Convert a timestamp as a long to struct tm

Question

I've a long as: 1459791437 and I want to transform it in this format: YYYYMMDD,HH. I don't know how to convert that long in a struct tm, I need that conversion to use the function: strftime.

Here you can see what I want to do:

 void find_date(long timestamp, int *date, int *hour){
    char buf[20];
    struct tm *time_struct = magic_function(timestamp);
    strftime(buf, sizeof(buf), "%Y%m%d,%H", time_struct);
    printf("Time: %s", buf);
}

I've tried this solution but it doesn't print anything:

void find_date(long timestamp, int *date, int *hour){
    char buf[20];
    struct tm *timeinfo;
    
    /* Conversion to time_t as localtime() expects a time_t* */
    time_t epoch_time_as_time_t = timestamp;
    /* Call to localtime() now operates on time_t */
    timeinfo = localtime (&epoch_time_as_time_t);
    
    strftime (buf, sizeof(buf), "%Y%m%d,%H", timeinfo);
    
    printf("Test: %s", buf);
}

What I cannot do is the struct tm *magic_function(long), could anyone help me?

Duplicate of http://stackoverflow.com/questions/14679022/converting-from-an-integer-epoch-time-value-to-local-time — Harry, Apr 04 '16 at 21:12
Possible duplicate of [How to convert a Unix timestamp to DateTime and vice versa?](http://stackoverflow.com/questions/249760/how-to-convert-a-unix-timestamp-to-datetime-and-vice-versa) — Iłya Bursov, Apr 04 '16 at 21:15
@Lashane C and C# are not the same language, I don't get how these 2 can be duplicate — Timmy, Apr 04 '16 at 21:22
@Timmy picked wrong question from the millions of the duplicates — Iłya Bursov, Apr 04 '16 at 21:22
@Harry I've tried that solution but gives me 2 warning and doesn't print anything — Timmy, Apr 04 '16 at 21:35
@Lashane I've looked for these duplicates, but I haven't found anyone working. — Timmy, Apr 04 '16 at 21:37
Your code fragments look fine. You may just be having the buffered output problem. Try `printf("Test: %s\n", buf)`, or `fflush(stdout)`. — Steve Summit, Apr 04 '16 at 21:56
@SteveSummit The program keep crashing and I don't get why. It doesn't print anything — Timmy, Apr 04 '16 at 22:06
Your code works for me, so your crash must be due to something you haven't shown us. How are you calling `find_date`? — Steve Summit, Apr 04 '16 at 22:18
Before attempting `strftime()`, insure the pointer `struct tm *timeinfo` is not `NULL`. — chux - Reinstate Monica, Apr 04 '16 at 22:26
@SteveSummit Thank you! I haven't included that library! Now everything works well. — Timmy, Apr 04 '16 at 22:39
Detail: `#include ` includes a header file for code. If does not cause a library to be included. — chux - Reinstate Monica, Apr 04 '16 at 22:50
Welcome to C, coding without a net and without training wheels. — chux - Reinstate Monica, Apr 04 '16 at 23:00
@Timmy Glad to hear that the `#include` line helped, but please also take chux's suggestion to check the return value from `localtime` to make sure it's not null. — Steve Summit, Apr 05 '16 at 02:35

Convert a timestamp as a long to struct tm

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