SWIG

In this time I will wrap c function with SWIG.

SWIG is a software development tool that connects programs written in C and C++ with a variety of high-level programming languages.

Frist of all, install SWIG on ubuntu:

sudo apt install SWIG

When you use SWIG, you need to a interface file called pyflags.i, i.e. the extension is normally i. which will be used by SWIG

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/* flags.c - Source file */
# include <stdio.h>
# inlcude "flags.h"

int gFlag = 0;

void welcome_msg(char *msg){
    printf("%s\n", msg);
    return;
}

int get_flag(){
    return flag;
}

void set_flag(int flag){
    gFlag = flag;
    return;
}

/* flags.h - Header file */
void welcome_msg(char *msg);
int get_flag();
void set_flag(int flag);

From now on, you have to write a interface file as shown below for the above c library.

/* pyflags.i - interface file*/
% module flags_wrap
%{
#include "flags.h"
%}

%include "flags.h"

Let’s type in commands to make output files _flags_wrap.so with input file pyflags.i.

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# Step to build python wrapper - build-python-wrapper-so.sh

swig -python pyflags.i
gcc -fPIC -c flags.c pyflags_wrap.c -l/usr/include/python3
ld -shared flags.o pyflags_wrap.o -o _flags_wrap.so

i.e. Let’s see normally how to use SWIG.

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$ cd <path-to-source>
$ swig -<wrap-language> <module-name>.i
$ gcc -c <module-name>.c <module-name>_wrap.c -I<path-to-wrap-language-headers>
$ ld -shared <module-name>.o <module-name>_wrap.o -o _<module-name>.so

After running “make all”,

the following files is created.

flags.o flags_wrap.py _flags_wrap.so pyflags_wrap.c pyflags_wrap.o

Let’s execute it on python interpreter.

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# In example.py
# from flags_wrap.py 
import flags_wrap
print(flags_wrap.welcome_msg("Hi C, I am from Python"))

flags_wrap.set_flag(1)
print(flags_wrap.get_flag())

OR You can improt so file.

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# In example2.py
# from _flags_wrap.so
import _flags_wrap
print(_flags_wrap.welcome_msg("Hi C, I am from Python"))

_flags_wrap.set_flag(1)
print(_flags_wrap.get_flag())

In here, I used pytho3, so if implement the python codes above with python2.7, it doesn’t work.

Let’s see the result of running the above python files.

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# hyunyoung2 @ hyunyoung2-desktop in ~/konltk/Hyunyoung2_Ctypes-CFFI-SWIG/SWIG/tutorial1 on git:master x [22:42:06] 
$ python3 example2.py  # this _flag_wrap.so
Hi C, I am from Python
None
1

# hyunyoung2 @ hyunyoung2-desktop in ~/konltk/Hyunyoung2_Ctypes-CFFI-SWIG/SWIG/tutorial1 on git:master x [22:43:22] 
$ python3 example.py  # this flag_wrap.py
Hi C, I am from Python
None
1

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Reference