How to time a C program

Question

I've read this post here and I followed the instructions, applying them to a simple program that sums all the numbers below 1000 divisible by 3 and 5.

#include <stdio.h> #include <time.h> clock_t begin, end; double time_spent; begin = clock(); int sumDivisibleBy (div, limit) { int h = (limit - 1)/div; return div*h*(h+1)/2; } int main(void) { int l = 1000; int s = sumDivisibleBy(3,l) + sumDivisibleBy(5,l) - sumDivisibleBy(15,l); printf("%d \n", s); } end = clock(); time_spent = (double)(end - begin) / CLOCKS_PER_SEC printf("%f \n", time_spent)

Now, when I type in the terminal "make 1" (the file is called 1.c), this is what I get:

cc 1.c -o 1 1.c:9:1: warning: data definition has no type or storage class [enabled by default] begin = clock(); ^ 1.c:9:1: error: conflicting types for ‘begin’ 1.c:6:9: note: previous declaration of ‘begin’ was here clock_t begin, end; ^ 1.c:9:1: error: initializer element is not constant begin = clock(); ^ 1.c:20:1: warning: data definition has no type or storage class [enabled by default] end = clock(); ^ 1.c:20:1: error: conflicting types for ‘end’ 1.c:6:16: note: previous declaration of ‘end’ was here clock_t begin, end; ^ 1.c:20:1: error: initializer element is not constant end = clock(); ^ 1.c:21:1: warning: data definition has no type or storage class [enabled by default] time_spent = (double)(end - begin) / CLOCKS_PER_SEC ^ 1.c:21:1: error: conflicting types for ‘time_spent’ 1.c:7:8: note: previous declaration of ‘time_spent’ was here double time_spent; ^ 1.c:21:1: error: initializer element is not constant time_spent = (double)(end - begin) / CLOCKS_PER_SEC ^ 1.c:21:1: error: expected ‘,’ or ‘;’ at end of input make: *** [1] Error 1

Why is it? Can somebody help please?

Jens · Accepted Answer · 2015-12-06 17:42:13Z

You can't use function calls to initialize file scope variables (those outside main).

You must move begin and end into main() (well, at least their initialization).

A C program doesn't execute top to bottom; it starts with main(). The values of initialized file scope variables must be known at compile-time, which is the reason you can't use function calls.

To get meaningful results, I also suggest you run the code to be tested many times in a loop, since the resolution of the clock is often too coarse to time a few instructions, i.e. do something like

 begin = ... for (j = 0; j < 10000; ++j) { code_to_test(); } end = ...

@Pigna That's good news. Now the next step on Stack Overflow is to accept the answer you deem most helpful by clicking the check mark to the left. This will earn you two reputation points.
Yep, I wanted to do it right away, but he asked me to wait 4 minutes @Jens
@Pigna In this case they're equivalent. I prefer ++j over j++ because mentally I read the code as "increment j" and not as "j, increment that".

Matthew Gunn · Accepted Answer · 2015-12-06 17:53:34Z

While you can initialize global variables outside of a code block, you can't do what you're doing (if you want code to work). In general, code shouldn't be sitting outside of functions. You want end = clock() to be executed at the end! To do this, it needs to be at the end of the main() function.

Move the code into main() so it reads:

int main(void) { begin = clock(); ... //your code here end = clock(); time_spent = (double)(end - begin) / CLOCKS_PER_SEC; printf("time spent %f \n", time_spent); }

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