We can use the function and structs in standard C library (sys/time.h) to calculate the elapsed time value between two time instances:
1. To get the processor time of the program, we use the C library clock_t struct to count the CPU clock ticks, and then divide the CLOCKS_PER_SEC to get the time:
e.g.
clock_t t;
t = clock();
// Do something
t = clock() - t;
double cpu_time_elapsed = (double)t/CLOCKS_PER_SEC;
2. To calculate the time interval in wall clock time, use the C library time_t struct and the time difference function:
double difftime(time_t end, time_t start)
, where the start and end time can be initialized by calling time_t time(time_t *time) function, and the return value is the seconds (in double) elapsed between start and end.
e.g.
time_t start, end;
time(&start);
// Do something
time(&end);
double time_elapsed = difftime(end, start);
3. To calculate the time interval more precisely (in higher resolution), we can define a C function like:
double getTimeElapsed(struct timeval end, struct timeval start)
{
return (end.tv_sec - start.tv_sec) + (end.tv_usec - start.tv_usec) / 1000000.00;
}
, where the struct timeval is elapsed time declared in the sys/time.h. It contains two members:
tv_sec (number of seconds elapsed)
and
tv_usec (remaining second fraction in microseconds).
To get the start and end time value, we call the gettimeofday() function.
e.g.
timeval start, end;
gettimeofday(&start, NULL);
// Do something
gettimeofday(&end, NULL);
double time_elapsed = getTimeElapsed(end, start);
Another time struct, timespec has two members tv_sec and tv_nsec, where tv_nsec represent remaining second fraction in nanosecond.
e.g.
clock_t t;
t = clock();
// Do something
t = clock() - t;
double cpu_time_elapsed = (double)t/CLOCKS_PER_SEC;
2. To calculate the time interval in wall clock time, use the C library time_t struct and the time difference function:
double difftime(time_t end, time_t start)
, where the start and end time can be initialized by calling time_t time(time_t *time) function, and the return value is the seconds (in double) elapsed between start and end.
e.g.
time_t start, end;
time(&start);
// Do something
time(&end);
double time_elapsed = difftime(end, start);
3. To calculate the time interval more precisely (in higher resolution), we can define a C function like:
double getTimeElapsed(struct timeval end, struct timeval start)
{
return (end.tv_sec - start.tv_sec) + (end.tv_usec - start.tv_usec) / 1000000.00;
}
, where the struct timeval is elapsed time declared in the sys/time.h. It contains two members:
tv_sec (number of seconds elapsed)
and
tv_usec (remaining second fraction in microseconds).
To get the start and end time value, we call the gettimeofday() function.
e.g.
timeval start, end;
gettimeofday(&start, NULL);
// Do something
gettimeofday(&end, NULL);
double time_elapsed = getTimeElapsed(end, start);
Another time struct, timespec has two members tv_sec and tv_nsec, where tv_nsec represent remaining second fraction in nanosecond.
Reference:
1. GNU C Library Manual: 21.2 Elapsed Time
2. difftime on cplusplus.com
