c++ - Is this an overflow?

Question

I have this segment of code:

struct timeval start, end;
gettimeofday(&start, NULL);
//code I'm timing
gettimeofday(&end, NULL);
long elapsed = ((end.tv_sec-start.tv_sec)*1000000 + end.tv_usec-start.tv_usec);
ofstream timeFile;
timeFile.open ("timingSheet.txt");
timeFile << fixed << showpoint;
timeFile << setprecision(2);
timeFile << "Duration: " << elapsed << "
";
timeFile.close();

Which will output the number of microseconds that has passed. However, if I change this line

long elapsed = ((end.tv_sec-start.tv_sec)*1000000 + end.tv_usec-start.tv_usec);

to this:

long elapsed = ((end.tv_sec-start.tv_sec)*1000000 + end.tv_usec-start.tv_usec)/1000000.0;

I get a negative value. Why does this happen?

Answer 1

You are dividing by a double: 1000000.0, and casting back into an integer type.

Presuming all your start and end variables are ints (or longs), there is an awkward casting into a double, and then back into a long.

Try:

double elapsed = (double)(end.tv_sec-start.tv_sec) + (double)(end.tv_usec-start.tv)/1000000.0;