How to best deal with nullPtr from alloca and friends?

Jason Dagit dagitj at
Tue Mar 29 19:55:14 CEST 2011

Attached is a patch to amend the documentation.  The diff is a bit hard to
read because I used vim to reflow the text after changing it.  The final
result looks good, but the diff looks bad.  Sorry about that!


On Tue, Mar 29, 2011 at 10:45 AM, Jason Dagit <dagitj at> wrote:

> Sending this question to Haskell-Cafe may have been a mistake on my part.
>  Is the library list the right place for this?
> Since posting this, I'm now convinced that the documentation in the
> haddocks are wrong.  It seems that alloca is designed to never return
> nullPtr whether that means crashing or throwing an exception.
> Do I just update the haddocks and submit a patch some where?
> ---------- Forwarded message ----------
> From: Jason Dagit <dagitj at>
> Date: Mon, Mar 28, 2011 at 9:43 PM
> Subject: How to best deal with nullPtr from alloca and friends?
> To: Haskell Cafe <haskell-cafe at>
> I was reading up on the documentation for alloca and friends[1], which
> says, "If any of the allocation functions fails, a value of nullPtr<> is
> produced."
> It seems like every example of FFI code that I find which uses alloca
> ignores the possibility of a nullPtr[2, 3, 4].
> So then I started trying out examples with the help of dmwit and others
> from #haskell.
> It seems that actually, alloca and friends throw exceptions:
> dmwit> main = allocaArray (2^30) (\ptr -> print ((nullPtr :: Ptr Double) ==
> ptr))
> <dmwit> lispy: alloca also throws an exception.
> <dmwit> lispy: Or rather, allocaBytes throws an exception, and alloca is
> implemented in terms of allocaBytes, so I'm *guessing* that alloca throws an
> exception.
> I'm on a 64bit version of windows here with more than 4GB of memory to
> spare for the GHC process. Unfortunately, allocaBytes takes an Int so I
> can't test it with a request larger than the amount of physical ram I have.
> Just playing around with different arguments to allocaBytes I can get
> different behavior:
>  <= 2^30 bytes, works perfectly
>  == 2^30 + 2^20 bytes, I get an "<interactive>: out of memory" message and
> ghci terminates
>  == 2^31-1, I get a crash where windows pops up a little dialog saying my
> program (ghci) has crashed.
> The behavior seems to be inconsistent with the documentation.
> What is the correct behavior for alloca and friends and should I be
> checking for nullPtr?
> Thanks,
> Jason
> [1]
> [2]
> [3]
> [4]
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