C - measuring computing time

问题:

is there any simple way how to measure computing time in C? I tried time utility when executed, but I need to measure specific part of a program.

Thanks

回答1:

You can use the clock function in <time.h> along with the macro CLOCKS_PER_SEC:

clock_t start = clock() ;
do_some_work() ;
clock_t end = clock() ;
double elapsed_time = (end-start)/(double)CLOCKS_PER_SEC ;

Now elapsed_time holds the time it took to call do_some_work, in fractional seconds.

回答2:

You can try the profiler "gprof". More information here: http://www.cs.utah.edu/dept/old/texinfo/as/gprof.html

回答3:

You can generally use the clock() function to get the start and end times of a single call to your function being tested. If, however, do_some_work() is particularly fast, it needs to be put in a loop and have the cost of the loop itself factored out, something like:

#define COUNT 10000

// Get cost of naked loop.

clock_t start_base = clock();
for (int i = count; i > 0; i--)
    ;
clock_t end_base = clock();

// Get cost of loop plus work.

clock_t start = clock();
for (int i = count; i > 0; i--)
    do_some_work() ;
clock_t end = clock();

// Calculate cost of single call.

double elapsed_time = end - start - (end_base - start_base);
elapsed_time = elapsed_time / CLOCKS_PER_SEC / COUNT;

This has at least two advantages:

• you'll get an average time which is more representative of the actual time it should take; and
• you'll get a more accurate answer in the case where the clock() function has a limited resolution.

回答4:

@codebolt - Thank you! very nice. On Mac OS X, I added an include of time.h, and pasted in your four lines. Then I printed the values of start, stop (integers) and elapsed time. 1mS resolution.

output:
3 X: strcpy .name, .numDocks: start 0x5dc   end 0x5e1   elapsed: 0.000005 
calloc: start 0x622   end 0x630   elapsed: 0.000014 

in my foo.c program I have

#include <libc.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>

but it works without explicitly including time.h. One of the others must bring it in.

Actual code:

clock_t start = clock() ;

strcpy( yard2.name, temp );  /* temp is only persistant in main... */
strcpy( yard1.name, "Yard 1");
strcpy( yard3.name, "3 y 3 a 3 r 3 d 3");
yard1.numDocks = MAX_DOCKS; /* or so I guess.. */
yard2.numDocks = MAX_DOCKS; /* or so I guess.. */
yard3.numDocks = MAX_DOCKS; /* or so I guess.. */

clock_t end = clock() ;
double elapsed_time = (end-start)/(double)CLOCKS_PER_SEC ;
printf("3 X: strcpy .name, .numDocks: start 0x%x   end 0x%x   elapsed: %-12:8f \n", start, end, elapsed_time );


start = clock() ;
arrayD = calloc( yard2.numDocks, sizeof( struct dock ) );   /* get some memory, init it to 0 */
end = clock() ;

elapsed_time = (end-start)/(double)CLOCKS_PER_SEC ;
printf("calloc: start 0x%x   end 0x%x   elapsed: %-12:8f \n", start, end, elapsed_time );