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SWIG.md

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SWIG

SWIG is an interface compiler that connects programs written in C and C++ with scripting languages. 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.

Download SWIG from their site and follow steps given here to complete installation.

Note : if you get swig: command not found error then go to /etc/ and source profile so that the environment variables are set.

The steps explained are for python.

“wrapper” functions are needed to access C/C++ which serve as a glue layer between languages.

After writing the .c and .h files for the C library, do this:

  1. $ swig -python example.i

  2. $ gcc -c example.c example_wrap.c \
    -I/usr/bin/python2.7
    This is because the python2.7 file is found in /usr/bin/ on the BeagleBone Black Debian

  3. On step #2, you might encounter this error:

    fatal error: Python.h: No such file or directory
# include <Python.h>
                  ^
compilation terminated.

    Following might be possible causes –

    You might not have ‘Python.h’ file or You are providing wrong location of ‘Python.h’ file to compiler

    To get ‘Python.h’ You must install Python-dev using following command –

    $ sudo apt-get install python-dev
    Most probably this will already be installed on the BBB.

    To find the correct path of ‘Python.h’ execute following command –
    $ python-config --cflags
    The output will be something like this -

    -I/usr/include/python2.7 -I/usr/include/arm-linux-gnueabihf/python2.7 -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-EEvVvy/python2.7-2.7.13=. -fstack-protector-strong -Wformat -Werror=format-security -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes

    I'm using g++ instead of gcc because gcc isn't allowing dynamic memory allocation in my program

    Now do $ g++ -c -fpic example_wrap.c example.c -I/usr/include/python2.7
    I thought I will have to provide -I/usr/bin/python2.7 but it clearly is -I/usr/include/python2.7

    This should remove the "Python.h" error and will generate example_wrap.o and example.o files.

  4. Now, at last, we have to link generated objects files together to create a shared object which is analogous to dll files in windows.
    $ g++ -shared example.o example_wrap.o -o _example.so: this will generate a “example.so” shared object file.(Note: Sometimes gcc will be needed instead of g++)

  5. Now we are ready to test out python wrapper by importing it. Make sure you are in directory having this wrapper file.