How to collect the processing time in C? - c

Question

I am collecting the total elapsed time by using two inline functions (specified and implemented in my .h file) as follows:
extern double _elapsed_time_mf;
extern double _elapsed_time_b;
//this function returns the elapsed time in order to compute the total elapsed time of an operation
static inline struct timeval get_current_time() {
struct timeval time;
gettimeofday(&time, NULL);
return time;
}
//calculate the total processed time and return the elapsed total time in seconds
static inline double get_elapsed_time(struct timeval start, struct timeval end) {
long int tmili;
tmili = (int) (1000.0 * (end.tv_sec - start.tv_sec) +
(end.tv_usec - start.tv_usec) / 1000.0);
return (double) (tmili / (double) 1000.0);
}
Then, when I would like to know the total elapsed time of an operation I do this:
void my_function() {
#ifdef COLLECT_STATISTICAL_DATA
struct timeval start;
struct timeval end;
start = get_current_time();
#endif
//a processing....
#ifdef COLLECT_STATISTICAL_DATA
end = get_current_time();
_elapsed_time_mf = get_elapsed_time(start, end);
#endif
}
_elapsed_time_mf is defined in only one .c file.
However, I am getting strange results. For instance, consider that I have another function, called function_b, which also collects its elapsed time (which is stored in other global variable). Then, this function makes a call to my_function (that collects its elapsed time according to my previous code). However, the total elapsed time of function_b is sometimes lesser than the total elapsed time of my_function. An example of this situations is:
void function_b() {
#ifdef COLLECT_STATISTICAL_DATA
struct timeval start;
struct timeval end;
start = get_current_time();
#endif
//a processing....
my_function();
//another processing...
#ifdef COLLECT_STATISTICAL_DATA
end = get_current_time();
_elapsed_time_b = get_elapsed_time(start, end);
#endif
}
Sometimes _elapsed_time_b is lesser than _elapsed_time_mf. Why?
I would like to collect both elapsed times in seconds according to the clock/date/timestamp (not the CPU elapsed time).

Answer 1

You might want to reconsider the implementation of get_elapsed_time. From here: http://www.gnu.org/software/libc/manual/html_node/Elapsed-Time.html
int timeval_subtract (struct timeval *result, struct timeval *x, struct timeval *y)
{
/* Perform the carry for the later subtraction by updating y. */
if (x->tv_usec < y->tv_usec) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
y->tv_usec -= 1000000 * nsec;
y->tv_sec += nsec;
}
if (x->tv_usec - y->tv_usec > 1000000) {
int nsec = (x->tv_usec - y->tv_usec) / 1000000;
y->tv_usec += 1000000 * nsec;
y->tv_sec -= nsec;
}
/* Compute the time remaining to wait.
tv_usec is certainly positive. */
result->tv_sec = x->tv_sec - y->tv_sec;
result->tv_usec = x->tv_usec - y->tv_usec;
/* Return 1 if result is negative. */
return x->tv_sec < y->tv_sec;
}

Answer 2

As Art has commented, I am using now clock_gettime. Thus, my code is now working as expected.
My functions are now written as:
static inline double get_elapsed_time(struct timespec start, struct timespec end) {
double start_in_sec = (double)start.tv_sec + (double)start.tv_nsec / 1000000000.0;
double end_in_sec = (double)end.tv_sec + (double)end.tv_nsec / 1000000000.0;
return end_in_sec - start_in_sec;
}
static inline struct timespec get_current_time() {
struct timespec time;
clock_gettime(CLOCK_MONOTONIC, &time);
return time;
}