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 please provide a sample code-snippet.

Pseudo code

Timer.Start()
foo();
Timer.Stop()
Display time elapsed in execution of foo()

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Environment details: - using gcc 3.4 compiler on a RHEL machine

Answer 1

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 2

On Intel and compatible processors you can use rdtsc instruction which can be wrapped into an asm() block of C code easily. It returns the value of a built-in processor cycle counter that increments on each cycle. You gain high resolution and such timing is extremely fast.

To find how fast this increments you'll need to calibrate - call this instruction twice over a fixed time period like five seconds. If you do this on a processor that shifts frequency to lower power consumption you may have problems calibrating.