Timer to find elapsed time in a function call in C

Question

I want to calculate time elapsed during a function call in C, to the precision of 1 nanosecond.

Is there a timer function available in C to do it?

If yes ple

Answer 1

Any timer functionality is going to have to be platform-specific, especially with that precision requirement.

The standard solution in POSIX systems is gettimeofday(), but it has only microsecond precision.

If this is for performance benchmarking, the standard way is to make the code under test take enough time to make the precision requirement less severe. In other words, run your test code for a whole second (or more).

Answer 2

Use clock_gettime(3). For more info, type man 3 clock_gettime. That being said, nanosecond precision is rarely necessary.

Answer 3

Can you just run it 10^9 times and stopwatch it?

Answer 4

May I ask what kind of processor you're using? If you're using an x86 processor, you can look at the time stamp counter (tsc). This code snippet:

#define rdtsc(low,high) \
     __asm__ __volatile__("rdtsc" : "=a" (low), "=d" (high))

will put the number of cycles the CPU has run in low and high respectively (it expects 2 longs; you can store the result in a long long int) as follows:

inline void getcycles (long long int * cycles)
{
  unsigned long low;
  long high;
  rdtsc(low,high);
  *cycles = high; 
  *cycles <<= 32; 
  *cycles |= low; 
}

Note that this returns the number of cycles your CPU has performed. You'll need to get your CPU speed and then figure out how many cycles per ns in order to get the number of ns elapsed.

To do the above, I've parsed the "cpu MHz" string out of /proc/cpuinfo, and converted it to a decimal. After that, it's just a bit of math, and remember that 1MHz = 1,000,000 cycles per second, and that there are 1 billion ns / sec.

Answer 5

You can use standard system calls like gettimeofday, if you are certain that your process gets 100% if the CPU time. I can think of many situation in which, while you are executing foo () other threads and processes might steal CPU time.