Time function and clock_ Differences between gettime() functions

stay Linux We have many functions of reading time in , But some functions get absolute time When our system time changes Calculating the time difference will have an impact . Now we're going to time（） and clock_gettime Function

function "clock_gettime" Is based on Linux C Time function of language , Can be used to calculate time , There are two kinds of precision, second and nanosecond .

The function prototype ：

int clock_gettime(clockid_t clk_id, struct timespec *tp);
among ,cld_id Four types ：   

a、CLOCK_REALTIME: System real time , It changes with the real time of the system
b、CLOCK_MONOTONIC, Start timing from the moment the system starts , Not affected by the change of system time by users
c、CLOCK_PROCESS_CPUTIME_ID, This process to the current code system CPU Time spent
d、CLOCK_THREAD_CPUTIME_ID, This thread to the current code system CPU Time spent

This article uses... By default CLOCK_REALTIME, The accurate timing of parallel programs can be realized .

among ,timespec The structure includes ：

struct timespec {

time_t tv_sec; /* second */

long tv_nsec; /* nanosecond */

};
 

and time（）

First look at the function declaration , This function is contained in time.h Header file , The return value type is time_t. Is from Epoch Seconds since . If the passed in parameter is not NULL, Then the seconds will be assigned to the passed in parameters .

#include <time.h>
time_t time(time_t *timep);
// returns number of seconds since the Epoch, or -1 on error

times（） function

First of all times function , The statement is as follows . You can see times The return type of the function is clock_t, And its definition is contained in the header file sys/time.h in . You can call sysconf(_SC_CLK_TCK) To get real time . Pay attention here times Functions and the above mentioned time Functions have different functions ,times Function is a way to get the process time ,time Function is a way to get the clock time .
 

The prototype is as follows ：
clock_t times(struct tms *buf);

tms The structure is as follows :
strace tms{
 clock_t tms_utime;
 clock_t tms_stime;
 clock_t tms_cutime;
 clock_t tms_cstime;
}

notes :
tms_utime It records the time that the process executes user code .
tms_stime It records the time that the process executes the kernel code .
tms_cutime It records the time when the subprocess executes the user code .
tms_cstime It records when a child process executes kernel code

If we want to calculate Every time It needs to be used sysconf(_SC_CLK_TCK)) function , This sysconf Function can go to the information of the system .  _SC_CLK_TCK  Yes 1 second The number of clocks .