time_t now = time(NULL);
struct tm * now_tm = gmtime(&now);
if( NULL==now_tm )
{
    printf("gmtime fail, now is %d\n",(int)now);
}

I am trying to get some time processing functions to work, and I am having difficulty getting gmtime to break a time_t value into a tm structure. I have the following code (having previously called set_time()): <<code>> time_t now = time(NULL); struct tm * now_tm = gmtime(&now); if( NULL==now_tm ) { printf("gmtime fail, now is %d\n",(int)now); } <</code>> This returns the text "gmtime fail, now is 1417599213". That value looks correct, so I don't understand why gmtime() is returning NULL. Have I called it properly, or is there some other setup that needs performing?

Did you resolve it because I meet it??

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

void test_gmtime(void)
{
    struct tm tm;
    struct tm *pT = NULL;
    time_t utc;
    memset(&tm, 0, sizeof(tm));

    tm.tm_sec = 5;
    tm.tm_min = 10;
    tm.tm_hour = 9;
    tm.tm_mday = 8;          /* Day-of-month. */
    tm.tm_mon = 11 - 1;       /* month 0..11 */
    tm.tm_year = 2016 - 1900;   /* years from 1900 */

    utc = mktime(&tm);

    printf("ctime = %s", ctime(&utc));

    pT = gmtime(&utc);

    if(pT == NULL) {
        printf("NULL answer\n");
    } else {
        printf("gmtime = %d-%d-%d %d:%d:%d\n",
               pT->tm_year + 1900,
               pT->tm_mon + 1,
               pT->tm_mday,
               pT->tm_hour,
               pT->tm_min,
               pT->tm_sec);
    }
}

int main()
{
	test_gmtime();
	return 0;
}

I am also seeing that gmtime() is basically broken. Here is a simple test program that shows it: <<code title=test_gmtime.c>> #include <stdio.h> #include <string.h> #include <time.h> void test_gmtime(void) { struct tm tm; struct tm *pT = NULL; time_t utc; memset(&tm, 0, sizeof(tm)); tm.tm_sec = 5; tm.tm_min = 10; tm.tm_hour = 9; tm.tm_mday = 8; /* Day-of-month. */ tm.tm_mon = 11 - 1; /* month 0..11 */ tm.tm_year = 2016 - 1900; /* years from 1900 */ utc = mktime(&tm); printf("ctime = %s", ctime(&utc)); pT = gmtime(&utc); if(pT == NULL) { printf("NULL answer\n"); } else { printf("gmtime = %d-%d-%d %d:%d:%d\n", pT->tm_year + 1900, pT->tm_mon + 1, pT->tm_mday, pT->tm_hour, pT->tm_min, pT->tm_sec); } } int main() { test_gmtime(); return 0; } <</code>> If you compile and run this on a normal PC (e.g. Linux here, but same for windows) it works: gcc -Wall test_gmtime.c ./a.out ctime = Tue Nov 8 09:10:05 2016 gmtime = 2016-11-8 9:10:5 But if run in the mded compiler on a target device (in my case DISCO_L476VG) the ctime() call works fine, but the gmtime() call is broken: ctime = Tue Nov 8 09:10:05 2016 NULL answer Is anyone going to explain what is wrong with the function?

Online builds use the ARM compiler v5.06 which doesn't support gmtime(). Taken from http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0378f/chr1358938934656.html: "Local time zones and Daylight Saving Time are not implemented. The values returned indicate that the information is not available. For example, the gmtime() function always returns NULL."