[Haskell-cafe] Need help with very simple C++ FFI call

[Yuras Shumovich]

Hello,

IIRC, strlen doesn't count the terminating '\0', so you'll need to
malloc(strlen(*ascii) + 1).

Not sure why python works, maybe just different memory layout hides the
bug.

Thanks,
Yuras

On Tue, 2013-11-19 at 12:59 -0200, Thiago Padilha wrote:
> Hi
> 
> I'm trying write a ffi binding to the v8 javascript engine (C++
> library). Theres one already at https://github.com/sol/v8 but I want
> to do it again as an exercise to improve by haskell skills.
> 
> I chose this library be because it will force me to deal with impure
> functions using monads. Since its not a trivial library, it will also
> improve my knowledge of writing FFI bindings and cabal packages in
> case I want to wrap more C++ libraries in the future.
> 
> My first step is to port the 'hello world' example in v8 embedders
> guide(https://developers.google.com/v8/get_started) as a haskell
> binding. So I have downloaded/compiled v8 and created the following
> directory structure:
> 
> src/V8.hs
> cbits/haskell-v8.cpp
> # v8 libraries
> deps/lib/libv8_base.a
> deps/lib/libv8_snapshot.a
> # v8 headers
> deps/include/
> 
> Here's the contents of the haskell-v8.cpp file:
> http://pastebin.com/RfYCCxFQ (basically a copy of the v8 embedders
> example function, except that it returns a char pointer).
> 
> and here's the haskell module: http://pastebin.com/fnXVbEeU
> 
> I compiled a shared library exposing the c "hello" function with the
> following command:
> 
>     $ g++ \
>       -shared -o libhaskellv8.so \
>       -I./deps/include \
>       -L./deps/lib \
>       ./cbits/haskell-v8.cpp \
>       -lv8_base \
>       -lv8_snapshot \
>       -lpthread \
>       -lrt
> 
> And loaded into ghci with the following:
> 
>     $ ghci -L. -lhaskellv8 src/V8.hs
> 
> The problem is that I get a segmentation fault when I call the 'hello'
> function from ghci. To ensure the library was compiled correctly, I
> successfully called it from python:
> 
>     $ python -c \
>       'from ctypes import CDLL, c_char_p; lv8=CDLL("libhaskellv8.so");
> lv8.hello.restype = c_char_p; print lv8.hello()'
>     Hello, World!
> 
> I have selectively uncommented lines from the c++ file, and the
> segmentation faults starts happening when I uncomment line 14
> (HandleScope handle_scope(isolate)).
> 
> What am I missing here?
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