How to check "struct tm" variable is empty?

[krishnampkkm]

Hello,

I have a struct tm variable named "file_timestamp" and I used it in different functions.

Two issues I face here,

Issue 1) Sometimes the date is printed (converted to string using strftime(), then used printf()) as 1900-01-00 00:00:00.
I do memset at few places, memset (&file_timestamp, '\0', sizeof (struct tm));
Will this memset trigger setting time as epoch time ?

Issue 2)
Before assigning some date&time to this variable "file_timestamp", I want to ensure that the variable is empty.
I want to do assigning date & time to this stuct tm variable only very first time. How can we do this?

I just tried this as follows. Will this work always?

Issue 2 code is as follows,

struct tm file_timestamp;

if (!(file_timestamp.tm_mday >= 1 && file_timestamp.tm_mday <= 31))
{
  /* Write the timestamp_str to struct tm file_timestamp */
  strptime (timestamp_str, "%Y/%m/%d %H:%M:%S",
            &file_timestamp);
}

Thanking you

[john.c]

Zeroing the tm struct sets all the fields to 0 and yields the date that you see (the year value has 1900 added to it and the month value has 1 added to it for display purposes). However, that date is not the epoch time. To see the epoch time (which is Midnight, Jan 1, 1970) you can do the following:

#include <stdio.h>
#include <time.h>
 
int main() {
    time_t t = 0; // 0 time_t value represents the epoch
    printf("%s\n", asctime(gmtime(&t))); // Thu Jan  1 00:00:00 1970
    return 0;
}

To ensure that the tm struct is zeroed (if that's what you mean by "empty") you need to use memset like you did. I guess you could check that tm_mday is still 0 to check that it hasn't been set since you zeroed it:

if (t.tm_mday == 0)  // day has not been set
{
}

[Salem]

Why don't you just do

struct tm file_timestamp = { 0 };

But normally, I would expect the thing that writes to file_timestamp to return a success/fail status of some sort (which you should check).