CS13002 PDS Lab, Spring 2003, Section 1/A

How to measure timings in a C program

• Include the header file <time.h>.
• Time variables have data type clock_t.
• If you want to measure the timimg of a block, note the time t₁ just before the block begins and the time t₂ just after the block exits. Use the C library call clock() to know a particular time.
• Divide (t₂ - t₁) by CLOCKS_PER_SEC to get the number of seconds the block took.
• If the block returns too fast so that you get a zero time, repeat the block some number of times (say, 100 or 1000 or ...) and record the timing for these many executions of the block.

The prototype example

#include <stdio.h> /* Include standard i/o header file */
#include <time.h>  /* Include the timing header file */

...

clock_t t1, t2;    /* Time variables */
int i,             /* Temporary index */
   repCount;       /* Repetition count */
float timeTaken;   /* Total time taken */

...

repCount = 1000;   /* Set repetition count */

t1 = clock();      /* Record timing before the block begins */

/* Execute the block repCount times */
for (i=0; i<repCount; i++) {
   /* Execute your block here. */ 
}

t2 = clock();     /* Record the time after the block exits */

/* Compute the time taken per execution of the block */
timeTaken = ( (float)(t2 - t1) / (float)CLOCKS_PER_SEC ) / (float)repCount;

printf("Time taken = %f seconds\n", timeTaken);

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