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How to use Ctypes with C++
8
• char * getGreetingStringWrapper(char *val) {
// char * to std::string
std::string str = getGreetingString(std::string(val));
// std::string to char *
char *ret = new char[str.length() + 1];
strcpy(ret, str.c_str());
return ret;
}
• #ifdef __cplusplus
extern "C" {
#endif
char *getGreetingStringWrapper (char *val);
#ifdef __cplusplus
}
#endif
• nm –D example.so
• 0000000000000bba T _Z4facti
0000000000000cad T
_Z9printLineNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
0000000000000bf2 T _Z9printLinev
U _ZNSaIcEC1Ev
U _ZNSaIcED1Ev
U_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEC
1EPKcRKS3
U_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEED
U _ZNSt8ios_base4InitD1Ev
U _ZSt4cout
U
_ZStlsIcSt11char_traitsIcESaIcEERSt13basic_ostreamIT_T0_ES7_RKN
St7__cxx1112basic_stringIS4_S5_T1_EE
C/C++ Workaround](https://image.slidesharecdn.com/wrappingcforpython121119editedfrazier-191114151546/85/Kostiantyn-Grygoriev-Wrapping-C-for-Python-8-320.jpg)
Kostiantyn Grygoriev "Wrapping C++ for Python"
- 1. www.luxoft.com Kostiantyn Grygoriev Technical Lead Wrapping C/C++ for Python
- 2. www.luxoft.com Growth of Major Programming Languages Based on Stack Overflow question views in World Bank high-income countries 2
- 3. www.luxoft.com Top Languages Over Time Based on GitHub statistics by contributors in public and private repositories, organizations of all sizes, and every region of the world. 3
- 5. www.luxoft.com Tools to be Overviewed: • Ctypes • SWIG • Pybind11 • Boost.Python
- 6. www.luxoft.com Ctypes 6 Ctypes is a foreign functions library for Python that provides C-compatible data types. Also, you can use ctypes with libraries written in any language that can export a C compatible API - e.g. Fortran, Rust. The pros and cons: • ctypes is already included with your Python installation • you don’t need to recompile a C/C++ library to use it from Python • you need to manually wrap the data and functions from the foreign library in Python code • not good C++ support
- 7. www.luxoft.com How to use Ctypes with C 7 • Compile C/C++ code into a shared library: gcc -o example.so -shared -fPIC example.c • Tell the system where to find the shared library: export LD_LIBRARY_PATH=. from ctypes import cdll my_lib = cdll.LoadLibrary(“example.so") foo = my_lib.fuct >> print foo(5) 120 • from ctypes import cdll my_lib = cdll.LoadLibrary(“example.so") foo = my_lib.getGreetingString >> print foo("world") 136040696 • from ctypes import c_char_p foo.restype = c_char_p And now it will work: >> print foo(“world”) hello, world Build Process Python Code Example 1 Python Code Example 2
- 8. www.luxoft.com How to use Ctypes with C++ 8 • char * getGreetingStringWrapper(char *val) { // char * to std::string std::string str = getGreetingString(std::string(val)); // std::string to char * char *ret = new char[str.length() + 1]; strcpy(ret, str.c_str()); return ret; } • #ifdef __cplusplus extern "C" { #endif char *getGreetingStringWrapper (char *val); #ifdef __cplusplus } #endif • nm –D example.so • 0000000000000bba T _Z4facti 0000000000000cad T _Z9printLineNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE 0000000000000bf2 T _Z9printLinev U _ZNSaIcEC1Ev U _ZNSaIcED1Ev U_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEC 1EPKcRKS3 U_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEED U _ZNSt8ios_base4InitD1Ev U _ZSt4cout U _ZStlsIcSt11char_traitsIcESaIcEERSt13basic_ostreamIT_T0_ES7_RKN St7__cxx1112basic_stringIS4_S5_T1_EE C/C++ Workaround
- 9. www.luxoft.com SWIG (Simple Wrapper Interface Generator) 9 SWIG is a software development tool that connects programs written in C and C++ with a variety of high-level programming languages. The pros and cons: • Capable of wrapping C/C++ in a large variety of languages (over 20)
- 10. www.luxoft.com C++ features in SWIG 10 SWIG currently supports most C++ features including the following: • Classes • Virtual and static functions • Public inheritance (including multiple inheritance) • Function and method overloading • Operator overloading for standard operators • References and smart pointers • Templates • Pointers to members • Default parameters • Overloaded versions of certain operators (new, delete, etc.) • Nested classes
- 11. www.luxoft.com How to Use SWIG 11 SWIG module file : api.i %module example %include <std_string.i> %{ #define SWIG_FILE_WITH_INIT #include “api.h” %} int fact(int n); std::string getLine(); void printLine(std::string line); • Build a Python module: swig -c++ -python api.i • Compile C/C++ code: gcc -fPIC -c api.c gcc -fPIC -c api_wrapper.c -I/usr/include/python2.7 gcc -shared api.o api_wrapper.o – o _api.so import example print(example.fact(5)) example.printLine(“linen”) print(example.getLine()) C/C++ Additional Code Build Process Python Code
- 12. www.luxoft.com Pybind11 12 Pybind11 is a lightweight header-only library that exposes C++ types in Python and vice versa, mainly to create Python bindings of existing C++ code. The pros and cons: • Header-only • Uses C++11 move constructors and move assignment operators whenever possible to efficiently transfer custom data types • Can automatically vectorize functions • Function signatures are precomputed at compile time (using constexpr), leading to smaller binaries • C++ types can be pickled and unpickled similar to regular Python objects
- 13. www.luxoft.com C++ features in Pybind11 13 The following core C++ features can be mapped to Python • Functions accepting and returning custom data structures per value, reference, or pointer • Instance methods and static methods • Overloaded functions • Arbitrary exception types • Enumerations • Callbacks • Custom operators • Single and multiple inheritance • STL data structures and smart pointers • C++ classes with virtual (and pure virtual) methods can be extended in Python
- 14. www.luxoft.com How to use Pybind11 14 • #include <pybind11/pybind11.h> namespace py = pybind11; PYBIND11_MODULE(my_client, m) { m.doc() = “My plugin for communication with C++"; m.def(“fact", &API::fact); m.def(“factorial", &API::fact,"This function calculates factorial",py::arg(“n")); m.def(“print", py::overload_cast<std::string&>(&API::printLine)); m.def(“print", py::overload_cast<int>(&API::printLine)); } • $ g++ -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` example.cpp -o example`python3-config --extension- suffix` C++ Additional Code Build Process
- 15. www.luxoft.com Boost.Python 15 The Boost Python Library is a framework for interfacing Python and C++. It allows you to quickly and seamlessly expose C++ classes functions and objects to Python, and vice-versa, using no special tools - just your C++ compiler.
- 16. www.luxoft.com Boost.Python vs Pybind11 16 • #include <pybind11/pybind11.h> namespace py = pybind11; PYBIND11_MODULE(my_client, m) { m.def(“fact", &API::fact); } • #include <boost/python.hpp> namespace py = boost::python; BOOST_PYTHON_MODULE(my_client) { py.def(" fact ", &API::fact); } Pybind11 Boost.Python
- 17. www.luxoft.com Boost dependencies 17 1. bind 2. config 3. conversion 4. core 5. detail 6. foreach 7. function 8. iterator 9. lexical_cast 10. mpl 11. numeric~conversion 12. preprocessor 13. smart_ptr 14. static_assert 15. tuple 16. type_traits 17. utility Primary dependencies for python:
- 18. www.luxoft.com Summary 18 • Ctypes. You need to use C code, and very few C++ functions. • SWIG. If you going to provide API for at least few languages. • Pybind11. For code on C++11 and later standards. You need to call C++ code from Python and vice-versa. • Boost.Python. In case you have to support old compilers or you’ve already had the whole boost at your project :) When to use?
- 19. www.luxoft.com Links to sources 1. Stack Overflow Trends 2. Octoverse GitHub overview 3. C/C++ из Python (ctypes) 4. SWIG documentation (Python) 5. SWIG documentation (C++) 6. Pybind11 7. Boostdep 8. Boost.Python 9. D-Bus documentation