What happens when you perform waitpid(-1, ...)?

ChildAutoReaper.c

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <stdbool.h>
#include <memory.h>

void signal_handler(int sig)
{
    if (sig != SIGCHLD)
    {
        return;
    }

    // Auto-reap all children.
    int wstatus;
    pid_t pid;
    do
    {
        while ((pid = waitpid(-1, &wstatus, WNOHANG)) == -1 && errno == EINTR) {}
    } while (pid > 0);
}

int main()
{
    struct sigaction act = {0};
    act.sa_handler = signal_handler;
    if (sigaction(SIGCHLD, &act, NULL))
    {
        perror("sigaction");
        return 1;
    }

    // This loop will intermittently print "waitpid: No child processes".
    while (true)
    {
        // Suppose below is a code fragment deep inside a library function.
        pid_t pid = fork();
        if (pid == -1)
        {
            perror("fork");
            return 1;
        }
        else if (pid == 0)
        {
            // child
            _exit(1);
        }
        else
        {
            // parent
            int wstatus;
            pid_t ret;
            while ((ret = waitpid(pid, &wstatus, 0)) == -1 && errno == EINTR) {}

            if (ret == -1)
            {
                // Failed to obtain the exit status. Hey, it's _MY_ child! BOOOO!
                perror("waitpid");
            }
            else
            {
                // Obtained the exit status. OK!
                // printf("Child (PID %d) exited: status = %08x\n", wstatus);
            }
        }
    }

    return 0;
}