Using fork(), how can I make child process run always first?

Question

Child and parent process execution is parallel and which starts first depends on OS scheduling. But what can be done to start child always before the parent?

This is

Answer 1

I don't really know why people keep telling not to rely on this behaviour, it's actually used a lot in tracing programs (strace, ldtrace, ...).

First, fork your process and get the child pid, stop the child, and resume it in the parent:

pid_t pid = fork();
if (pid == -1)
    abort();
else if (pid == 0) {
    raise(SIGSTOP); // stop the child
} else {
    waitpid(pid, NULL, WUNTRACED); // wait until the child is stopped
    kill(pid, SIGCONT); // resume the child
}