Is there a way to dump a C struct?

Question

I've written a program to probe the limits of a system's C time.h functions and dump them out in JSON. Then other things which depend on those functions can know their limits.

# system time.h limits, as JSON
{
    "gmtime": { "max": 2147483647, "min": -2147483648 },
    "localtime": { "max": 2147483647, "min": -2147483648 },
    "mktime": {
        "max": { "tm_sec": 7, "tm_min": 14, "tm_hour": 19, "tm_mday": 18, "tm_mon": 0, "tm_year": 138, "tm_wday": 1, "tm_yday": 17, "tm_isdst": 0 },
        "min": { "tm_sec": 52, "tm_min": 45, "tm_hour": 12, "tm_mday": 13, "tm_mon": 11, "tm_year": 1, "tm_wday": 5, "tm_yday": 346, "tm_isdst": 0 }
    }
}

gmtime() and localtime() are simple enough, they just take numbers, but mktime() takes a tm struct. I wrote a custom function to turn a tm struct into a JSON hash.

/* Dump a tm struct as a json fragment */
char * tm_as_json(const struct tm* date) {
    char *date_json = malloc(sizeof(char) * 512);
#ifdef HAS_TM_TM_ZONE
    char zone_json[32];
#endif
#ifdef HAS_TM_TM_GMTOFF
    char gmtoff_json[32];
#endif

    sprintf(date_json,
            "\"tm_sec\": %d, \"tm_min\": %d, \"tm_hour\": %d, \"tm_mday\": %d, \"tm_mon\": %d, \"tm_year\": %d, \"tm_wday\": %d, \"tm_yday\": %d, \"tm_isdst\": %d",
            date->tm_sec, date->tm_min, date->tm_hour, date->tm_mday,
            date->tm_mon, date->tm_year, date->tm_wday, date->tm_yday, date->tm_isdst
    );

#ifdef HAS_TM_TM_ZONE
    sprintf(&zone_json, ", \"tm_zone\": %s", date->tm_zone);
    strcat(date_json, zone_json);
#endif
#ifdef HAS_TM_TM_GMTOFF
    sprintf(&gmtoff_json", \"tm_gmtoff\": %ld", date->tm_gmtoff);
    strcat(date_json, gmtoff_json);
#endif

    return date_json;
}

Is there a way to do this generically, for any given struct?

Note: C, not C++.

Answer 1

Not in C—at least in general. But if the C module is compiled with debug symbols, and the object module is available, you could parse that and discover everything about the structure. I bet there's a library for your system to assist with that.