How to exit child process correctly?

Question

With this code example I expect that my child processes will exit normally:

int main(void)
{
    for (int i = 0; i < 100; ++i)
    {
        int pid = fork();
        printf("pid %d\n", pid);
        sleep(1);
        if (pid == 0)
            exit(0);
    }
}

But when I run ps, I get the following output:

5089 pts/0    00:00:00 test
5090 pts/0    00:00:00 test <defunct>
5091 pts/0    00:00:00 test <defunct>
5092 pts/0    00:00:00 test <defunct>
5094 pts/0    00:00:00 test <defunct>

What should I do to make child process exit normally?

Answer 1

The parent process must call one of the wait functions (wait, waitpid, waitid, wait4), to release child process resources. See then manpage of waitpid:

WAIT(2)

NAME
       wait, waitpid, waitid - wait for process to change state

SYNOPSIS
       #include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait(int *wstatus);

       pid_t waitpid(pid_t pid, int *wstatus, int options);

       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
                       /* This is the glibc and POSIX interface; see
                          NOTES for information on the raw system call. */

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       waitid():
           Since glibc 2.26: _XOPEN_SOURCE >= 500 ||
               _POSIX_C_SOURCE >= 200809L
           Glibc 2.25 and earlier:
               _XOPEN_SOURCE
                   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
                   || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION

…

       In the case of a terminated child, performing a wait
       allows the system to release the resources associated with the child;
       if a wait is not performed, then the terminated child remains in a
       "zombie" state (see NOTES below).