Understanding how fork() system call works

Question

I have this C code sequence:

    printf("\nThe PID of this (main) process is: %d\n", getpid());

    if(fork() != -1) { // #1
        printf("\n\nParent 1 PID: %d\n", getpid());
        for(int i = 0; i < 10; i++) {
            printf("\n%d", i);
        }

        if(fork() != -1) { // #2
            //sleep(1);
            printf("\n\nParent 2 PID: %d\n", getpid());
            for(char i = 'a'; i != 'f'; i++) {
                printf("\n%c", i);
            }
        }
        else { // #3
            sleep(3);
            printf("\n\nChild 2 PID: %d\n", getpid());
            for(char i = 'F'; i != 'J'; i++) {
                printf("\n%c", i);
            }
        }
    }
    else { // #4
        sleep(4);
        printf("\n\nChild 1 PID: %d\n", getpid());
        for(int i = 10; i < 20; i++) {
            printf("\n%d", i);
        }
    }

I expect that I will have 4 processes: two parents and two childs. At line #1 I call fork() for first time, and everything from line #1 to line #4 will be executed in first parent process. In the parent process (1) I call fork() one more time, so from line #2 to line #3 I will have the parent 2 process, and from line #3 to #4 child 2 process.

What I expect to be printed:

Parent 1 PID: ....
0
1
2
3
4
5
6
7
8
9
Parent 2 PID: ....
a
b
c
d
e
Child 2 PID: ....
F
G
H
I
Child 1 PID: ....
10
11
12
13
14
15
16
17
18
19

What I actually got:

Parent 1 PID: 3877

0
1
2
3
4
5
6
7
8


Parent 1 PID: 3878

0
1
2
3
4
5
6
7
8
9

Parent 2 PID: 3877

a
b
c
d
e9

Parent 2 PID: 3878
9


a
b
c
d
Parent 2 PID: 3879

a
b
c
d
e9

eParent 2 PID: 3880

a
b
c
d
e

What I do wrong?

Answer 1

This line isn't doing what you think:

if(fork() != -1) { // #1

That will succeed for both the parent and the child (as long as fork is possible, which is almost always the case). You mean to test against 0 here. The parent will get 0, the child will get >0. -1 is an error.

In your case, what you've marked as the "child" legs should never be executed unless there are errors. I don't think that's what you meant. What you're seeing are the initial 2 (parent and child) forks plus 4 (parent+child * 2) second forks. That's 6 forks, which is what the output indicates.

Answer 2

from man fork :

RETURN VALUE
       On success, the PID of the child process is returned in the parent, and 0 is returned in the child.  On failure, -1 is returned in the parent, no child process is created, and errno
       is set appropriately.

this means, that you should expect 0 in the child process and the childs pid in the parent process, so your code should look something like this:

switch(pid = fork()) {
  case -1:  //error handling here
  case 0:   // child process code here
  default:  // parent process code here.
}

merry christmas :)

Answer 3

For understanding detail functioning of how Fork System call works ? you can go to this link : http://linuxtrainers.wordpress.com/2014/12/31/how-fork-system-call-works-what-is-shared-between-parent-and-child-process/