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[very noob question] how can i calculate the hours passed between 2 different times and between days in C (self.programming)

submitted 2 years ago by Vilian to c/[email protected]

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hello, i'm new to programming in i'm trying to solve this exercise in C, basically it's the amount of passed hours between the start of a game and it's end, if the game started at 16 and ended at 2 the result is a game with 10 hours(in different days) i know i can to it more manually, but i wanted to somehow use the <time.h> to learn how to use a header etc, can someone help me?, thank you all

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[–] tunetardis 1 points 2 years ago (1 children)

Okay, I've read up on this a bit an you raise some valid points.

For whatever reason, posix has decided that CLOCKS_PER_SEC must be exactly 1000000 on every system that wants to comply. That doesn't leave much room on a 32-bit system for maximum clock duration. It will wrap around well before the 10 hours the OP was giving by example.

The other is I gather on some systems, clock() may be measuring the uptime of a particular process, which is not necessarily the same as real world time in multicore environments. (Fwiw I'm on a Mac where I don't think it works that way? You would need some special apis to get that kind of info.)

I'm trying to think of the last time I programmed straight C and used time.h and the like. It was probably in developing scientific instrumentation? iirc the clock in that case was keyed to the VBL refresh of the display, meaning CLOCKS_PER_SEC would have been something like 60 Hz. And I doubt time() would have yielded any useful value. I wonder what the OP's use case is?

[–] [email protected] 2 points 2 years ago

Most of my hobby programming is in ANSI C and C99, so I'm unfortunately far too aware of the weird and counter-intuitive things the C and POSIX standards say. :P

clock() is fantastic for sub-second timings, such as deltatimes in games, or peripheral synchronization, which matches the use case you mention very well. I recommended time() over it as OP's use case is for calculating the amount of hours a user has had their software open, and unix timestamps are the perfect mechanism to do that in my opinion.