How do I use fork in C?

The only way to create a new process in UNIX is with the fork system call, which duplicates the current process. Example:

#include <stdio.h>
#include <unistd.h>
int main(void) {
  pid_t pid = fork();
  if (pid == 0) {
    printf("I'm the child process.\n");
  } else {
    printf("I'm the parent process; the child got pid %d.\n", pid);
  }
  return 0;
}

This prints:

% ./a.out
I'm the parent process; the child got pid 45055.
I'm the child process.

A call to fork() instructs the operating system to duplicate the calling process. The new process is identical, with one main difference: the new process gets a new process ID, and this ID is returned to the caller of fork(). The new process is returned the value 0, by which it knows that it is the child, because 0 is not a valid process ID.

Notice we get one line of output from each process. The order of the lines here is non-deterministic, since they come from different processes! We receive both lines because both processes’ stdout descriptor reference the same pipe. The fork() system call copies all of the parent process’s descriptors, including the standard pipes (stdin, stdout, and stderr).

The fork system call is often combined with execve as a way to start a new process from a program file. I’ll cover fork/execve in a future post.

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