ByteString I/O Performance

Donald Bruce Stewart dons at
Mon Sep 3 00:02:52 EDT 2007

> -----Original Message-----
> From: libraries-bounces at [mailto:libraries-bounces at] On Behalf Of Bryan O'Sullivan
> Sent: Sunday, September 02, 2007 11:23 PM
> To: Peter Simons
> Cc: libraries at
> Subject: Re: ByteString I/O Performance
> Peter Simons wrote:
> > One way to get malloc() out of the picture would be to provide a
> > variant of hGet that takes an existing, pre-allocated buffer as an
> > argument, so that the user can allocate a ByteString once and re-use
> > it for every single hGet and hPut.
> This is already quite easy to do.  See unsafeUseAsCStringLen in 
> Data.ByteString.Base, and hGetBuf in System.IO.
> Is it possible without resorting to an unsafeXXX function?

They're all 'unsafe' for different reasons :) 
The question should be: why is this unsafe? 

(It's unsafe because it doesn't copy the C string, so you need to have a
side condition that the string isn't modified by C).

-- Don

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