Thread: Timing code in C?

How do I account for time in C? I mean if I have 20 lines of code and I want to know how long time it takes to execute them in nano secs, how do I get this time?

Thanks
/Micke

Thanks, sound good, hmm, but I'm kind of new to macros, how do I use that?

like:

Code:

#define rdtscll(val) \
      __asm__ __volatile__("rdtsc" : "=A" (val))

long long test;
long long diff;

test = rdtscll(val);

//some code to time
//some more code ...

diff = rdtscll(val) - test;

where diff is what I want?

Or how do I use it?

I know in solaris there is gethrtime(), is there something similar in linux?

Originally Posted by micke_b

Code:

#define rdtscll(val) \
      __asm__ __volatile__("rdtsc" : "=A" (val))

long long test, test2;
long long diff;

rdtscll(test);

//some code to time
//some more code ...

rdtscll(test2);
diff = test2 - test;

I believe it's like this. "val" is a parameter. Basically, it translates "val" in the macro with whatever you pass as argument.

Be aware that if you are in a multitask environnement, the operating system switch from process to process every x amout of time and thus, if your 20 lines of code takes more than a certain y amout of time (let's say microseconds, but i really don't know) because there's loop or fonction call in it (or beause your CPU has been interrupt while it was executing this part of the code), well, you might have some "incorrect" results.

It will still give you an idea. But if you really need nano-second precision, well, you are losing your time because you'll never really have it.

But i guess if you only want to know how much time it takes to execute some 20 simples C instructions (which i guess would be under 100 ns on a decent x86 processor), well, guess you'll be mostly fine.

Someone with more knowledge than me could talk more about this. Just wanted to raise the point.