Styjun

I read an article form geeksforgeeks.The code shows a funciton to measure the time function cost.

In my machine ,I always got 0.000000 no matter how long I press enter.

I print t = clock() - t; t always equal to 0.00000,I rewrite the statement to this,still got 0.000000.

clock_t m;
m = clock() - t;

gcc version in centos7

[root@localhost log]# gcc --version
gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

/* Program to demonstrate time taken by function fun() */
#include <stdio.h> 
#include <time.h> 

// A function that terminates when enter key is pressed 
void fun() 
 
 printf("fun() starts n"); 
 printf("Press enter to stop fun n"); 
 while(1) 
 
 if (getchar()) 
 break; 
 
 printf("fun() ends n"); 
 

// The main program calls fun() and measures time taken by fun() 
int main() 
 
 // Calculate the time taken by fun() 
 clock_t t; 
 t = clock(); 
 fun(); 
 t = clock() - t; 
 double time_taken = ((double)t)/CLOCKS_PER_SEC; // in seconds 

 printf("fun() took %f seconds to execute n", time_taken); 
 return 0; 
 

Hello try this instead:

unsigned int t=time(0);
fun();
unsigned int result=time(0)-t; // result is the time taken by fun

In case you're trying to measure "CPU time used" (and not "wall clock time spent waiting for user to press a key"), then...

Imagine if the OS has a timer that generates an IRQ 10 times per second, and the IRQ handler just does ticks++; and clock() is just return ticks;. In this case, CLOCKS_PER_SEC would be 10.

Now, if you call clock() once just before the timer IRQ occurs and again immediately after the timer IRQ occurs, the difference between the values returned might be 1 (equal to 100 ms) even significantly less time passed between calls to clock().

Alternatively; if you call clock() once immediately after the timer IRQ occurs and again just before the timer IRQ occurs again, the difference between the values returned might be 0, even significantly more time passed between calls to clock().

Essentially, the difference between values returned by clock() is "up to 1/CLOCKS_PER_SEC sooner than you think" and "up to 1/CLOCKS_PER_SEC longer than you think".

Note that the value of CLOCKS_PER_SEC is implementation defined. Depending on various things (which version of which OS running on which hardware, with which C library) CLOCKS_PER_SEC can be anything, and could be as low as 10 and could be as high has 4 billion.

clock() measures the CPU time, that is the time that the processor has been active. You may want to measure wall clock time instead, which is the number of seconds from the start to the end of the function run:

struct timeval start;
gettimeofday(&start, 0); 
fun();
struct timeval end;
gettimeofday(&end, 0); 
double time_taken = (end.tv_sec - start.tv_sec)
 + (end.tv_usec - start.tv_usec)*1e-6;

To make this work, you must also include <sys/time.h>