Process Graph for the program clearly showing the parent and child processes execution from creation to termination

Question

pid_t pid;
printf("Begin");
if (fork() == 0) {
    printf("First if fork==0");
    if (fork() != 0) 
        printf("First if fork!=0");
}

if (fork() != 0) {   
    printf("second if fork!=0");
    if (fork() == 0) 
        printf("second if fork==0");
}

printf("End if\n");

I try to understand how fork works. Can you draw a process graph to be understandable for me and for others?

Answer 1

Your example has four forks. Typically this is done differently

pid_t child_pid = fork();
// both parent and child process return from `fork()` and continue here
if (child_pid == -1) {
    // still parent process, error handling
    // no child process was created
} else if (child_pid == 0) {
    // this is the child process
} else {
    // this is the parent process, child_pid > 1
}

Now you can clearly see, which code is run in the parent, and which one is run in the child process.

---

Back to your example, you have a main process doing the forks (ignoring error handling)

if (fork() == 0) {
    printf("First child process\n");
    if (fork() != 0)
        printf("First child process (parent part)\n");
    /*
    else
        printf("Grandchild\n");

    // fall through for both first child and grandchild
    */
}

// Main process, and first child, and grandchild
if (fork() != 0) {
    // also first child and grandchild ("parent" part)
    printf("Main process (parent part)\n");
    if (fork() == 0)
        // also third grandchild, and second great-grandchild
        printf("Third child process\n");
}
/*
else
    // also second grandchild, and first great-grandchild
    printf("Main process, second child\n");
*/

This means, you have a main process, having three children. The first of the child processes does a fork too, creating a grandchild.

---

But, both the first child and the first grandchild continue with the same code to the second outer if, and so create additional children, grandchildren, and great-grandchildren.

So, in the end the main process has three children. From the first child, three grandchildren descend, of which the first grandchild has two great-grandchildren of its own.

Unless I forgot one or two or so, of course :-)