[Haskell-cafe] Re: Efficient way to edit a file

Maurício briqueabraque at yahoo.com
Mon Jun 19 16:59:47 EDT 2006

David Roundy wrote:
> On Fri, Jun 02, 2006 at 12:34:51PM +1000, Donald Bruce Stewart wrote:
>>>>  Hi,
>>>>  I need to edit big text files (5 to 500 Mb). But I just need to 
>>>>change one or two small lines, and save it. What is the best way to do 
>>>>that in Haskell, without creating copies of the whole files?
>>Thinking further, since you want to avoid copying on the disk, you need
>>to be able to keep the edited version in memory. So the strict
>>bytestring would be best, for example:
> dons is right here, but I'd add that it's hard to safely edit a big
> file without creating a copy, if you want your program to leave the
> file in a consistent state even if it crashes (power failure, kill,
> file server failure).  dons' suggestion could leave you with a deleted
> file (if power goes down at the beginning of a write).  If you aren't
> changing the size of the file, opening it ReadWrite will allow you to
> modify it reasonably safeily.  If you *are* changing its size, then
> doing something explicit would probably be the way to go (and I'd
> probably actually use mmap and memmove to make the change, if you do
> need to modify the file size).  But then, I'm thinking posix (as I
> generally do), which may not be the case for you.  And perhaps you
> don't need to be careful.  I've found that if bad things can happen,
> they do.  But that's largely because darcs has lots of users...

   I like very much the idea of memory mapping the file. In some 
situations, random access to the file would help a lot. Can I do that on 
   Also: safety is not a concern. I can delete those big files as much 
as I need, they are only temporary data.


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