How to debug - Python CTypes causing glibc detected

Question

I've compiled a C++ program into a dynamic library, which I'm now calling in python via ctypes. However, I get the following error:

*** glibc detected *** ../../bin/python: free(): invalid pointer: 0x00007fbdf8ae3500 ***

How do I figure out if this is an error in the C++ code, or in how I'm using ctypes?

The C++ library runs succesfully when it is linked with a C program, or if I compile it as a standalone program. When combined with ctypes it always crashes at this C++ line:

// ... A bunch of code
if(tempBuffer) {
    delete[] tempBuffer;
    // More code below that never runs...

Python code that fails:

def test(fileName):
    feedbackLib = cdll.LoadLibrary("./libpitch.so")

    # Set return type to a character pointer
    feedbackLib.getPitchString.restype = c_char_p
    feedbackLib.getPitchString.argtypes = [c_char_p]

    # Crashes Here!
    feedbackStr = feedbackLib.getPitchString(fileName)
    return feedbackStr

For some reason, I can get the python program to work successfully if I change things around like so:

def test(fileName):
    # I've only changed this line
    feedbackLib = CDLL("./libpitch.so")

    # Set return type to a character pointer
    feedbackLib.getPitchString.restype = c_char_p
    feedbackLib.getPitchString.argtypes = [c_char_p]

    # No longer crashes!
    feedbackStr = feedbackLib.getPitchString(fileName)
    return feedbackStr

However, if I start adding things to the above function (which I need to do) it'll give me other glibc errors.

Here's information on how I compiled the C++ in case it's relevant.

Header of function I'm calling, using extern "C" seems to be how to get C++ code working with CTypes:

extern "C" char* getPitchString(char* filename);

g++ commands:

$(CXX) $(LDFLAGS) -fPIC -c $(SRCS) $(LIBS)
$(CXX) -shared -Wl,-soname,libpitch.so -o libpitch.so $(OBJS) $(LIBS)

The Python code looks correct. Why is getPitchString calling free? — Mark Tolonen
– Mark Tolonen, Commented Mar 28, 2012 at 0:33
getPitchString calls a bunch of code from within the C++ library. — Alexander Soto
– Alexander Soto, Commented Mar 28, 2012 at 7:07

Alexander Soto · Accepted Answer · 2012-03-28 07:06:41Z

2

Turns out there was an error in the C++ portion of the code. I managed to get that part to segfault independently of Python/CTypes, and after debugging it seems to work.

answered Mar 28, 2012 at 7:06

Alexander Soto

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