Compiling a shared library

Note

This brief introduction uses C as the reference programming language and GCC (GNU) as the reference compiler.

Compiling code as a shared library may actually be easier than making a full program in C that requires a main function, plus its own procedures to handle I/O (input & output). The later is noticeably tedious to do in some lower level languages, while Python can make it almost trivial. That means we can write C code that performs the expensive calculations and write the rest of the program in Python.

Even old programs can be interfaced from Python, provided that they were originally written in a modular way that separates the “guts” from the rest of the program. We can keep the parts of the original code that we need and build a new interface from Python.

The following code snippet can be saved into a text file example.c and compiled into a shared library.

int add_doubled_integers (int a, int b) {
    int c;
    c = 2 * a + 2 * b;
    return c;
}

On a Unix system, we simply type the following into the terminal:

$ gcc -shared -fPIC example.c -o example.so

• -shared tells the compiler to compile it as a shared library

• -fPIC tells the compiler to compile it as position independent code

The .so extension is short for shared object and is commonly used in shared libraries compiled for Unix systems. For Windows systems, .dll is the commonly used extension. This is just a convention, but it is worth following so other people know what these files are within a project. Please, do not use .exe if you work in Windows.