安丘网站建设报价/电脑优化软件

背景：

        需要将起始时间存储下来（要求格式为年-月-日-时:分:秒），以便下次进入循环时能与当前时间做比较。

解决方案：

        显然就是调用linux的localtime时间函数(参考文献1)的，于是乎写出了如下的代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>struct tm* old_time;void InitTime(void)
{time_t timep;time(&timep);old_time = localtime(&timep);	
}int main(int argc, char* argv[])
{time_t timep;struct tm* p;InitTime();while(1){time(&timep);p = localtime(&timep);if(memcmp(p, old_time, sizeof(struct tm))==0){printf("Same %d\n",old_time->tm_sec);}else{printf("Different %d\n", old_time->tm_sec);memcpy(old_time, p, sizeof(struct tm));}int i, j;for(i=0; i<100000;++i)for(j=0; j<1000;++j);}return 0;
}

        但运行结果，总是打印“Same”而且秒数一直在递增，很是奇怪。

       经过分析以及调研后，发现localtime返回的是一个地址，指向内核申请的一个固定地址，所以old_time和p都是指向同一个地方，所以memcmp的返回值肯定是0。

        为了达到目的，可行的解决方法就是将每次localtime返回后立即将“时间内容”存储到本地保存。如下所示。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>struct tm* old_time;void InitTime(void)
{time_t timep;time(&timep);//old_time = localtime(&timep);old_time = (struct tm*)malloc(sizeof(struct tm));memcpy(old_time, localtime(&timep), sizeof(struct tm));
}int main(int argc, char* argv[])
{time_t timep;struct tm* p;p = (struct tm*)malloc(sizeof(struct tm));InitTime();while(1){time(&timep);//p = localtime(&timep);memcpy(p, localtime(&timep), sizeof(struct tm));if(memcmp(p, old_time, sizeof(struct tm))==0){printf("Same %d\n",old_time->tm_sec);}else{printf("Different %d\n", old_time->tm_sec);memcpy(old_time, p, sizeof(struct tm));}int i, j;for(i=0; i<100000;++i)for(j=0; j<1000;++j);}return 0;
}

       于是，对localtime更进一步的探寻，本来是希望能够找出localtime的内部实现的，但从GUN的FTP上下下来后表示看不懂，待看懂的时候再来更新此博文。

/* Convert `time_t' to `struct tm' in local time zone.Copyright (C) 1991,92,93,95,96,97,98,2002 Free Software Foundation, Inc.This file is part of the GNU C Library.The GNU C Library is free software; you can redistribute it and/ormodify it under the terms of the GNU Lesser General PublicLicense as published by the Free Software Foundation; eitherversion 2.1 of the License, or (at your option) any later version.The GNU C Library is distributed in the hope that it will be useful,but WITHOUT ANY WARRANTY; without even the implied warranty ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNULesser General Public License for more details.You should have received a copy of the GNU Lesser General PublicLicense along with the GNU C Library; if not, write to the FreeSoftware Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA02111-1307 USA.  */#include <time.h>/* The C Standard says that localtime and gmtime return the same pointer.  */
struct tm _tmbuf;/* Return the `struct tm' representation of *T in local time,using *TP to store the result.  */
struct tm *
__localtime_r (t, tp)const time_t *t;struct tm *tp;
{return __tz_convert (t, 1, tp);
}
weak_alias (__localtime_r, localtime_r)/* Return the `struct tm' representation of *T in local time.  */
struct tm *
localtime (t)const time_t *t;
{return __tz_convert (t, 1, &_tmbuf);
}
libc_hidden_def (localtime)

总结

        折腾好一会儿总算是解决了，感觉还是自己太马虎，既然返回的是一个地址，就应该敏锐注意到不能随意使用，毕竟所调用的函数也掌握了该地址，可以对其任意更改甚至销毁。

参考文献

[1] http://www.cnblogs.com/xmphoenix/archive/2011/05/09/2041546.html