What happens after the parent of zombie process terminates?

Question

I'm just curious, what happens to zombie process, if it's parent doesn't care to wait for it.

Suppose, we've a parent and a child. Child terminates before parent does.

From APUE:

The kernel keeps a small amount of information for every terminating process...Minimally
this information consists of the process ID, the termination status of the process....

Parent is required to fetch this information using waitpid().
But if, parent exits without waiting for child, what happens:

Does the kernel delete this information (certainly it's of no use)?
Or, it keeps collecting this junk?
Is this implementation specific?
Or, is there a standard way to deal with this situation?

Answer 1

Orphan processes are automatically adopted by init which has a standard SIGCHLD handler that just discards any exit status of a dead process.

In your case if the parent of a zombie process dies the zombie orphan will be adopted by init and cleaned up.

The following code tests this:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>


int main() {
    pid_t child_pid;
    if (child_pid = fork()) { // fork a child, child will exit straight away
        char name[128];
        sprintf(name, "/proc/%d/stat", child_pid);
        char line[2048];

        // read childs /proc/pid/stat, field 3 will give its status
        FILE * fp = fopen(name, "r");

        while (fgets(line, sizeof(line), fp))
            puts(line);

        fclose(fp);

        usleep(5000000);

        // by now the child will have exited for sure, repeat
        fp = fopen(name, "r");

        while (fgets(line, sizeof(line), fp))
            puts(line);

        fclose(fp);

        // make another child to repeat the process and exit the parent
        if (!fork()) {
            usleep(5000000);
            // both parent and child will have exited by now

            fp = fopen(name, "r");

            // this should fail because init has already cleaned up the child
            if (!fp) {
                perror("fopen");
                return -1;
            }

            while (fgets(line, sizeof(line), fp))
                puts(line);

            fclose(fp);
        }

    }

    return 0;
}