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Hello,
I want to find the difference between two times in C. I am using difftime to calculate the difference but it appears that the results are being returned in seconds. I need to get the exact difference between the two times (nano, micro, milli, etc). Can anyone provide some suggestions on how this can be done?
Thanks
You're asking for too much precision. There's no function in the C library that does that.
And I'm not sure there's such a function that gives you all that precision either. There may be, but again, it may not. It depends on the platform and the hardware, most likely.
The time you get from time() only has second resolution in the first place. There's no way difftime() could hope to get anything better.
You need platform-specific high-resolution timers. In Windows, you get that from QueryPerformanceCounter(). Under POSIX, you need gettimeofday().
Hello, thanks for all your suggestions. I am using Linux. I will think about using POSIX.
Precise, short-term timing is not possible with a user-mode program on a PC because of the multitasking operating system... While your program is trying to time something, the operating system can interrupt your program to run another process/thread. This can happen even when you are only running one application because the O.S. is always running little background operations.
With Windows, I think you typically get errors in the ballpark of 30ms, assuming that another program (or driver) isn't hogging the CPU.
In order to take control away from the operating system, you need to write a kernel mode driver, or you can use a real time operating system (RTOS). There are RTOS versions of Linux, but AFAIK they only run on embedded systems, not on a PC.
Usually, things that require precise timing (like USB or serial ports, soundcards, video cards, etc.) use a combination of a driver, a separate hardware-clock, and a data-buffer.