std::clock

 

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Date and time library

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Date and time library

Time point time_point (C++11) clock_time_conversion (C++20)   clock_cast (C++20) Duration duration (C++11) Clocks system_clock (C++11) steady_clock (C++11) is_clock (C++20) utc_clock (C++20) tai_clock (C++20) high_resolution_clock (C++11) gps_clock (C++20) file_clock (C++20) local_t (C++20) Time of day is_amis_pm (C++20)(C++20) make12make24 (C++20)(C++20) hh_mm_ss (C++20)

Calendar day (C++20) month (C++20) year (C++20) weekday (C++20) operator/ (C++20) year_month_day (C++20) year_month_day_last (C++20) year_month_weekday (C++20) year_month_weekday_last (C++20)   weekday_indexed (C++20) weekday_last (C++20) month_day (C++20) month_day_last (C++20) month_weekday (C++20) month_weekday_last (C++20) year_month (C++20) last_speclast (C++20)(C++20) chrono I/O parse (C++20)

Time zone tzdb (C++20) tzdb_list (C++20) get_tzdbget_tzdb_listreload_tzdbremote_version (C++20)(C++20)(C++20)(C++20) sys_info (C++20) local_info (C++20) nonexistent_local_time (C++20) ambiguous_local_time (C++20) locate_zone (C++20) current_zone (C++20) time_zone (C++20) choose (C++20) zoned_traits (C++20) zoned_time (C++20) time_zone_link (C++20) leap_second (C++20) leap_second_info (C++20) get_leap_second_info (C++20) C-style date and time

 

C-style date and time utilities

Functions

Time manipulation

difftime

time

clock

timespec_get (C++17)

Format conversions

asctime

ctime

strftime

wcsftime

gmtime

localtime

mktime

Constants

CLOCKS_PER_SEC

Types

tm

time_t

clock_t

timespec (C++17)

 

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.

Return value

Processor time used by the program so far.

• If the processor time used is not available, returns (std::clock_t)(-1).
• If the value of the processor time used cannot be represented by std::clock_t, returns an unspecified value.

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 implementations. For example, on such an implementation, if std::clock_t is a signed 32-bit integer and CLOCKS_PER_SEC is 1'000'000, 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 <chrono>#include <ctime>#include <iomanip>#include <iostream>#include <thread>   // The function f() does some time-consuming work.void f(){volatiledouble d =0;for(int n =0; n !=10000;++n)for(int m =0; m !=10000;++m) d += d * n * m;}   int main(){conststd::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();conststd::clock_t c_end = std::clock();constauto 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)<<'\n';}

Possible output:

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

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)
C documentation for clock