Measuring execution time of a program - profiling

Question

The link here says that gettimeofday() sets a structure which contains number of seconds and microseconds since Epoch (please tell me what Epoch is). With that thing in mind I set a structure before and after calling sleep function with parameter 3. So the total time difference setting of these structure is 3 seconds or 3000000 microseconds but it seem to give some wrong output. Where am I getting wrong?
#include<iostream>
#include<ctime>
#include<unistd.h>
#include<cstdio>
#include<sys/time.h>
using namespace std;
int main()
{
struct timeval start,end;
gettimeofday(&start,NULL);
sleep(3);
gettimeofday(&end,NULL);
cout<<start.tv_usec<<endl;
cout<<end.tv_usec<<endl;
cout<<end.tv_usec-start.tv_usec;
return 0;
}

Answer 1

Here's the point you're missing:
unsigned long time_in_micros = 1000000 * tv_sec + tv_usec;
To get the elapsed time in microseconds, you need to ADD "seconds" to "microseconds". You can't just ignore the tv_sec field!
Sample code:
#include <unistd.h>
#include <stdio.h>
#include <time.h>
#include <sys/time.h>
int main(int argc, char *argv[])
{
struct timeval start,end;
gettimeofday(&start,NULL);
sleep(3);
gettimeofday(&end,NULL);
printf ("start: %ld:%ld\n", start.tv_sec, start.tv_usec);
printf ("end: %ld:%ld\n", end.tv_sec, end.tv_usec);
printf ("diff: %ld:%ld\n",
end.tv_sec-start.tv_sec, end.tv_usec-start.tv_usec);
gettimeofday(&start,NULL);
sleep(10);
gettimeofday(&end,NULL);
printf ("start: %ld:%ld\n", start.tv_sec, start.tv_usec);
printf ("end: %ld:%ld\n", end.tv_sec, end.tv_usec);
printf ("diff: %ld:%ld\n",
end.tv_sec-start.tv_sec, end.tv_usec-start.tv_usec);
return 0;
}
Corresponding output:
start: 1459100430:214715
end: 1459100433:215357
diff: 3:642
start: 1459100433:215394
end: 1459100443:217024
diff: 10:1630
gettimeofday() links:
http://linux.die.net/man/2/gettimeofday
https://blog.habets.se/2010/09/gettimeofday-should-never-be-used-to-measure-time
Measure time in Linux - time vs clock vs getrusage vs clock_gettime vs gettimeofday vs timespec_get?