problem with the output in my C program (gives unexpected output) - c

Question

What is the problem with this program it is supposed to calculate the elapsed time of each function call but to my surprise, the elapsed time is always ZERO because the begin and end are exactly the same. Does anyone have an explanation for this?
This is the output I get:
TIMING TEST: 10000000 calls to rand()
2113 6249 23817 12054 7060 9945 26819
13831 6820 14149 13035 30858 13924 26467
4268 11314 28400 5239 4496 27757 21452
10878 25064 9049 6508 29612 11373 29913
10234 31769 16167 24553 1875 23992 30606
2606 19539 2184 14832 27089 27474 23310
, .. , ,
End time: 1610034404
Begin time: 1610034404
Elapsed time: 0
Time for each call:,10f
Here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define NCALLS 10000000
#define NCOLS 7
#define NLINES 7
int main(void) {
int i, val;
long begin, diff, end;
begin = time(NULL);
srand(time(NULL));
printf("\nTIMING TEST: %d calls to rand()\n\n", NCALLS);
for (i = 1; i <= NCALLS; ++i) {
val = rand();
if (i <= NCOLS * NLINES) {
printf("%7d", val);
if (i % NCOLS == 0)
putchar('\n');
} else
if (i == NCOLS * NLINES + 1)
printf("%7s\n\n", ", .. , ,");
}
end = time(NULL);
diff = end - begin;
printf("%s%ld\n%s%ld\n%s%ld\n%s%,10f\n\n",
"End time: ", end,
"Begin time: ", begin,
"Elapsed time: ", diff,
"Time for each call:", (double)diff / NCALLS);
return 0;
}

Answer 1

instead of time(NULL) you can use clock()
time_t t1 = clock();
// your code
time_t t2 = clock();
printf("%f", (double)(t2 - t1) / CLOCKS_PER_SEC); // you have to divide it to CLOCKS_PER_SEC (1 000 000) if you want time in seconds
time() measures in seconds, so if your program doesn't take 1 second you won't see difference
Someone in stackoverflow has already answered difference between them time() vs clock()

Answer 2

Changing your code just to spend some random time inside the loop using a system call a few times, using
struct stat file_stat;
for( int j = 0; j < rand()%(1000); j+=1) stat(".", &file_stat);
and we get on a very old machine (for 10,000 and not 10,000,000 cycles as in your code)
toninho#DSK-2009:/mnt/c/Users/toninho/projects/um$ gcc -std=c17 -Wall tlim.c
toninho#DSK-2009:/mnt/c/Users/toninho/projects/um$ ./a.out
TIMING TEST: 10000 calls to rand()
953019096 822572575 552766679 1101222688 890440097
348966778 1483436091 1936203136 1060888701 936990601
524198868 554412390 1109472424 51262334 723194231
353522463 1808580291 673860068 818332399 350403991
442567054 1054917195 229398907 420744931 620127925
1975661852 812007818 1400791797 1471940068 1739247840
1364643097 529639947 1569398779 20035674 92849903
1060567289 1126157009 2111376669 324165122 338724259
719809477 977786583 510114270 981390269 2029486195
1551025212 1112929616 2091082251 1066603801 1722106156
, .. , ,
End time: 1610044947
Begin time: 1610044942
Elapsed time: 5
Time for each call:500.000000
Using
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <time.h>
#define NCALLS 10 * 1000
#define NCOLS 5
#define NLINES 10
int main(void)
{
int i, val;
long begin, diff, end;
begin = time(NULL);
srand(210701);
printf("\nTIMING TEST: %d calls to rand()\n\n", NCALLS);
for (i = 1; i <= NCALLS; ++i)
{
val = rand();
// spend some time
struct stat file_stat;
for( int j = 0; j < rand()%(1000); j+=1) stat(".", &file_stat);
if (i <= NCOLS * NLINES)
{
printf("%12d", val);
if (i % NCOLS == 0)
putchar('\n');
}
else if (i == NCOLS * NLINES + 1)
printf("%7s\n\n", ", .. , ,");
//printf("%7d: ", i);
//fgetc(stdin);
}; // for
end = time(NULL);
diff = end - begin;
printf("%s%ld\n%s%ld\n%s%ld\n%s%10f\n\n", // %
"End time: ", end,
"Begin time: ", begin,
"Elapsed time: ", diff,
"Time for each call:", (double)diff / NCALLS);
return 0;
}