C++sleep()和usleep()的区别

#include <time.h>
#include <stdio.h>
#include <iOStream>
#include <unistd.h>

using namespace std;
int main(int arGC, char *argv[]){
    int sleep_time = 100;
    cout << "sleep before time: " << time(NULL) << endl;
    sleep(sleep_time);
    cout << "sleep after time: " << time(NULL) << endl;

    cout << "======================" << endl;

    cout << "usleep before time: " << time(NULL) << endl;
    usleep(sleep_time);
    cout << "usleep after time: " << time(NULL) << endl;
}

#include <stdio.h>
#include <iostream>
#include <unistd.h>
#include <sys/time.h>

using namespace std;
int main(int argc, char *argv[]){
    int sleep_time = 100;
    cout << "sleep before time: " << time(NULL) << endl;
    sleep(sleep_time);
    cout << "sleep after time: " << time(NULL) << endl;

    cout << "======================" << endl;

    cout << "usleep before time: " << time(NULL) << endl;
    usleep(sleep_time);
    cout << "usleep after time: " << time(NULL) << endl;

    cout << "======================" << endl;
    struct timeval tv;  
    gettimeofday(&tv,NULL); 
    cout << "not usleep before time: " << tv.tv_sec<<"s,"<<tv.tv_usec<<"微秒"<<endl;
    usleep(0);
    gettimeofday(&tv,NULL); 
    cout << "not usleep after time: " << tv.tv_sec<<"s,"<<tv.tv_usec<<"微秒"<<endl;
    cout << "======================" << endl;
    gettimeofday(&tv,NULL); 
    cout << "usleep before time: " << tv.tv_sec<<"s,"<<tv.tv_usec<<"微秒"<<endl;
    usleep(sleep_time);
    gettimeofday(&tv,NULL); 
    cout << "usleep after time: " << tv.tv_sec<<"s,"<<tv.tv_usec<<"微秒"<<endl;
}