Question

The following program calls the fork() system call to create a child process.

Suppose that the actual pids of the parent process and child process are as follows:

Parent process: 801

Child process:   802 (as returned by fork)

Assume all supporting libraries have been included.

=> Use the answer text field to write the values of the pids printed at lines 1, 2, 3, and 4 (indicate each line number and the value printed).

int main()

{

     pid_t pid, pid1;

     pid = fork(); /* fork a child process */

     if (pid < 0) {

          fprintf(stderr, "Error creating child.\n");

          exit(-1);

     }

     else if (pid > 0) {

          wait(NULL); /* move off the ready queue */

          pid1 = getpid();

          printf(pid); /* Line 1 */

          printf(pid1); /* Line 2 */

     }

     else {

          pid1 = getpid();

          printf(pid); /* Line 3 */

          printf(pid1); /* Line 4 */

     }

     return 0;

}

Answer 1

Output:

Line 1: 802

Line 2: 801

Line 3: 0

Line 4: 802

Explanation:

Parent Process Id = 801 (Given)

Child Process Id = 802 (Given)

Flow of the program and initialisation of all pid variables:

• The "else if block" (for pid > 0) executes for the parent process, it stays on hold until child process terminates because of invocation of wait system call.
• The control moves to the child process i.e. the "else block" (i.e. pid == 0) in it and calls getpid() which makes pid1 = Child Process id = 802.
• After that, the wait ends and the parent process resumes i.e. its "else if block" again. It has its variable pid = 802 (the process id of the child process returned from fork()) and pid1 = 801 (i.e. process id of the parent process' itself got from calling getpid() function).

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