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;
    }
}

Compilers' default (i.e. no options except source file name given) behavior tested on my PC...

Win7 32bit, standalone:

• Cygwin GCC 4.5: Fail
• MinGW GCC 4.5: Fail
• MS Visual C++ 2010 Express: Pass
• Digital Mars C 8.52: Fail
• Borland C++ Compiler 5.5: Fail (Include and Library paths must be specified)
• Tiny C Compiler 0.9.25: Fail
• Open Watcom 1.9: Unsigned
• SDCC 3.2.1: Won't compile

Xubuntu 12.04 LTS 32bit, on VMware Player:

• GCC 4.6.3: Fail
• Clang 3.0: Fail

Fedora 17 LXDE spin 32bit, on VMware Player:

• GCC 4.7.2: Fail
• Clang 3.0: Fail
• Portable C compiler 1.1.0: Fail
• NQC 3.1: Won't compile
• SDCC 3.0.0: Won't compile

Arch Linux 32bit, on VMware Player:

• GCC 4.7.1: Fail
• Clang 3.1: Fail

OpenSUSE 12.2 64bit, standalone:

• GCC 4.7.1: Pass
• Clang 3.1: Pass

FreeBSD 9.0-RELEASE 32bit, on VMware Player:

• GCC 4.2.1: Fail
• GCC 4.7.0: Fail
• Clang 3.0: Fail
• Tiny C Compiler 0.9.25: Fail
• UCC 1.6.0: Won't compile
• Bruce's C compiler 1995.03.12: Won't compile
• C Interpreter 5.16.19: Won't compile

Legend

• "Pass": time_t is 64-bit integer, and will work after mid-January 2038.
• "Fail": time_t is 32-bit signed integer, and won't work after 2038.
• "Unsigned": time_t seems to be 32-bit unsigned integer.
• "Won't compile": There is a compile-time issue, and cannot be tested.

FreeBSD 11.1-RELEASE-p5 32bit, standalone:

• Clang 4.0.0: Fail