clock

 

C

Language

Standard Library

Type support

Dynamic memory management

Error handling

Program utilities

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

 

Date and time utilities

Functions

Time manipulation

difftime

time

clock

timespec_get (C11)

Format conversions

asctime

ctime

strftime

wcsftime

gmtime

localtime

mktime

Constants

CLOCKS_PER_SEC

Types

tm

time_t

clock_t

timespec (C11)

 

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

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.

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 <math.h>
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
 
// the function f() does some time-consuming work
void f()
{
    volatile double d=0.0; // we don't want subsequent computation to be optimized out
    for (int n=0; n<10000; ++n)
       for (int m=0; m<10000; ++m)
           d += d*n*m;
}
 
int main(void)
{
    clock_t t = clock();
    if (t == ((clock_t)-1))
    {
       fprintf(stderr,"clock() failed in file %s at line # %d\n", __FILE__,__LINE__-3);
       exit(EXIT_FAILURE);
    }
 
    f();
    t = clock() - t;
    printf("Time used: %f seconds\n",((float)t)/CLOCKS_PER_SEC);
 
    return 0;
}

Output:

Time used: 1.010000 seconds

See also

ctime	converts a time_t object to a textual representation (function)
time	returns the current calendar time of the system as time since epoch (function)
C++ documentation for clock