How to Use clock() Function in C for Measuring Time

In C, use the clock() function from time.h to measure CPU time consumed by your program. It returns the processor time used since the program started as a clock_t value, which you convert to seconds by dividing by CLOCKS_PER_SEC.

Syntax

The clock() function is declared in the time.h header. It returns a clock_t value representing the processor time used by the program since it started.

To get the time in seconds, divide the returned value by the constant CLOCKS_PER_SEC.

c

#include <time.h>

clock_t clock(void);

Example

This example shows how to measure the time taken by a simple loop using clock(). It calculates the elapsed CPU time in seconds.

c

#include <stdio.h>
#include <time.h>

int main() {
    clock_t start, end;
    double cpu_time_used;

    start = clock();

    // Simple loop to waste some CPU time
    for (long i = 0; i < 100000000; i++) {
        // do nothing
    }

    end = clock();

    cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;

    printf("CPU time used: %f seconds\n", cpu_time_used);

    return 0;
}

Output

CPU time used: 0.400000 seconds

Common Pitfalls

• Not including time.h will cause compilation errors.
• Dividing the clock() result by a wrong value instead of CLOCKS_PER_SEC leads to incorrect time.
• Using clock() to measure real elapsed (wall) time is not reliable; it measures CPU time used by the program.
• On some systems, clock() may return (clock_t)-1 if the processor time is not available.

c

#include <stdio.h>
#include <time.h>

int main() {
    clock_t start, end;
    double cpu_time_used;

    start = clock();

    // Some code

    end = clock();

    // Wrong: dividing by 1000 instead of CLOCKS_PER_SEC
    cpu_time_used = ((double) (end - start)) / 1000;

    printf("Wrong CPU time used: %f seconds\n", cpu_time_used);

    // Correct:
    cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
    printf("Correct CPU time used: %f seconds\n", cpu_time_used);

    return 0;
}

Output

Wrong CPU time used: 0.000400 seconds
Correct CPU time used: 0.000004 seconds

Quick Reference

Term	Description
clock()	Returns processor time used since program start as clock_t
clock_t	Data type to store processor time values
CLOCKS_PER_SEC	Constant to convert clock_t units to seconds
time.h	Header file where clock() and related constants are declared

Key Takeaways

Use clock() to measure CPU time used by your program, not real elapsed time.

Always divide the difference of clock() values by CLOCKS_PER_SEC to get seconds.

Include time.h to use clock() and related constants.

Check for clock() returning (clock_t)-1 which means the processor time is not available.

For measuring real elapsed time, consider other functions like time() or platform-specific timers.