Question

I have a Makefile that looks something like this:

sometarget: 
    command_one    # calls fork()
    command_two

Here’s the problem I’m running into when I do make sometarget:

1. command_one starts and eventually calls fork().

2. The child process execs something and finishes early, returning control to make before all the processing of command_one is done.

3. command_two then executes before the parent completes, causing the sequence to fail (as it depends on command_one finishing completely).

I can change command_one (the fork() and exec() have to stay, though), and I’d rather not change the Makefile if possible. Is there a way to prevent the child process from returning (on Linux)? I’m thinking the answer is no, but I’ve been wrong before…

Answer 1

It sounds like your command_one looks something like this pseudo-code:

main() {
    pid_t child = fork();    /* ignore error for sake of example */
    if (child) {
        /* some work in the parent */
        exit;
    } else {
        /* some work in the child */
    }
    exit;
}

If you insert a waitpid(2) or wait(2) (or any member of the family) immediately before the parent’s exit, it’ll make sure both child and parent are finished before make(1) moves onto the next command. It’ll look something more like this:

main() {
    pid_t child = fork();    /* ignore error for sake of example */
    if (child) {
        /* some work in the parent */
        exit;
    } else {
        /* some work in the child */
    }
    waitpid(child, &status, 0);  /* NEW LINE */
    exit(&status);
}