David Roundy wrote:
On Fri, Jun 02, 2006 at 12:34:51PM +1000, Donald Bruce Stewart wrote:
dons:
briqueabraque:
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 Windows? Also: safety is not a concern. I can delete those big files as much as I need, they are only temporary data. Maurício