OT: recommended way of timing two pieces of code in C

[Rick Stevens]

On 02/27/2016 06:48 AM, Ranjan Maitra wrote:
> On Sat, 27 Feb 2016 14:09:25 +0000 "Patrick O'Callaghan" <pocallaghan at gmail.com> wrote:
>
>> On Sat, 2016-02-27 at 07:44 -0600, Ranjan Maitra wrote:
>>> #define INTERVAL 1        /* number of milliseconds to go off */
>>>
>>> int main() {
>>>      double sum = 0;
>>>      struct itimerval initial, updated;
>>>
>>>      initial.it_value.tv_sec     = INTERVAL/1000000;
>>>      initial.it_value.tv_usec    = (INTERVAL/1000000) * 1000000;
>>
>> To start with, these are both integer values, so you're initializing
>> them to 0. There may be other bugs but I stopped looking when I saw
>> this.
>>
>> poc
>> --
>
> OK, thanks! So, I tried this:
>
> #include <sys/time.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <limits.h>
>
> #define INTERVAL 1        /* number of milliseconds to go off */
>
> int main() {
> 	double sum = 0;
> 	struct itimerval initial, updated;
> 	
> 	initial.it_value.tv_sec     = INTERVAL;
> 	initial.it_value.tv_usec    = INT_MAX;
> 	initial.it_interval = initial.it_value;

If you want the timer to go off every millisecond:

	initial.it_value.tv_sec = 0;
	initial.it_value.tv_usec = 1000;

The ".tv_usec" must fall in the range 0 <= .tv_usec <= 999999.

>
> 	printf("%ld\n", initial.it_value.tv_usec);
> 	
> 	if (setitimer(ITIMER_VIRTUAL, &initial, NULL) == -1) {
> 		perror("error calling setitimer()");
> 		exit(1);
> 	}
>
> 	for (unsigned int i; i < 100000; i++)
> 		sum += 1./i;
> 	
> 	if (getitimer(ITIMER_REAL, &updated) == -1) {

Uh, why are you setting ITIMER_VIRTUAL, then reading ITIMER_REAL?

> 		perror("error calling getitimer()");
> 		exit(1);
> 	}
>
> 	printf("Time started = %ld\n; Time taken = %ld\n: Time taken = %ld\n",
> 	       initial.it_value.tv_usec, updated.it_value.tv_usec,
> 	       initial.it_value.tv_usec - updated.it_value.tv_usec);
> 	return 0;
> }
>
>
> But now setitimer does not execute:-(
>
> $gcc -o timer -std=c99 -Wall -pedantic getitimer.c -lrt -O3
> $./timer
> 2147483647
> error calling setitimer(): Invalid argument

That error message is pretty conclusive and indicates what I said at the
top.
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