gettimeofday()

scorpio2002 · #1 01-Nov-2006, 13:09

Hi there! I'm trying to port a piece of code from windows to linux. In this code i use the function timeGettime() from a Microsoft library. I checked for the exsistence of a similar function that could return the time since an epoch in milliseconds. The only thing I foud is gettimeofday()... ---> www.penguin-soft.com

But it seems to have a lot of problems and a strange behaviour... Take a look at here:

CPP / C++ / C Code:

#include <sys/time.h>
#include <stdio.h>

main() {
	double 	frame_count,
	start_time,
	last_time;
	struct timeval tempo;

	gettimeofday(&tempo, NULL);
	start_time = (double)tempo.tv_usec;
	while (getchar() != 'q') {
		
	}

	gettimeofday(&tempo, NULL);
	printf("Running for %f milliseconds\n", ((double)tempo.tv_usec - start_time)/1000);
}

If you run this, you'll notice that at the end what you get is not the time elasped since the program run, but a casual value, sometime also negative...

What is wrong with that?

davekw7x · #2 01-Nov-2006, 13:55

Quote:

Originally Posted by scorpio2002

But it seems to have a lot of problems and a strange behaviour... Take a look at here:

If you run this, you'll notice that at the end what you get is not the time elasped since the program run, but a casual value, sometime also negative...

What is wrong with that?

In Linux, one of the best resources is "man"

Enter "man gettimeofday" at a shell prompt, and you will discover that there are two counters: a seconds counter and a microseconds counter. The two counters together indicate the number of seconds and microseconds since the epoch. The total elapsed time between two function calls (in microseconds) would be given by

1000000 * (difference in seconds) + (difference in microseconds)

Let's assume that at the start, we have this:

seconds counter = 1162406644, usecs counter = 702525

and after a few seconds we enter 'q' and we see that

seconds counter = 1162406647, usecs counter = 103550

The difference in the seconds counter is 3; the difference in the usecs counter is -598975

Then the elapsed time is 1000000 * 3 - 598975 = 2401025 microseconds

Try it yourself:

CPP / C++ / C Code:

#include <sys/time.h>
#include <stdio.h>

int main()
{
    struct timeval tempo1, tempo2;

    long elapsed_utime;    /* elapsed time in microseconds */
    long elapsed_mtime;    /* elapsed time in milliseconds */
    long elapsed_seconds;  /* diff between seconds counter */
    long elapsed_useconds; /* diff between microseconds counter */

    printf("Enter q to quit timing loop: ");
    gettimeofday(&tempo1, NULL);
    while (getchar() != 'q') {

    }

    gettimeofday(&tempo2, NULL);
    printf("tempo2.tv_sec = %ld, tempo2_tv_usec = %ld\n", 
            tempo2.tv_sec, tempo2.tv_usec);
    printf("tempo1.tv_sec = %ld, tempo1_tv_usec = %ld\n", 
            tempo1.tv_sec, tempo1.tv_usec);
    elapsed_seconds  = tempo2.tv_sec  - tempo1.tv_sec;
    elapsed_useconds = tempo2.tv_usec - tempo1.tv_usec;

    printf("Elapsed time = %ld seconds + %ld microseconds\n",
            elapsed_seconds, elapsed_useconds);

    elapsed_utime = (elapsed_seconds) * 1000000 + elapsed_useconds;
     elapsed_mtime = ((elapsed_seconds) * 1000 + elapsed_useconds/1000.0) + 0.5;

    printf("Elapsed time = %ld microseconds\n", elapsed_utime);
    printf("Elapsed time = %ld milliseconds\n", elapsed_mtime);

    return 0;
}

Notice that for milliseconds, I rounded off to the nearest millisecond.

A few runs:

Code:

Enter q to quit timing loop: q
tempo2.tv_sec = 1162407046, tempo2_tv_usec = 230398
tempo1.tv_sec = 1162407044, tempo1_tv_usec = 795110
Elapsed time = 2 seconds + -564712 microseconds
Elapsed time = 1435288 microseconds
Elapsed time = 1435 milliseconds
.
.
.
tempo2.tv_sec = 1162407049, tempo2_tv_usec = 212860
tempo1.tv_sec = 1162407048, tempo1_tv_usec = 612037
Elapsed time = 1 seconds + -399177 microseconds
Elapsed time = 600823 microseconds
Elapsed time = 601 milliseconds
.
.
.
tempo2.tv_sec = 1162407571, tempo2_tv_usec = 745646
tempo1.tv_sec = 1162407570, tempo1_tv_usec = 188905
elapsed time = 1 seconds + 556741 microseconds
Elapsed time = 1556741 microseconds
Elapsed time = 1557 milliseconds

Regards,

Dave

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Measuring time? Help!	smoothdogg00	C++ Forum	1	10-Apr-2006 09:19
Click Ticks	dayrinni	C Programming Language	1	20-Feb-2006 12:50

gettimeofday()

Re: gettimeofday()