Python call C++ function by SWIG

Python call C++ function by SWIG

This note shows how to use SWIG to enable python invoking C++ codes, which might be wanted if high-efficiency is needed, such as create cplex model

What is SWIG

SWIG is an interface compiler that connects programs written in C and C++ with scripting languages such as Perl, Python, Ruby, and Tcl. It works by taking the declarations found in C/C++ header files and using them to generate the wrapper code that scripting languages need to access the underlying C/C++ code.

Install

Ref: https://blog.csdn.net/LIANGJIANGLI/article/details/72832154 (Mac Swig 3.0.12 安装)

Steps

• Download, Extract
• ./configure --prefix=/Users/y0w02p1/wy/programs/swig // 指定安装目录
  • 如果想让默认安装的话，可以直接执行 ./configure
• make // compiling
• make install // do install
• Configurate path: nano ~/.bash_profile

    export SWIG_HOME=/Users/y0w02p1/wy/programs/swig
    export PATH=${PATH}:${SWIG_HOME}/bin
  

• Test: $ swig -version 显示版本号说明安装成功！

Example (Python to C)

Codes

/* File: example.i */
%module example
%{
    /* Put header files here or function declarations like below */
    extern double My_variable;
    extern int fact(int n);
    extern int my_mod(int x, int y);
    extern char *get_time();
    extern char *getTime();
%}

extern double My_variable;
extern int fact(int n);
extern int my_mod(int x, int y);
extern char *get_time();
extern char *getTime();

/* File : example.c */
#include <time.h>
double My_variable = 3.0;

int fact(int n) {
    if (n <= 1) return 1;
    else return n*fact(n-1);
}

int my_mod(int x, int y) {
    return (x%y);
}

char *get_time()
{
    time_t ltime;
    time(&ltime);
    return ctime(&ltime);
}

char *getTime() {
    time_t tVal;
    time_t* p = &tVal;
    time(p);
    return ctime(p);
}

Compile

• Create example_wrap.c and example.py using swig

swig -python example.i

• Compile .c to .o file

gcc  -c example.c  example_wrap.c -I/usr/local/anaconda3/envs/django/include/python3.6m

• Link .o to .so library

gcc  -bundle -undefined dynamic_lookup -L/usr/local/anaconda3/envs/django/lib example.o example_wrap.o  -o _example.so

Python test

Note: Since we compile it using conda - django - Python3.6, before test, need to activate the conda env first

conda activate django
python

# --------------------------------------
# Test example module
# --------------------------------------
import example
print(example.fact(5))      # 120
print(example.my_mod(5,3))  # 2
print(example.get_time())   # current time
print(example.getTime())    # current time

To address the issue in compiling

Ref: https://stackoverflow.com/questions/25882150/python-h-not-found-using-swig-and-anaconda-python

You can use the command python-config --cflags to get the full set of recommended compilation flags:

python-config --cflags

-I/Users/myuser/anaconda/include/python2.7 -I/Users/myuser/anaconda/include/python2.7 -fno-strict-aliasing -I/Users/myuser/anaconda/include -arch x86_64 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes

Let distutils figure out all the compiling and linking options for you.

# File: setup.py
from distutils.core import setup, Extension
example_module = Extension('_example', sources=['example.c', 'example.i'])
setup(name='example', ext_modules=[example_module], py_modules=["example"])

Then you can run:

python setup.py build_ext --inplace

Reference

• SWIG Official

http://www.swig.org/exec.html (What is SWIG) http://www.swig.org/tutorial.html (Tutorial)

• 利用 SWIG 对 C++ 库进行 Python 包装

https://segmentfault.com/a/1190000013219667

• 如何实现 C/C++ 与 Python 的通信？

https://www.zhihu.com/question/23003213/answer/56121859

本文分4个部分C/C++

• 调用 Python （基础篇）— 仅讨论Python官方提供的实现方式Python
• 调用 C/C++ （基础篇）— 仅讨论Python官方提供的实现方式C/C++
• 调用 Python （高级篇）— 使用 CythonPython
• 调用 C/C++ （高级篇）— 使用 SWIG