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

[Thiago Padilha]

Thanks for pointing the strlen issue, but as I stated before, the
segmentation fault occurs even before the copying the resulting
string. To illustrate the error better, heres a very trimmed(20 lines
long) version of the c++ file that still causes the segmentation
fault: http://pastebin.com/cJvkLQPg . Commenting line 14 will fix the
segfault.

On Tue, Nov 19, 2013 at 1:06 PM, Yuras Shumovich <shumovichy at gmail.com> wrote:
>
> 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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>
>