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

[Yuras Shumovich]

Ok, then I don't see anything obviously wrong.

I was embeding v8 into haskell project few months ago, and almost the
same code works for me.

I don't have v8 installed right now, so I can't try your code at the
moment, but I see only one not trivial difference with my code. I'm
creating isolate manually, and you are using the default one. Check that
Isolate::GetCurrent() returns anything. I can imaging that your code is
running in unbound green thread, or it is bound to OS thread other then
the main one. v8 use thread local storage, that can be the issue.

On Tue, 2013-11-19 at 14:19 -0200, Thiago Padilha wrote:
> 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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> >> http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >