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

[Micah J. Cowan]

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.
> 
> Reading isn't a problem -- I can use fseek or lseek or whatever and read
> the data I want using fread or read.  No problem.
> 
> Writing is another matter.  When I seek to the spot and write the modified
> data, it destroys the data that comes afterwards.
> 
> There's another catch: I gotta use 64-bit safe calls because the file I'm
> reading/writing is quite big (> 2GB).  And since the file is so big, I
> can't store the data that follows to prevent it from getting overwritten.
> 
> Anybody know how to do this?  I'm so disheartened.  Please help!!  Thanks!

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. The
temporary file is now the file you want, so just replace the original
with it.

This is the only real way to do what you wish with large files: this
is why large database files typically are structured so that changes
are not made in place, but appended to the end of the file. The PDF
format is also structured this way: modifications can appended to the
end of the file (no idea if this actually gets used much).

-Micah