NAME¶

std::clock - std::clock

Synopsis¶

Defined in header <ctime>
std::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 std::clock is
meaningful, as the beginning of the std::clock era does not have to coincide with
the start of the program. std::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, std::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, std::clock time may
advance faster than wall clock.

Parameters¶

(none)

Return value¶

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

Exceptions¶

Throws nothing.

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 non-conforming
implementations. For example, on such an implementation, if std::clock_t is a signed
32-bit integer and CLOCKS_PER_SEC is 1000000, it will wrap after about 2147 seconds
(about 36 minutes).

Example¶

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

// Run this code

#include <iostream>
#include <iomanip>
#include <chrono>
#include <ctime>
#include <thread>

// the function f() does some time-consuming work
void f()
{
volatile double d = 0;
for(int n=0; n<10000; ++n)
for(int m=0; m<10000; ++m) {
double diff = d*n*m;
d = diff + d;
}
}

int main()
{
std::clock_t c_start = std::clock();
auto t_start = std::chrono::high_resolution_clock::now();
std::thread t1(f);
std::thread t2(f); // f() is called on two threads
t1.join();
t2.join();
std::clock_t c_end = std::clock();
auto t_end = std::chrono::high_resolution_clock::now();

std::cout << std::fixed << std::setprecision(2) << "CPU time used: "
<< 1000.0 * (c_end - c_start) / CLOCKS_PER_SEC << " ms\n"
<< "Wall clock time passed: "
<< std::chrono::duration<double, std::milli>(t_end-t_start).count()
<< " ms\n";
}

Possible output:¶

CPU time used: 1590.00 ms
Wall clock time passed: 808.23 ms

See also¶

ctime converts a std::time_t object to a textual representation
(function)
time returns the current time of the system as time since epoch
(function)