how to measure pipe syscall time in milliseconds?

问题

I want to see the time of my pipe program system call. I need to measure it for analyzing results. How can i measure the time of system call in milliseconds?

For example, this is simple pipe program:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/timeb.h>
#include <time.h>

void main() {

    FILE *read_fp;
    char buffer[10];
    int cnt;

    memset(buffer,'\0',sizeof(buffer));
    read_fp=popen("uname -a","|");

    if (read_fp!=NULL) {
        cnt=fread(buffer,sizeof(char),9,read_fp);
        if (cnt>0) printf(buffer,"olololo");
            pclose(read_fp);
    }
}

And what is the reason of time differences, executing it more than 4 times? What's the reason of time "jumps"?

来源：https://stackoverflow.com/questions/47725553/how-to-measure-pipe-syscall-time-in-milliseconds