clock

Defined in header `<time.h>`
clock_t clock(void);

Returns the approximate processor time used by the process since the beginning of an implementation-defined era related to the program's execution. To convert result value to seconds, divide it by CLOCKS_PER_SEC.

Only the difference between two values returned by different calls to clock is meaningful, as the beginning of the clock era does not have to coincide with the start of the program. clock time may advance faster or slower than the wall clock, depending on the execution resources given to the program by the operating system. For example, if the CPU is shared by other processes, clock time may advance slower than wall clock. On the other hand, if the current process is multithreaded and more than one execution core is available, clock time may advance faster than wall clock.

Parameters

(none)

Return value

Processor time used by the program so far or (clock_t)(-1) if that information is unavailable or its value cannot be represented.

Notes

On POSIX-compatible systems, clock_gettime with clock id CLOCK_PROCESS_CPUTIME_ID offers better resolution.

The value returned by clock() may wrap around on some implementations. For example, on a machine with 32-bit clock_t, it wraps after 2147 seconds or 36 minutes.

Example

This example demonstrates the difference between clock() time and real time.

Run this code

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <threads.h> // pthread.h in POSIX
 
// the function f() does some time-consuming work
int f(void* thr_data) // return void* in POSIX
{
    volatile double d = 0;
    for (int n=0; n<10000; ++n)
       for (int m=0; m<10000; ++m)
           d += d*n*m;
    return 0;
}
 
int main(void)
{
    struct timespec ts1, tw1; // both C11 and POSIX
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts1); // POSIX
    clock_gettime(CLOCK_MONOTONIC, &tw1); // POSIX; use timespec_get in C11
    clock_t t1 = clock();
 
    thrd_t thr1, thr2;  // C11; use pthread_t in POSIX
    thrd_create(&thr1, f, NULL); // C11; use pthread_create in POSIX
    thrd_create(&thr2, f, NULL);
    thrd_join(thr1, NULL); // C11; use pthread_join in POSIX
    thrd_join(thr2, NULL);
 
    struct timespec ts2, tw2;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts2);
    clock_gettime(CLOCK_MONOTONIC, &tw2);
    clock_t t2 = clock();
 
    double dur = 1000.0*(t2-t1)/CLOCKS_PER_SEC;
    double posix_dur = 1000.0*ts2.tv_sec + 1e-6*ts2.tv_nsec
                       - (1000.0*ts1.tv_sec + 1e-6*ts1.tv_nsec);
    double posix_wall = 1000.0*tw2.tv_sec + 1e-6*tw2.tv_nsec
                       - (1000.0*tw1.tv_sec + 1e-6*tw1.tv_nsec);
 
    printf("CPU time used (per clock(): %.2f ms\n", dur);
    printf("CPU time used (per clock_gettime()): %.2f ms\n", posix_dur);
    printf("Wall time passed: %.2f ms\n", posix_wall);
}

Possible output:

CPU time used (per clock(): 1580.00 ms
CPU time used (per clock_gettime()): 1582.76 ms
Wall time passed: 792.13 ms

References

C11 standard (ISO/IEC 9899:2011):

7.27.2.1 The clock function (p: 389)

C99 standard (ISO/IEC 9899:1999):

7.23.2.1 The clock function (p: 339)

C89/C90 standard (ISO/IEC 9899:1990):

4.12.2.1 The clock function

Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Atomic operations (C11)
Thread support (C11)
Technical Specifications

Functions
Time manipulation
difftime
time
clock
timespec_get (C11)
Format conversions
asctimeasctime_s (C11)
ctimectime_s (C11)
strftime
wcsftime (C95)
gmtimegmtime_s (C11)
localtimelocaltime_s (C11)
mktime
Constants
CLOCKS_PER_SEC
Types
tm
time_t
clock_t
timespec (C11)