. But never something like ABABABABABABAB.... In fact, sometimes I even get BBBBB....BBBBAAAA....AAAAA.

So why am I experiencing this behavior? Perhaps I'm going about it completely wrong.

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

void my_char(char n) {
    write(1, &n, 1);
}

int main() {
    int status;
    pid_t child1, child2;

    if (!(child1 = fork())) {
        // first childi
        int a;
        for (a = 0; a < 50; a++) {
            my_char('A'); 
        }
        exit(0);
    } else if (!(child2 = fork())) {
        // second child
        int a;
        for (a = 0; a < 50; a++) {
            my_char('B');
        }
        exit(0);
    } else {
        // parent
        wait(&child1);
        wait(&child2);
        my_char('\n');
    }

    return 0;
}

`fork` is probably not what most people think of when they say "concurrent programming"... — Kerrek SB, Sep 20 '11 at 01:47
`fork` creates a new, single-threaded process. "Concurrency" usually refers to multithreading inside one single process. — Kerrek SB, Sep 20 '11 at 01:52
Your processes do nothing but fight over one resource, the output device. So how can they run concurrently? — David Schwartz, Feb 29 '16 at 23:14

Mysticial · Accepted Answer · 2011-09-20T01:58:35.290

8

They are running concurrently, but the processes end almost immediately after being started. In other words, they're too short to actually get any real overlap.

EDIT:

The time needed to start another process is longer than the time it takes to run them. Therefore the chance of overlap is small. (there are also buffering issues which I'll omit)

You need each process to do more work than that. Try printing more than 50. Printing more than 10000 will probably be enough.

edited Sep 20 '11 at 01:58

answered Sep 20 '11 at 01:48

Mysticial

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Perfect. [proof](http://ideone.com/msGZE). Thanks so much. I was getting worried that I hadn't a clue how fork worked. – Jordan Scales Sep 20 '11 at 01:59
2

Or you can call sleep a bit inside the loop. Try [usleep](http://pubs.opengroup.org/onlinepubs/7908799/xsh/usleep.html) or [Sleep](http://msdn.microsoft.com/en-us/library/ms686298%28v=VS.85%29.aspx) according to your OS / Compiler. – Anthony Accioly Sep 20 '11 at 02:16

score 3 · Answer 2 · answered Jun 21 '12 at 09:28

I think this is much easier to figure how fork() works:

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

int main() {
    pid_t child1, child2;
    printf("Start\n");
    if (!(child1 = fork())) {
        // first childi
        printf("\tChild 1\n");
        sleep(5);
        exit(0);
    } else if (!(child2 = fork())) {
        // second child
        printf("\tChild 2\n");
        sleep(5);
        exit(0);
    } else {
        // parent
        printf("Parent\n");
        wait(&child1);
            printf("got exit status from child 1\n");
        wait(&child2);
            printf("got exit status from child 2\n");
    }

    return 0;
}

...and here is the output:

    Start
        Child 1
    Parent
        Child 2
    got exit status from child 1
    got exit status from child 1

How to run two child processes simultaneously in C?

2 Answers2

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