How to measure execution time in C on Linux

Question

I am working on encryption of realtime data. I have developed encryption and decryption algorithm. Now i want to measure the execution time of the same on Linux platform in C. How can i correctly measure it ?. I have tried it as below

             gettimeofday(&tv1, NULL);
         /* Algorithm Implementation Code*/
             gettimeofday(&tv2, NULL);

        Total_Runtime=(tv2.tv_usec - tv1.tv_usec) +          
                      (tv2.tv_sec - tv1.tv_sec)*1000000);

which gives me time in microseconds. Is it correct way of time measurement or i should use some other function? Any hint will be appreciated.

Answer 1

Measuring the execution time of a proper encryption code is simple although a bit tedious. The runtime of a good encryption code is independent of the quality of the input--no matter what you throw at it, it always needs the same amount of operations per chunk of input. If it doesn't you have a problem called a timing-attack.

So the only thing you need to do is to unroll all loops, count the opcodes and multiply the individual opcodes with their amount of clock-ticks to get the exact runtime. There is one problem: some CPUs have a variable amount of clock-ticks for some of their operations and you might have to change those to operations that have a fixed amount of clock-ticks. A pain in the behind, admitted.

If the single thing you want is to know if the code runs fast enough to fit into a slot of your real-time OS you can simply take to maximum and fill cases below with NOOPs (Your RTOS might have a routine for that).

Answer 2

Read time(7). You probably want to use clock_gettime(2) with CLOCK_PROCESS_CPUTIME_ID or CLOCK_MONOTONIC. Or you could just use clock(3) (for the CPU time in microseconds, since CLOCK_PER_SEC is always a million).

If you want to benchmark an entire program (executable), use time(1) command.

Answer 3

clock(): The value returned is the CPU time used so far as a clock_t;

Logic

Get CPU time at program beginning and at end. Difference is what you want.

Code

#include <time.h>

/* ... */

clock_t begin = clock();

/****  code ****/

clock_t end = clock();
double time_spent = (double)(end - begin) //in microseconds

_{To get the number of seconds used,we divided the difference by CLOCKS_PER_SEC.}

More accurate

In C11 timespec_get() provides time measurement in the range of nanoseconds. But the accuracy is implementation defined and can vary.