localtime 和 localtime_r 的区别

转自：http://blog.csdn.net/maocl1983/article/details/6221810
#include <cstdlib>
#include <iostream>
#include <time.h>
#include <stdio.h>

using namespace std;

int main(int argc, char *argv[])
{
    time_t tNow =time(NULL);
    time_t tEnd = tNow + ;
    //注意下面两行的区别
    struct tm* ptm = localtime(&tNow);
    struct tm* ptmEnd = localtime(&tEnd);

    char szTmp[] = {};
    strftime(szTmp,,"%H:%M:%S",ptm);
    char szEnd[] = {};
    strftime(szEnd,,"%H:%M:%S",ptmEnd);

    printf("%s /n",szTmp);
    printf("%s /n",szEnd);

    system("PAUSE");
    return EXIT_SUCCESS;
}

最后出来的结果是：:: ::  和最初想法不一致。查阅localtime的文档，发现这段话：This structure is statically allocated and shared by the functions gmtime and localtime. Each time either one of these functions is called the content of this structure is overwritten.也就是说每次只能同时使用localtime()函数一次，要不就会被重写！The localtime() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.因此localtime()不是可重入的。同时libc里提供了一个可重入版的函数localtime_r()；Unlike localtime(), the reentrant version is not required to set tzname。  修改程序：
#include <cstdlib>
#include <iostream>
#include <time.h>
#include <stdio.h>

using namespace std;

int main(int argc, char *argv[])
{
    time_t tNow =time(NULL);
    time_t tEnd = tNow + ;

    //在这里修改程序
    //struct tm* ptm = localtime(&tNow);
    //struct tm* ptmEnd = localtime(&tEnd);
    struct tm ptm = {  };
    struct tm ptmEnd = {  };
    localtime_r(&tNow, &ptm);
    localtime_r(&tEnd, &ptmEnd);

    char szTmp[] = {};
    strftime(szTmp,,"%H:%M:%S",&ptm);
    char szEnd[] = {};
    strftime(szEnd,,"%H:%M:%S",&ptmEnd);
    printf("%s /n",szTmp);
    printf("%s /n",szEnd);

    system("PAUSE");
    return EXIT_SUCCESS;
}

最后出来的结果是：::  ::