本文将汇总Linux用户态程序获取时间的接口，本文内容主要转载自clock()、time()、clock_gettime()和gettimeofday()函数的用法和区别。

1. clock()

https://man7.org/linux/man-pages/man3/clock.3.html

1.1 概述

clock函数的返回值类型是clock_t,它除以CLOCKS_PER_SEC来得出时间,一般用两次clock函数来计算进程自身运行的时间.

clock有三个问题:

  1. 如果超过一个小时,将要导致溢出.
  2. 函数clock没有考虑CPU被子进程使用的情况.
  3. 也不能区分用户空间和内核空间.

所以clock函数在linux系统上变得没有意义.

1.2 测试

编写程序,测试采用clock函数的输出与time程序的区别.

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#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main( void )
{
   long i=1000L;
   clock_t start, finish;
   double  duration;
   printf( "Time to do %ld empty loops is ", i );
   start = clock();
   while (--i){
    system("cd");
   }
   finish = clock();
   duration = (double)(finish - start) / CLOCKS_PER_SEC;
   printf( "%f seconds\n", duration );
   return 0;
}

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$ gcc test1.c -o test1
$ time ./test1
Time to do 1000 empty loops is 0.180000 seconds

real    0m3.492s
user    0m0.512s
sys     0m2.972s

1.3 总结

  1. 程序调用system("cd")，这里主要是子进程的消耗,test1程序不能体现这一点.
  2. 0.180000 seconds秒的消耗是两次clock()函数调用除以CLOCKS_PER_SEC.
  3. clock()函数返回值是一个相对时间，而不是绝对时间.
  4. CLOCKS_PER_SEC是系统定义的宏，由GNU标准库定义为1000000.

2. times()

https://man7.org/linux/man-pages/man2/times.2.html

2.1 概述

原型如下：
clock_t times(struct tms *buf);

tms结构体如下:

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struct tms{
 clock_t tms_utime;
 clock_t tms_stime;
 clock_t tms_cutime;
 clock_t tms_cstime;
}

tms_utime记录的是进程执行用户代码的时间.
tms_stime记录的是进程执行内核代码的时间.
tms_cutime记录的是子进程执行用户代码的时间.
tms_cstime记录的是子进程执行内核代码的时间.

2.2 测试

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#include <sys/times.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

static void do_cmd(char *);
static void pr_times(clock_t, struct tms *, struct tms *);

int main(int argc, char *argv[]){
        int i;
        for(i=1; argv[i]!=NULL; i++){
                do_cmd(argv[i]);
        }
        exit(1);
}
static void do_cmd(char *cmd){
        struct tms tmsstart, tmsend;
        clock_t start, end;
        int status;
        if((start=times(&tmsstart))== -1)
                puts("times error");
        if((status=system(cmd))<0)
                puts("system error");
        if((end=times(&tmsend))== -1)
                puts("times error");
        pr_times(end-start, &tmsstart, &tmsend);
        exit(0);
}
static void pr_times(clock_t real, struct tms *tmsstart, struct tms *tmsend){
        static long clktck=0;
        if(0 == clktck)
                if((clktck=sysconf(_SC_CLK_TCK))<0)
                           puts("sysconf err");
        printf("real:%7.2f\n", real/(double)clktck);
        printf("user-cpu:%7.2f\n", (tmsend->tms_utime - tmsstart->tms_utime)/(double)clktck);
        printf("system-cpu:%7.2f\n", (tmsend->tms_stime - tmsstart->tms_stime)/(double)clktck);
        printf("child-user-cpu:%7.2f\n", (tmsend->tms_cutime - tmsstart->tms_cutime)/(double)clktck);
        printf("child-system-cpu:%7.2f\n", (tmsend->tms_cstime - tmsstart->tms_cstime)/(double)clktck);
}
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$ gcc test2.c -o test2
$ time ./test2 "dd if=/dev/zero f=/dev/null bs=1M count=10000"
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB) copied, 4.93028 s, 2.1 GB/s
real:   4.94
user-cpu:   0.00
system-cpu:   0.00
child-user-cpu:   0.01
child-system-cpu:   4.82

real    0m4.943s
user    0m0.016s
sys     0m4.828s

2.3 总结

  1. 通过这个测试,系统的time程序与test2程序输出基本一致了.
  2. (double)clktck是通过clktck=sysconf(_SC_CLK_TCK)来取的,也就是要得到user-cpu所占用的时间,就要用(tmsend->tms_utime - tmsstart->tms_utime)/(double)clktck)
  3. clock_t times(struct tms *buf)返回值是过去一段时间内时钟嘀嗒的次数.
  4. times()函数返回值也是一个相对时间.

3. gettimeofday()

https://man7.org/linux/man-pages/man2/gettimeofday.2.html

3.1 概述

gettimeofday()可以获得当前系统的时间,是一个绝对值

原型如下：
int gettimeofday (struct timeval * tv , struct timezone * tz)

timeval结型体的原型如下:

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struct timeval {
               time_t      tv_sec;    
               suseconds_t tv_usec;   
};

所以它可以精确到微秒

3.2 测试

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#include <sys/time.h>
#include <stdio.h>
#include <unistd.h>
int
main(){
        int i=10000000;
        struct timeval tvs,tve;
        gettimeofday(&tvs,NULL);
        while (--i);
        gettimeofday(&tve,NULL);
        double span = tve.tv_sec-tvs.tv_sec + (tve.tv_usec-tvs.tv_usec)/1000000.0;
        printf("time: %.12f\n",span);
        return 0;
}
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$ gcc test5.c
$ ./a.out
time: 0.041239000000

4. clock_gettime()

https://www.man7.org/linux/man-pages/man3/clock_gettime.3.html

4.1 概述

原型如下：
int clock_gettime(clockid_t clk_id, struct timespec *tp);

有以下特点:

  1. 有一个时间结构体:timespec,timespec计算时间的精度是ns.

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    struct timespec{
            time_t tv_sec;
            long tv_nsec;
    }

  2. clockid_t确定哪个时钟类型.
    CLOCK_REALTIME

    CLOCK_MONOTONIC

    CLOCK_PROCESS_CPUTIME_ID和CLOCK_THREAD_CPUTIME_ID.

4.2 测试:

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#include<time.h>
#include<stdio.h>
#include<stdlib.h>

#define MILLION 1000000


int main(void)
{
        long int loop = 1000;
        struct timespec tpstart;
        struct timespec tpend;
        long timedif;

        clock_gettime(CLOCK_MONOTONIC, &tpstart);

        while (--loop){
                system("cd");
        }

        clock_gettime(CLOCK_MONOTONIC, &tpend);
        timedif = MILLION*(tpend.tv_sec-tpstart.tv_sec)+(tpend.tv_nsec-tpstart.tv_nsec)/1000;
        fprintf(stdout, "it took %ld microseconds\n", timedif);

        return 0;
}
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$ gcc test3.c -lrt -o test3

$ time ./test3
it took 3463843 microseconds

real    0m3.467s
user    0m0.512s
sys     0m2.936s

5. 比较

5.1 精度

  • clock()的精度是10毫秒(ms)
  • times()的精度是10毫秒(ms)
  • gettimofday()的精度是微秒(μs)
  • clock_gettime()的精度是纳秒(ns)

5.2 测试

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#include    <stdio.h>
#include    <stdlib.h>
#include    <unistd.h>
#include    <time.h>
#include    <sys/times.h>
#include    <sys/time.h>
#define WAIT for(i=0;i<298765432;i++);
#define MILLION    1000000
    int
main ( int argc, char *argv[] )
{
    int i;
    long ttt;
    clock_t s,e;
    struct tms aaa;

    s=clock();
    WAIT;
    e=clock();
    printf("clock time : %.12f\n",(e-s)/(double)CLOCKS_PER_SEC);


    long tps = sysconf(_SC_CLK_TCK);
    s=times(&aaa);
    WAIT;
    e=times(&aaa);
    printf("times time : %.12f\n",(e-s)/(double)tps);


    struct timeval tvs,tve;
    gettimeofday(&tvs,NULL);
    WAIT;
    gettimeofday(&tve,NULL);
    double span = tve.tv_sec-tvs.tv_sec + (tve.tv_usec-tvs.tv_usec)/1000000.0;
    printf("gettimeofday time: %.12f\n",span);


    struct timespec tpstart;
    struct timespec tpend;

    clock_gettime(CLOCK_REALTIME, &tpstart);
    WAIT;
    clock_gettime(CLOCK_REALTIME, &tpend);
    double timedif = (tpend.tv_sec-tpstart.tv_sec)+(tpend.tv_nsec-tpstart.tv_nsec)/1000000000.0;
    printf("clock_gettime time: %.12f\n", timedif);

    return EXIT_SUCCESS;
}
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$ gcc -lrt test4.c -o test4
$ ./test4
clock time : 1.190000000000
times time : 1.180000000000
gettimeofday time: 1.186477000000
clock_gettime time: 1.179271718000