[Haskell-cafe] How to best deal with nullPtr from alloca and friends?

Jason Dagit dagitj at gmail.com
Tue Mar 29 06:43:10 CEST 2011

I was reading up on the documentation for alloca and friends[1], which says,
"If any of the allocation functions fails, a value of

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

It seems that actually, alloca and friends throw exceptions:
dmwit> main = allocaArray (2^30) (\ptr -> print ((nullPtr :: Ptr Double) ==
<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

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?


[2] http://en.wikibooks.org/wiki/Haskell/FFI
[3] http://www.haskell.org/haskellwiki/HSFFIG/Examples
[4] http://book.realworldhaskell.org/read/interfacing-with-c-the-ffi.html
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