C语言纳秒级计时

struct timespec{
    time_t tv_sec ; // seconds
    long   tv_nsec; // nanoseconds
};
#define TIME_UTC 1  //time.h 头文件里面定义的，而且只有这一个

#include<iostream>
#include<time.h>
using namespace std;
int main(){
    clock_t start_ms, end_ms;         // 毫秒级计时
    struct timespec start_ns, end_ns; // 纳秒级计时 C11标准库
    
    start_ms = clock();
    timespec_get(&start_ns,TIME_UTC);
    
    // do something
    for(int i=0;i<1000;i++){
        for(int j=0;j<1000;j++) ; 
    }
    
    end_ms = clock();
    timespec_get(&end_ns,TIME_UTC);
    
    // output
    cout << "clock function: " << end_ms - start_ms << endl;
    std::cout << "timespec_get function: " << ((end_ns.tv_sec - start_ns.tv_sec) * 1e9 + (end_ns.tv_nsec - start_ns.tv_nsec)) / 1e6 << "ns" <<std::endl;
    return 0;
}