Multiprocessing: First Program

• New system call: fork
  • Here is the simplest of programs that knows how to create other processes. It uses a system call named fork.
  • Code is in lecture examples folder: processes/basic-fork.c. Code is also right here.

  static const int kForkFailed = 1;
  int main(int argc, char *argv[]) {
    printf("Greetings from process %d! (parent %d)\n", getpid(), getppid());
    pid_t pid = fork();
    exitIf(pid == -1, kForkFailed, stderr, "fork function failed.\n");
    printf("Bye-bye from process %d! (parent %d)\n", getpid(), getppid());
    return 0;
  }

• fork is called once, but it returns twice.
  • fork knows how to clone the calling process, create a virtually identical copy of it, and schedule it as if the second copy of the original were running all along. All segments (data, bss, init, stack, heap, text) are faithfully replicated, and all open file descriptors from the first are duplicated and donated to the clone.
  • The only difference: fork's return value in the new process (the child) is 0, and fork's return value in the spawning process (the parent) is the child's process id. The return value can be used to dispatch each of the two processes in a different direction (although in this introductory example, we don't do that).
  • As a result, the output of the above program is really the output of two processes. We should expect to see a single greeting but two separate bye-byes.
  • Key observation: Each of the bye-byes is inserted into the console by two different processes. The order each line executes, in principle, cannot be predicted. The system's scheduler dictates whether the child or the parent gets to print its bye-bye first (although on multiple core machines, child and parent could be running in parallel).