sleep() 和 usleep()主要区别前者单位为秒，后者为微妙（*1e6）
C++ sleep() 和 usleep()
代码如下：设置一个时间参数，分别向sleep()和usleep()方法传入该参数，打印sleep和usleep前后的系统时间戳
#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;
}运行结果：
sleep before time: 1560257692
sleep after time: 1560257792
======================
usleep before time: 1560257792
usleep after time: 1560257792
从运行结果来看，sleep()方法执行前后的时间戳相减正好是sleep_time大小，打印的时间戳是以秒为单位的，所以sleep()方法将程序挂起使用的单位是秒级别的；从usleep()执行前后的时间戳来看，感觉上程序并没有被挂起，这是因为usleep()方法将程序挂起的使用的单位是微秒级别的（1,000,000 微秒 = 1秒）所以程序看起来并没有被挂起，下面我们将打印的时间戳改为微秒级别的看看。
可以通过<sys/time.h>库中的gettimeofday()方法来拿到当前系统的微秒级别时间。
#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;
}运行结果
sleep before time: 1560260201
sleep after time: 1560260301
======================
usleep before time: 1560260301
usleep after time: 1560260301
======================
not usleep before time: 1560260301s,491772微秒
not usleep after time: 1560260301s,491948微秒
======================
usleep before time: 1560260301s,491962微秒
usleep after time: 1560260301s,492191微秒
从使用usleep(0)和使用了usleep(100)方法来看，执行前后的时间差差不多相差100，从而可以看出usleep()是以微秒级别为单位的。
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