ByteString I/O Performance

Seth Kurtzberg seth at
Mon Sep 3 00:08:06 EDT 2007

-----Original Message-----
From: Donald Bruce Stewart [mailto:dons at] 
Sent: Monday, September 03, 2007 12:03 AM
To: Seth Kurtzberg
Cc: libraries at
Subject: Re: ByteString I/O Performance

> -----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).

OK.  Assume that I'm not doing any C coding, so that the only C code that is
invoked is called from within the implementation (in this case the
implementation of System.IO).  Can I assume that no implementation code
modifies the string?  In other words, is it valid to assume that the side
condition is never violated so long as I don't violate the side condition in
my own C code (if any)?

-- Don

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