请教：fopen的文件使用方式"a"的问题

bobzone

有个数据文件powermeter，假设原来为10KByte。
我要从第100Byte到第199Byte改写信息，还要在文件开始处的第2000Byte开始追加100Byte的信息。

我之前的做法是：
fp=fopen(filename,"a");
fseek(fp,0,SEEK_SET);//我以为此时光标位置是文件开头，结果发现光标位置是文件的末尾，也就是说“a”只能以追加方式写，只能定位到原文件末尾，而不能是原文件中间的某个位置


请问：我现在需要改写原文件，又要追加信息，该怎么办？

先谢谢啦！

yjh777

fopen("filename", "r+");

man fopen

emmoblin

用rw打开，然后用seek定位就行了

yjh777

原帖由 emmoblin 于 2009-10-13 23:36 发表
用rw打开，然后用seek定位就行了

提醒：fopen 没有 rw ，用rw和r一样；读写是r+

DESCRIPTION
       The fopen() function opens the file whose name is the string pointed to by path and associates a stream with it.
       The argument mode points to a string beginning with one of the following sequences (Additional characters may follow these sequences.):
       r      Open text file for reading.  The stream is positioned at the beginning of the file.
       r+     Open for reading and writing.  The stream is positioned at the beginning of the file.
       w      Truncate file to zero length or create text file for writing.  The stream is positioned at the beginning of the file.
       w+     Open for reading and writing.  The file is created if it does not exist, otherwise it is truncated.  The stream is positioned at the begin-
              ning of the file.
       a      Open for appending (writing at end of file).  The file is created if it does not exist.  The stream is positioned at the end of the file.
       a+     Open for reading and appending (writing at end of file).  The file is created if it does not exist.  The initial file position for  reading
              is at the beginning of the file, but output is always appended to the end of the file.

@sky

rw记得可以用

yjh777

原帖由 @sky 于 2009-10-14 17:53 发表
rw记得可以用

记错了吧，至少标准C里没有，，

bobzone

谢谢大家。新手就是新手，还没有养成man的习惯。
吸取教训。