Fork to Execute Processes Using Wait in C++

The fork() system call is used to create a process commonly known as a child process if the fork() returns 0. Otherwise, the created process is known as the parent process.

All processes created with fork() execute in parallel. But what if we want the last process to be executed first? In this case, the parent process would execute last because of bottom-to-top execution. This can be done using the wait() system call.

The wait system call is used to process handling. It pauses the execution of the calling process until the child process has finished its execution. It's commonly used in a scenario where the parent process creates a child process using fork().

Example of Bottom-up Execution of Processes using fork() and wait()

Following is the program to demonstrate bottom-up execution of processes using fork() and wait() ?

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

using namespace std;
int main() {
   //Creates 4 process using two consecutive fork. The main process, two children and one grand child
   pid_t id1 = fork();
   pid_t id2 = fork();

   if (id1 > 0 && id2 > 0) {
      //when both ids are non zero, then it is parent process
      wait(NULL);
      wait(NULL);

      cout << "Ending of parent process" << endl;
   } else if (id1 == 0 && id2 > 0) {
      //When first id is 0, then it is first child
      sleep(2); //wait 2 seconds to execute second child first
      wait(NULL);

      cout << "Ending of First Child" << endl;
   } else if (id1 > 0 && id2 == 0) {
      //When second id is 0, then it is second child
      sleep(1); //wait 1 seconds

      cout << "Ending of Second child process" << endl;
   } else {
      cout << "Ending of grand child" << endl;
   }
   return 0;
}

Following is the output of the above code ?

Ending of grand child
Ending of Second child process
Ending of First Child
Ending of parent process

Example for Creating Parent, Child, and Grandchild Processes

The following is another example that uses fork() to create multiple processes, following a parent-child-grandchild hierarchy. The grandchild completes first, followed by the child, and finally the parent process ?

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

using namespace std;
int main() {
   // First fork - creates child
   pid_t id1 = fork();

   if (id1 < 0) {
      cerr << "Fork failed!" << endl;
      return 1;
   }

   if (id1 == 0) {
      // Second fork - creates grandchild
      pid_t id2 = fork();

      if (id2 < 0) {
         cerr << "Fork failed!" << endl;
         return 1;
      }

      if (id2 == 0) {
         // Grandchild process
         cout << "Grandchild is calculating sum..." << endl;
         int sum = 0;
         for (int i = 1; i <= 10; i++) {
            sum += i;
         }
         cout << "Grandchild finished: Sum = " << sum << endl;
      } else {
         // First child waits for grandchild
         wait(NULL);
         cout << "Child is printing a message..." << endl;
         cout << "Hello from the Child Process!" << endl;
      }
   } else {
      // Parent process
      wait(NULL);
      cout << "Parent is finalizing..." << endl;
      cout << "Parent Process Terminated!" << endl;
   }
   return 0;
}

Following is the output of the above code ?

Grandchild is calculating sum...
Grandchild finished: Sum = 55
Child is printing a message...
Hello from the Child Process!
Parent is finalizing...
Parent Process Terminated!