[Haskell-cafe] Lazy IO and closing of file handles

Donald Bruce Stewart dons at cse.unsw.edu.au
Thu Mar 15 01:20:26 EDT 2007

> > When using readFile to process a large number of files, I am exceeding
> > the resource limits for the maximum number of open file descriptors on
> > my system.  How can I enhance my program to deal with this situation
> > without making significant changes?
> I note that if you use mmap(2) to map a disk file into virtual memory,
> you may close the file descriptor afterwards and still access the data.
> That might also be relatively economical in other respects.  Pardon me
> if this has already been suggested.
> I should know how to make ByteStrings from offsets into a mapped region,
> but all I can say right now is, I'm pretty sure that would be no problem.
> Don't give them any finalizer.

In fact, the (commented out code) for mmapFile :: FilePath -> IO ByteString
is in Data.ByteString now. You can also provide munmap as the finaliser,
and let the GC take care of that.

-- Don

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