reap.c

#include <stdio.h>
#include <thread.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>

int reaper_thread( void *arg ) {
    int pid, status;
    while ( 1 ) {
        pid = wait(&status);
        switch ( pid ) {
            case -1:
                fprintf(stderr, "error reaping child: %s\n", strerror(errno) );
                break;
            case 0:
                printf("nothing to reap\n");
                break;
            default:
                printf("reaped pid %i\n", pid);
                break;
        }
    }
}

int main( void ) {
    long int r;
    thrd_t t;

    /* set up signal stuff ... */

    thrd_create( &t, reaper_thread, (void*)r);

    /* spawn stuff */
}

if your toolchain doesn't support c11threads, change <thread.h> to "c11threads.h" and download that header file from here:

https://github.com/jtsiomb/c11threads/blob/master/c11threads.h

and link with -lpthread

this is licensed public domain, CC0, and MIT, use as you need