Hello everyone,

I'm currently calling Haskell code from C. My goal is to apply a Haskell function to each element of some dataset that I get from outside Haskell-land. For now, before I make this fancier and more efficient, the plan is to just bring the RTS up with hs_init, call my Haskell function (exporter to C with 'foreign export ccall [...]') on the element and finally shut the RTS down, doing this for every element.

When running this, I realized the RTS was running into a segfault when calling the Haskell function on the second element. This led me to believe it wasn't possible to call hs_init()/hs_exit() multiple times in the same program. But then I checked the Haskell 2010 report, FFI section [1], and it says:

The function hs_init() initialises the Haskell system and provides it with the available command line arguments. Upon return, the arguments solely intended for the Haskell runtime system are removed (i.e., the values that argc and argv point to may have changed). This function must be called during program startup before any Haskell function is invoked; otherwise, the system behaviour is undefined. Conversely, the Haskell system is deinitialised by a call to hs_exit(). Multiple invocations of hs_init() are permitted, provided that they are followed by an equal number of calls to hs_exit() and that the first call to hs_exit() is after the last call to hs_init(). In addition to nested calls to hs_init(), the Haskell system may be de-initialised with hs_exit() and be re-initialised with hs_init() at a later point in time. This ensures that repeated initialisation due to multiple libraries being implemented in Haskell is covered.

Which means, if I understand correctly, that what I want, while very inefficient, should work fine.

I've put together a minimal example that exhibits the problem, which can be found at https://github.com/alpmestan/simple-c-export :

- https://github.com/alpmestan/simple-c-export/blob/master/run-haskell.c shows the C code that brings the RTS up and down, with some printf statements to show what's going on.

- https://github.com/alpmestan/simple-c-export/blob/master/Foo.hs shows the trivial Haskell function I'm exposing.

- https://github.com/alpmestan/simple-c-export/blob/master/simple-c-export.cabal contains the build options I'm compiling the code with

When running this on my machine (OS X, ghc 7.8.3 and 7.10.2), I always get:

$ cabal run simple-c
Preprocessing executable 'simple-c' for simple-c-export-0.1...
Linking dist/build/simple-c/simple-c ...
Running simple-c...
#0 - Launching RTS...
#0 - RTS started! Calling Haskell function...
0
#0 - Killing RTS now...
#0 - RTS killed!
#1 - Launching RTS...
#1 - RTS started! Calling Haskell function...
Segmentation fault: 11

Is there something special I should do to make this work, that I'm overlooking? Or is this a bug (that I should report on Trac, I guess) ?

Thanks in advance for any clarification on this.

--

Alp Mestanogullari

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