Parent pid_t of a child process created after fork()

Question

I have the following function:

void test_fork()
{
    pid_t id;

    printf("Parent process id: %d\n", getpid());

    id = fork();

    if (id == 0)
        printf("\nChild:");
    else if (id > 0)
        printf("\nParent:");
    else
        exit(EXIT_FAILURE);

    printf("\nprocess id: %d\n", getpid());
    printf("parent process id: %d\n", getppid());
}

My doubt is related to its output, in particular sometimes I have for example the following and straightforward output:

Parent process id: 879

Parent:
process id: 879
parent process id: 878

Child:
process id: 881
parent process id: 879

Where the last id is equal to its parent id (as would be expected).

But sometimes the output is for example:

Parent process id: 858

Parent:
process id: 858
parent process id: 857

Child:
process id: 860
parent process id: 1

Where the last id is not equal to its parent process id, but it's equal to 1 (that should be the id of the init process).

How can it be possible?

Answer 1

It is because the parent process has already finished when the child asks for its parent's pid, so the child's parent defaults to the init process. init process has a pid of 1.

Try putting a wait() in the parent process !!

Answer 2

It's because the parent process exits before the child process could run, leaving the child process a child of the first process instead.