subject_35335.htm

一些关于vc的问答

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序号：35335 发表者：铁骑柔肠 发表日期：2003-04-05 20:42:50
<br>主题：关于fseek
<br>内容：#include"stdio.h"<BR>#include"ctype.h"<BR><BR>int main(void)<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;FILE *fp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;char a;<BR>&nbsp;&nbsp;&nbsp;&nbsp;if((fp=fopen("rrr","r+b"))==NULL)<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("Can't open %s!\n","rrr");<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit(1);<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR><BR>a=fgetc(fp);<BR>while(a!=EOF)<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp; if(islower(a))<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a=toupper(a);<BR>&nbsp;&nbsp;&nbsp;&nbsp; fseek(fp,-1,1);<BR>&nbsp;&nbsp;&nbsp;&nbsp; printf("%c",a);<BR>&nbsp;&nbsp;&nbsp;&nbsp; fputc(a,fp);<BR>&nbsp;&nbsp;&nbsp;&nbsp; fseek(fp,0,1);&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; /* 为什么一定要这句呢？？（去掉了就死循环了）*/<BR>&nbsp;&nbsp;&nbsp;&nbsp; a=fgetc(fp); /* if placed in the first line? */<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR>}
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回复者：悌悌 回复日期：2003-04-07 14:00:57
<br>内容：铁骑柔肠:<BR> 知道了怎么回事么？我单步跟踪了一下，并用ftell()来跟踪文件指针，我也想不通是怎么回事。我修改后的代码如下：<BR>#include "stdafx.h"<BR>#include "stdlib.h"<BR>#include"stdio.h"<BR>#include"ctype.h"<BR><BR>int main(void)<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;long m;<BR>&nbsp;&nbsp;&nbsp;&nbsp;FILE *fp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;char a;<BR>&nbsp;&nbsp;&nbsp;&nbsp;if((fp=fopen("c:\\12.txt","r+b"))==NULL)<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("Can't open %s!\n","rrr");<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exit(1);<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR><BR>&nbsp;&nbsp;m=ftell(fp);//跟踪文件指针<BR>a=fgetc(fp);<BR>&nbsp;&nbsp;m=ftell(fp);//跟踪文件指针<BR>while(a!=EOF)<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp; if(islower(a))<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a=toupper(a);<BR>&nbsp;&nbsp;&nbsp;&nbsp; fseek(fp,-1,1);<BR>&nbsp;&nbsp;&nbsp;&nbsp; m=ftell(fp);//跟踪文件指针<BR>&nbsp;&nbsp;&nbsp;&nbsp; printf("%c",a);<BR>&nbsp;&nbsp;&nbsp;&nbsp; fputc(a,fp);<BR>&nbsp;&nbsp;&nbsp;&nbsp; m=ftell(fp);//跟踪文件指针<BR>&nbsp;&nbsp;&nbsp;&nbsp; fseek(fp,0,1);&nbsp;&nbsp;/* 为什么一定要这句呢？？（去掉了就死循环了）*/<BR>&nbsp;&nbsp;&nbsp;&nbsp; m=ftell(fp);//跟踪文件指针<BR>&nbsp;&nbsp;&nbsp;&nbsp; a=fgetc(fp); /* if placed in the first line? */<BR>&nbsp;&nbsp;&nbsp;&nbsp; m=ftell(fp);//跟踪文件指针<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR>return 0;<BR>} 
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回复者：yingpf 回复日期：2003-04-08 14:59:13
<br>内容：去掉了fseek(fp,0,1); 没有死循环啊，只不过只转换了第一个字符。<BR>为什么移动0 个字符也会有影响呢？真想不通。。。。。
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回复者：悌悌 回复日期：2003-04-09 08:54:37
<br>内容：怎么没有人来解决呢？高手请多帮忙呀！
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回复者：neilgan 回复日期：2003-04-09 11:57:13
<br>内容：Very interesting question.<BR><BR>The reason is: fputc doesn't really put the char to the file, it just puts it into a buffer, but fgetc still uses the old content, so it returns the second character repeatly.<BR><BR>The solution is: put a fflush(fp) after fputc to force the flush of the stream.<BR><BR>fseek calls fflush internally.<BR>
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回复者：悌悌 回复日期：2003-04-11 13:48:12
<br>内容：请问neilgan哪里有相关的文档说明么？能介绍一下么？多谢的了。不过，我还是觉得不是很能让人信服。fputc()是把字符送入到buffer，但在fputc()后我对文件指针的测试即：m=ftell()得到的结果是文件指针指向了下一个字符。按你的说法，ftell(）判断的也是buffer里的指针？显然不对的呀。希望得到解答，多谢<BR>2003-4-11 13:57:00