Thread-safe dubs checking

I'm writing a program for recognizing and validating a specific set of integer values called ``dubs''. If the message handler is ready for dubs, it sends a CHECK_EM message to the thread. When it receives a CHECK_EM, it replies with a NICE_DUBS message. How can I avoid a TOCTOU (Time-of-check Time-of-use) race condition by making sure that no other messages are sent to the thread between the time it enters the dubs ready state and the time the dubs are checked?

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

const time_t seconds = 1,
      minutes = 60 * seconds,
      hours = 60 * minutes,
      days = 24 * hours;

char *saydate(time_t when) {
    struct tm *td = localtime(&when);
    char *ret;
    asprintf(&ret, "%d/%d/%d",
        td->tm_mon + 1,
        td->tm_mday,
        td->tm_year + 1900);

    // Leaks memory if not freed
    return ret;
}

int main(int argc, char **argv) {
    time_t when = time(NULL),
           start = 1239610380,
           offset = 612;

    if (argc > 1) offset = atoi(argv[1]);

    printf("%s + %ld days = %s\n",
        saydate(start), offset,
        saydate(start + offset * days));

    return 0;
}