Question

I’m sure this question is answered elsewhere, but I cannot find it on Google or SO, so here goes.

In C/C++, I want to convert a relative time in format dd-hh:mm:ss provided by

ps -o etime

to an absolute UTC formatted date.

This doesn’t seem like it should be very hard. Supposing I have already got a function to produce the relative time stored in struct tm format:

struct tm *starting_rel_time = my_reltime_converstion(...);
time_t t = time(0);
struct tm *current_abs_time =  localtime(&t);

what I want is basically the opposite of difftime:

struct *tm starting_abs_time = current_abs_time - starting_rel_time;

Now, I can write my own function to do the conversion, but it’s a nightmare because of all the carry operations and special conditions (leap years etc.). Surely there is a way to do this in the C/C++ libraries?

Answer 1

Convert the DD-HH:MM::SS to seconds with simple math; it’s relative-time, so just multiply and add. Then, query the current time() in seconds (assuming it’s “relative to now”), and add them. Then use gmtime to convert back to a struct tm.