14. Process time

Concept

Process time is process After creation The use of CPU The amount of time . For the purpose of recording , The kernel puts CPU Time is divided into the following two parts .

• user CPU Time is the amount of time spent executing in user mode , Also known as virtual time （virtual time）, This is for programs , Yes, it has been CPU Time for .
• System CPU Time is the amount of time spent executing in kernel mode . This is the time that the kernel uses to perform system calls or other tasks on behalf of programs .
  When we run a shell Program , have access to time(1) command , To get the time of these two parts and the actual time required to run the program at the same time .

Such as ：

time ./a.out

times() call

system call times() Retrieve process time information , And pass the result through buf The structure pointed to returns i.

#include <sys/time.h>
clock_t times(struct tms  *buf);
	Returns number of clock ticks(sysconf(_SC_CLK_TCK)) since "arbitrary"
		time in past pn success, or (clock_t)-1 on error.

buf Point to the TMS The structure has the following format ：

struct tms{
    
	clock_t tms_utime;	/*User CPU used by caller*/
	clock_t tms_stime;	/*System CPU time used by caller*/
	clock_t tms_cutime;	/*User CPU time of all (waited for) children*/
	clock_t tms_cstime;	/*System CPU time of all (waited for) children*/
};

Parameter description

tms The first two fields of the structure return the user and system components used by the calling process so far CPU Time . The last two fields are ： The parent process （ such as ,times() The caller ） A system call was executed wait() Used by all terminated subprocesses of CPU Time .

data type clock_t It is a clock timing unit （closk tick） The unit measurement is the integer value of time , Used for calculation tms Structure of the 4 A field . You can call sysconf(_SC_CLK_TCK) To get the number of clock timing units per second , Then divide this number by clock_t Convert to seconds .

Return value

If it works ,times() Returns the time elapsed at any point in the past, in units of clock timing units （ Actual ） Time .SUSv3 There is no definition of what this is , Just say , Zhejiang is a fixed point in the life cycle of the calling process .

therefore , The only use of this return value is by calculating a pair times() The difference between the values returned by the call , To calculate the time consumed by the process execution .( However , Even this is not reliable , Because it may overflow clock_t The effective range of , At this time times() The return value of will again be from 0 Start calculating ). The reliable way is to use functions gettimeofday()

clock()

function clock() It provides a simple interface for obtaining process time , It returns a value that describes the total... Used by the call CPU Time （ Including users and systems ）.

#include <time.h>
clock_t clock(void);
	Returns total CPU time used by calling process measured in 
		CLOCKS_PER_SEC, or (clock_t)-1 on error.

The unit of measurement is CLOCK_PER_SEC, Divide by this value to get the number of seconds .
In some UNIX On the system clock() The returned time includes all the time waiting for the child process , But in Linux On , barring .