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How I resolve Cppyy load_library giving Runtime error?

Okay so according to the answer I found here to the question titled "Calling C/C++ from Python?" here, and also on the cppyy documentation website, I made some sample classes in .h and .cpp files and tried to include them in Python. While the .h file gets included easily, when I try to use the cppyy.load_library() function, it gives me a runtime error for some reason. Can someone please help? I've tried to look for solutions online but apparently no one has got a similar problem before. This is what I'm running in Jupyter Notebook:
import cppyy
cppyy.include("foo.h")
cppyy.load_library("libfoo")
the final line is giving me the following error:
RuntimeError Traceback (most recent call last)
<ipython-input-3-eea6173ad08e> in <module>
----> 1 cppyy.load_library("libfoo")
~\anaconda3\lib\site-packages\cppyy\__init__.py in load_library(name)
219 sc = gSystem.Load(name)
220 if sc == -1:
--> 221 raise RuntimeError('Unable to load library "%s"%s' % (name, err.err))
222 return True
223
RuntimeError: Unable to load library "libfoo"
This is my .h file:
class Foo {
public:
void bar();
};
And here is my .cpp file:
#include "foo.h"
#include <iostream>
void Foo::bar() { std::cout << "Hello" << std::endl; }
I'm using the commands g++ -c -fPIC foo.cpp -o foo.o and g++ -shared -Wl,-soname,libfoo.so -o libfoo.so foo.o to compile my cpp code.
Please can someone help?

The example works just fine for me.
To debug, first make sure to start python in the same directory where libfoo.so is located, or to add the directory where libfoo.so lives to LD_LIBRARY_PATH (for any process to use), or call cppyy.add_library_path() with the path as argument to add it for use by cppyy only. As concerns the name, the .so extension is automatically added as needed, as is the lib part, so either one of foo, libfoo, foo.so, or libfoo.so is fine.
If that still fails, a reasonable way on Linux (only) of getting more information for what could be going wrong, is to use ctypes:
$ python
>>> import cypes
>>> lib = ctypes.CDLL("libfoo.so")
which will show you if there are different problems such as missing symbols or missing dependent libraries (but neither is the case here).

pybind with third-party C++ on Windows (ImportError: DLL load failed)

I'm trying to use a third-party c++ lib from python using pybind11.
I have managed to do it using pure C++ code, but, when I add the third-party code I get the following error:
ImportError: DLL load failed while importing recfusionbind
Here's my code:
#include <RecFusion.h>
#include <pybind11/pybind11.h>
using namespace RecFusion;
bool isValidLicense() {
return RecFusionSDK::setLicenseFile("License.dat");
}
namespace py = pybind11;
PYBIND11_MODULE(recfusionbind, m) {
m.def("isValidLicense", &isValidLicense, R"pbdoc(
Check if there is a valid license file.
)pbdoc");
#ifdef VERSION_INFO
m.attr("__version__") = VERSION_INFO;
#else
m.attr("__version__") = "dev";
#endif
}
If i change, for testing purposes, the return of the function to simply true it works just fine. But calling the third-party library gives the above mentioned error. I have put RecFusion.dll on the current directory (where the python script is), with same results.
Any tips on what I'm missing will be welcomed. Lib is 64 bits, same as my code.

This seems to me like a linking problem. Look at your dll's import table (with tools like PE explorer or IDA). Now validate that the function RecFusionSDK::setLicenseFile is indeed imported from a dll named RecFusion.dll and not some other dll name.
Also validate that the mangled function name in RecFusion.dll's export table and in your dll match. If not, this could be an ABI problem.

Swig/python : when SWIG_init() is needed?

Hi everyone and thanks for trying to help me !
I encounter trouble when trying to import a python module generated by swig.
I have a basic library "example" containing few methods.
Next to it I have a main program dynamically linked to python.
This program imports the generated module and calls a function in it.
If my library example is a shared one, named _example.so, everything works perfectly, and I can import it in python.
But if my library is static, _example.a, and linked to the main program, then I will have the error "no module named _example was found" unless I add a call to SWIG_init() in the main function.
What exactly does SWIG_init() , and when should I use it ? It seems quite weird to me because it's never said in the documentation to do such a call.
I know that dealing with a .so shared library is better but I try to reproduce the behavior of what I have on a big project at work, so I really have to understand what happens when the module is static.
Here is my main file :
#include "Python.h"
#include <iostream>
#if PY_VERSION_HEX >= 0x03000000
# define SWIG_init PyInit__example
#else
# define SWIG_init init_example
#endif
#ifdef __cplusplus
extern "C"
#endif
#if PY_VERSION_HEX >= 0x03000000
PyObject*
#else
void
#endif
SWIG_init(void);
int main (int arc, char** argv)
{
Py_Initialize();
SWIG_init(); // needed only using the statically linked version of example ?
PyRun_SimpleString("print \"Hello world from Python !\"");
PyRun_SimpleString("import sys");
PyRun_SimpleString("sys.path.append(\"/path/to/my/module\")");
PyRun_SimpleString("import example");
PyRun_SimpleString("a = example.Example()");
PyRun_SimpleString("print a.fact(5)");
}
Here is how things are generated :
swig -c++ -python example.i
g++ -fpic -c example.cpp example_wrap.cxx -I/include/python2.7 -lstdc++
ar rvs libexample.a example.o example_wrap.o
// to generate dynamic instead of static : g++ -shared example.o example_wrap.o -o _example.so
g++ main.cpp -I/include/python2.7 libexample.a -lstdc++ -L/lib/python -lpython2.7 -o main

What you are calling is the init function of the native python module _example that is loaded by the SWIG generated python wrapper. For python 2 this function is named init_example, and for python 3 it is named PyInit__example.
Every python extension with C or C++ needs such a function, it basically initializes everything and registers the name of the module and all the methods available for it. In your case SWIG has generated this function for you.
The reason you have to call this function yourself when you compiled the library statically is simply that the python wrapper example imports the native module _example which is by the python convention a shared object, which you did not compile, and which is thus not found.
By calling SWIG_init, you "preload" the module, so python does not try to reimport it, so it works even though there is no shared object anywhere on the python module path.
If you have the shared object for your module, python will call this function for you after loading the shared object and you don't have to worry about this.

Compile c++ Programm with CMake that uses Boost/python.hpp

I'm currently trying to compile a c++ file using CMake.
But since I'm using Boost::python it won't compile.
I set up a little test file to figure out what I need to do but I just can't get it to work.
Any Help would be greatly appreciated!!
The test file:
#include <Python.h>
#include <boost/python.hpp>
#include <iostream>
using std::cout;
using std::endl;
int main()
{
namespace py = boost::python;
Py_Initialize();
// Retrieve the main module's namespace
py::object global(py::import("__main__").attr("__dict__"));
py::exec("print 'Hello from Python!' \n", global, global);
return 0;
}
It will compile if I just use,
clang++ -I/usr/include/python2.7 -lpython2.7 -lboost_python -std=c++11 boosttest.cpp -o boosttest
I tried this CMakeLists.txt to get it to work.
cmake_minimum_required(VERSION 3.2)
FIND_PACKAGE(PythonLibs)
FIND_PACKAGE(Boost)
INCLUDE_DIRECTORIES(${Boost_INCLUDE_DIRS} ${PYTHON_INCLUDE_DIRS})
LINK_LIBRARIES(${Boost_LIBRARIES} ${PYTHON_LIBRARIES})
add_executable(Test1 boosttest.cpp)
target_link_libraries(Test1 ${Boost_LIBRARIES} ${PYTHON_LIBRARIES})
and what I get is
undefined reference to `boost::python::import(boost::python::str)'
and a couple more of the same category.

Thanks for your help Mark, thanks to the new errors after I included
find_package(Boost REQUIRED python)
I was able to figure out that the problem was that CMake selected the libs for python 3.4 but Boost was build against 2.7.
So the Solution was to include the version as so:
FIND_PACKAGE(PythonLibs 2.7 REQUIRED)

Did you try
find_package(Boost REQUIRED python)
also run with verbosity to see what is going on
cmake . --debug-output
make VERBOSE=1

Building a DLL with MinGW and loading it with Python's ctypes module

I am trying to build a C DLL which can be loaded within python using ctypes.windll.loadlibrary(...)
I can create a DLL and a client program all in C which work following the MinGW tutorial at http://www.mingw.org/wiki/MSVC_and_MinGW_DLLs.
When I try to load the same DLL within python I get an error:
OSError: [WinErrror 193] %1 is not a valid Win32 application
Can someone give me some idea as to what I am doing incorrectly?
Here are the files:
noise_dll.h
#ifndef NOISE_DLL_H
#define NOISE_DLL_H
// declspec will identify which functions are to be exported when
// building the dll and imported when 'including' this header for a client
#ifdef BUILDING_NOISE_DLL
#define NOISE_DLL __declspec(dllexport)
#else
#define NOISE_DLL __declspec(dllimport)
#endif
//this is a test function to see if the dll is working
// __stdcall => use ctypes.windll ...
int __stdcall NOISE_DLL hello(const char *s);
#endif // NOISE_DLL_H
noise_dll.c
#include <stdio.h>
#include "noise_dll.h"
__stdcall int hello(const char *s)
{
printf("Hello %s\n", s);
return 0;
}
I build the DLL with:
gcc -c -D BUILDING_NOISE_DLL noise_dll.c
gcc -shared -o noise_dll.dll noise_dll.o -Wl,--out-implib,libnoise_dll.a
The python code is simply:
import ctypes
my_dll = ctypes.windll.LoadLibrary("noise_dll")
and I get the error above: '%1 is not not a valid Win32 application'
I know the DLL is not completely wrong, because if i create a client file:
noise_client.c
#include <stdio.h>
#include "noise_dll.h"
int main(void)
{
hello("DLL");
return 0;
}
and build with:
gcc -c noise_client.c
gcc -o noise_client.exe noise_client.o -L. -lnoise_dll
I get a working executable. I have some understanding of all of what goes on in the code, options and preprocessor directives above, but am still a little fuzzy on how the .dll file and the .a file are used. I know if I remove the .a file I can still build the client, so I am not even sure what its purpose is. All I know is that it is some kind of archive format for multiple object files
I can ctypes.windll.loadlibrary(...) an ordinary windows DLL found in the windows/system32 without an issue.
One final point:
I am using 64 bit python 3.3. I am using the version of minGW tat comes with the recommended installer (mingw-get-inst-20120426.exe). I am not sure if it is 32 bit, or if that matters.
Thanks!

Use 32-bit python - your DLL probably wasn't compiled as 64 bit code.