Adding a few seconds of offset to stat()

Question

I am using stat to get the last modified time of a file, but I would like to add a few seconds to the overall time.

if (stat(file_path, &st) == 0)
{
    char estimate_time[50];
    strftime(estimate_time, 50, "\'%F %T\'", localtime(&st.st_mtime)
}

So, this code works fine in getting the last modified time of the file, but I would just like to add a few seconds to the overall time. Is there a way to get "st_mtime" in seconds so I can simply add to that value and then use strftime to convert into a formatted output?

Thanks

See http://stackoverflow.com/questions/471248/what-is-ultimately-a-time-t-typedef-to — Barmar, May 08 '17 at 20:14
Are you trying to report a different mtime (modification time) for a file, or do you want to set/change the modification time? — ChuckCottrill, May 08 '17 at 21:09

chux - Reinstate Monica · Answer 1 · 2017-05-08T22:25:16.137

1

To portable add a few seconds to a struct tm, simple add to the tm_sec member and call mktime() to normalize the structure.

  struct stat st;
  struct tm *tm = localtime(&st.st_mtime);
  if (tm == NULL) Handle_Error();
  tm->tm_sec += offset;
  time_t time_new = mktime(tm);  // adjust fields.
  if (time_new == (time_t) -1) Handle_Error();
  strftime(estimate_time, sizeof estimate_time, "\'%F %T\'", tm);

Per the [stat] tag, OP is on a POSIX system which requires time_t is in seconds, so POSIX code may add to st.st_mtime.

  time_t new_time = st.st_mtime + offset;
  struct tm *tm = localtime(&new_time);
  if (tm == NULL) Handle_Error();
  strftime(estimate_time, sizeof estimate_time, "\'%F %T\'", tm);

edited May 08 '17 at 22:25

answered May 08 '17 at 20:31

chux - Reinstate Monica

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POSIX requires that `time_t` be in seconds, so why wouldn't you be confident? – Barmar May 08 '17 at 21:34
`&st.st_mtime + offset` is incorrect. You are adding to its address, not its value. – user253751 May 08 '17 at 21:44
@Barmar OP's post is likely POSIX compliant, but that was not stated nor tagged as such. Had it been, I would have answered that way. – chux - Reinstate Monica May 08 '17 at 21:45
@chux He's using the `stat()` function -- that implies it's POSIX. – Barmar May 08 '17 at 21:47
@Barmar A reasonable conclusion, yet only an implication - even if a good one. Other platforms may use `stat()` in a non-POSIX fashion. – chux - Reinstate Monica May 08 '17 at 21:49
The tag `stat` says "The stat(2) system call on POSIX systems". So he DID indirectly tag it POSIX. – Barmar May 08 '17 at 21:53

Adding a few seconds of offset to stat()

1 Answers1