Simple Fork Example – Checking Return Value

#include <unistd.h>
#include <stdio.h>

int main() {
  pid_t ret;
  printf("welcome..pid=%d\n", getpid());
  ret = fork();
  if (ret < 0) {
    perror("fork");
    exit(1);
  }
  if (ret == 0) {
    printf("Hello, this is child process\n");
    printf("child--welcome,pid=%d,ppid=%d\n", getpid(), getppid());
    exit(0);
  } else // ret>0
  {
    printf("Hello, this is parent process\n");
    printf("parent--hello,pid=%d,ppid=%d\n", getpid(), getppid());
    sleep(1);
  }
  // printf("thank you,pid=%d,ppid=%d\n", getpid(),getppid());
  return 0;
}


Notes

• Creates a new process known as child process
• New pid, PCB/PD will be allocated to child (new entry in process table)
• Duplicates resources from parent to child
• fork returns zero to child, non zero to parent
• Child resumes from next statement after fork
• Parent & child run concurrently

 
Observations and Understanding
 

• Can you distinguish between parent and child in above code, and control over which process should execute which code?
• Mapping child’s ppid with parent’s pid
• What is parent’s ppid, to which process it belongs to?
• Check the pid, ppid values from the output of ps, pstree commands (Ensure program won’t terminate by using sleep/getchar)
• What is the meaning of +ve ret value returned to parent process?
• When do fork fails with negative return value
• What if any code written unconditionally outside if-else
• Create an array before fork, try modifying in one process (whichever executes first, may be parent) and check the values in other.
• What is your observation, whether array is shared between parent and child or two copies are maintained.