Question

I have a source file. When I compile the code, I want the executable file to remember when it was built. I am wondering if it is possible. For example:

 int main(){
    time_t t = ???  // Time when this line is compiled
    //print out value of t in certain format. 
    return t 
 } 

Answer 1

You can use the __TIME__ and __DATE__ macros to get the time the preprocessor ran at. It’s a string, so yo need to convert it to a time_t from there.

A quick example I put together:

#include <time.h>
#include <iostream>
#include <cassert>

time_t build_time() {
  static const char *built = __DATE__" "__TIME__;  
  struct tm t;
  const char *ret = strptime(built, "%b %d %Y %H:%M:%S", &t);
  assert(ret);
  return mktime(&t);
}

int main() {
  std::cout << build_time() << std::endl;
}

I was a little worried about how this interacted with different locales, so I had a quick look in a recent C standard and found the following passage:

__DATE__ The date of translation of the preprocessing translation unit: a character string literal of the form “Mmm dd yyyy”, where the
names of the months are the same as those generated by the asctime
function, and the first character of dd is a space character if the
value is less than 10. If the date of translation is not available, an
implementation-defined valid date shall be supplied.

asctime is quite clear that:

… The abbreviations for the months are
“Jan”, “Feb”, “Mar”, “Apr”, “May”, “Jun”, “Jul”, “Aug”, “Sep”, “Oct”,
“Nov”, and “Dec” …

But %b of strptime() says:

%b or %B or %h

The month name according to the current locale, in abbreviated
form or the full name.

So you need to be aware that this is making an assumption about what the locale will be set to at run time.

(You could in theory write a constexpr function or two to do that at compile time in C++11, but that’s non-trivial to say the least!)