#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

int main() {
  // The program that will get run. It is similar to the command-line:
  // echo "Hello, world!"
  char *const cmdline[] = { "echo", "Hello, world!", NULL };

  // Create a new process that's a clone of this one. The original process is
  // known as the parent, and the new process is known as the child. The child
  // process activates in the `fork` call, meaning both the parent and the child
  // will return from fork and assign a value to the variable `pid`.
  pid_t pid = fork();

  // When the child process returns from `fork`, the value assigned to `pid`
  // will be 0. In the parent process, `pid` will be the process ID (a non-0
  // number) of the child process. It is important to note that the process ID
  // of the child isn't actually 0. The call to `fork` only returns 0 to
  // allow code to determine if it is the child. 
  if(pid == 0) {
    // Since this block only executes if `pid` is 0, the call to `execvp` will
    // only be run by the child process. This is done because the `exec` family
    // of functions replace the process' instructions with the instructions
    // found in `cmdline[0]` (in this case, `echo`). The `v` in `execvp` means
    // vector, which in this case refers to the array `cmdline`. The `p` means
    // path, indicating that it will search all of the directories found in the
    // environment vaiable `PATH` for the program (in this case, `execvp` is
    // searching the directories for `echo`).
    execvp(cmdline[0], cmdline);
  } else {
    // In the parent process, pause until the child process exits. The `wait`
    // function takes a pointer to an integer that contains information about
    // how the child process exited. If the exit information isn't necessary,
    // `NULL` can be passed instead. Several macros are available to obtain
    // further information about how the process exited.
    int status = 0;
    wait(&status);
    printf("Process %d exited with status %d.\n", pid, status);
  }

  // The parent process is now finished, and will return the number `0` to
  // indicate it was successful. The child process will never reach this point
  // in this program, since its instructions were replaced with `echo`s
  // instructions earlier.
  return 0;
}