Check if your time_t implementation will work after 2038

test2038.c

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

int main () {
    time_t time_num[5] = {(time_t)0x00000000, (time_t)0x7fffffff, (time_t)0x80000000, (time_t)0xffffffff,};
    struct tm* parsed_time; unsigned int i;
    printf("sizeof(time_t) is %d.\n", sizeof(time_t));
    for (i = 0; i < 4; i++) {
        parsed_time = gmtime(&(time_num[i]));
        if (parsed_time == NULL) {
            if (sizeof(time_t) >= 8) printf("(time_t)0x%016llx is null.\n", time_num[i]);
            else printf("(time_t)0x%08x is null.\n", time_num[i]);
        } else {
            if (sizeof(time_t) >= 8)
                printf("(time_t)0x%016llx is %04lld/%02d/%02d %02d:%02d:%02d UTC.\n",
                    time_num[i],
                    (long long)parsed_time->tm_year+1900, parsed_time->tm_mon+1, parsed_time->tm_mday,
                    parsed_time->tm_hour     , parsed_time->tm_min  , parsed_time->tm_sec);
            else
                printf("(time_t)0x%08x is %04d/%02d/%02d %02d:%02d:%02d UTC.\n",
                    time_num[i],
                    parsed_time->tm_year+1900, parsed_time->tm_mon+1, parsed_time->tm_mday,
                    parsed_time->tm_hour     , parsed_time->tm_min  , parsed_time->tm_sec);
        }
    }
    if (sizeof(time_t) >= 8) {
        printf("This C implementation will work well after January 2038.\n");
        return 0;
    } else if (sizeof(time_t) == 4) {
        if ((time_t)0x80000000 < (time_t)0) {
            printf("time_t seems to be signed integer.\n");
            printf("This C implementation might not work after January 2038.\n");
            return 1;
        } else {
            printf("time_t seems to be unsigned integer. Huh?\n");
            printf("This C implementation might work until beginning of February 2106.\n");
            return 2;
        }
    } else {
        printf("Huh?\n");
        return 2;
    }
}

FreeBSD 11.1-RELEASE-p5 32bit, standalone:

• Clang 4.0.0: Fail