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2003 May 21 11:37

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Re: [vox-tech] [C] randomly accessing file data in C
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Re: [vox-tech] [C] randomly accessing file data in C





--On Wednesday, May 21, 2003 09:05:15 -0700 "Micah J. Cowan" <micah@cowan.name> wrote:

On Tue, May 20, 2003 at 06:45:53PM -0700, Mark K. Kim wrote:
Hello...

I'm trying to access data in a file in C.  I want to read some portion
of it, modify it, then rewrite that portion back into the file.
I don't think what you're trying to do is possible. What you *can* do,
is read in the file in chunks of an appropriate size for keeping in
memory, and writing them out to a new (temporary) file as you go. As
soon as you reach the spot you need to modify, you just modify the
data you are going to write out to the new file, and then copy the
tail end of the original, chunk-by-chunk, into the temporary file.
Er, what? Overwriting part of a file is a normal operation. Kim's problem is that he's opening the output file in "w+" mode, which truncates the file.

The copy-and-modify scheme you outline is generally used to replace a chunk of the file with a different-sized chunk, e.g. to replace some text within a text file.
--
Kenneth Herron Kherron@newsguy.com 916-366-7338
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