How to record elaspsed wall time in C? - c

Question

I'm trying to record the time my program takes to finish in seconds, with sub-second accuracy.
I'm not recording CPU time or cycles, I simply want to be able to mark a start point (Wall Clock time), then at the end of my program mark a finish (Wall Clock time), and calculate the delta.

Answer 1

Have a look at the function:
int clock_gettime(clockid_t clk_id, struct timespec *tp);
The function will fill the structure struct timespec you provide. Here is its definition:
struct timespec {
time_t tv_sec; /* secondes */
long tv_nsec; /* nanosecondes */
};
So the returned time in nanosecondes is: tp->tv_sec * 1e9 + tp->tv_nsec.
You can find all the possible clk_id in the man. I would recommend you to use CLOCK_MONOTONIC as it guarantees you that the time given will always be continuous, even if the time of the system is modified.

Answer 2

Just call time(NULL) to get the current time and use difftime to calculate the time between two points.
#include <time.h>
// ...
time_t start = time(NULL);
// do stuff here
time_t end = time(NULL);
printf("Took %f seconds\n", difftime(end, start));

Answer 3

This displays start/end time stamps and calculates a delta in seconds.
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
void print_timestamp(char *, time_t);
int main (int argc, char *argv[]) {
time_t start = time(0);
print_timestamp("Start: ", start);
sleep(2);
time_t end = time(0);
print_timestamp("End: ", end);
double diff = difftime(end, start);
printf("Elapsed: %5.2lf seconds\n", diff);
}
void
print_timestamp(char *msg, time_t time) {
struct tm *tm;
if ((tm = localtime (&time)) == NULL) {
printf ("Error extracting time stuff\n");
return;
}
printf ("%s %04d-%02d-%02d %02d:%02d:%02d\n",
msg,
1900 + tm->tm_year,
tm->tm_mon+1,
tm->tm_mday,
tm->tm_hour,
tm->tm_min,
tm->tm_sec);
}
Sample output:
Start: 2017-02-04 15:33:36
End: 2017-02-04 15:33:38
Elapsed: 2.00 seconds

Answer 4

You may also be able to use the time command available on (at least) Unix systems.
After compiling your program, run the command like this:
# compile your code
$ gcc foo.c -o foo
# compute the time
$ time ./foo

Answer 5

You can use the clock() function to record the number of ticks taken and then convert this to seconds:
#include <time.h>
#include <stdio.h>
#include <math.h>
int main () {
clock_t start_t = clock();
double a=0;
for(int i=0; i< 10000000; i++) {
a+=sqrt(a);
}
clock_t end_t = clock();
double total_t = (double)(end_t - start_t) / CLOCKS_PER_SEC;
printf("Total time taken by CPU: %lf\n", total_t );
return(0);
}