clock() precision in time.h

Question

I am trying to calculate the number of ticks a function uses to run and to do so an using the clock() function like so:

unsigned long time = clo         


        

Answer 1

The most precise (but highly not portable) way to measure time is to count CPU ticks.

For instance on x86

 unsigned long long int asmx86Time ()
 {
   unsigned long long int realTimeClock = 0;
   asm volatile ( "rdtsc\n\t"         
                  "salq $32, %%rdx\n\t"    
                  "orq %%rdx, %%rax\n\t"   
                  "movq %%rax, %0"         
                  : "=r" ( realTimeClock ) 
                  : /* no inputs */
                  : "%rax", "%rdx" );
   return realTimeClock;
 }

 double cpuFreq ()
 {
   ifstream file ( "/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq" );
   string sFreq; if ( file ) file >> sFreq;
   stringstream ssFreq ( sFreq ); double freq = 0.;
   if ( ssFreq ) { ssFreq >> freq; freq *= 1000; } // kHz to Hz
   return freq;
 }

 // Timing

 unsigned long long int asmStart = asmx86Time ();
 doStuff ();
 unsigned long long int asmStop  = asmx86Time ();
 float asmDuration = ( asmStop - asmStart ) / cpuFreq ();

If you don't have an x86, you'll have to re-write the assembler code accordingly to your CPU. If you need maximum precision, that's unfortunatelly the only way to go... otherwise use clock_gettime().