It is pretty trivial to call our library functions with C-compatible languages (C, C++ and Objective-C) and no additional code is required. To use it, just compile all the files into a single executable or link the library as a shared library.

Generally, writing the glue code by hand is error prone and complex and the final code tends to be hard to write. To automate this process, SWIG automagically generates the wrapper code of multiple programming languages. It currently supports Ruby, Python, Java, Javascript and many, many others.

SWIG, of course, is far from being a silver bullet. For example, this answer from Stackoverflow summarizes that one of the disadvantages is that the generated C code is ugly and may be tricky to set up a complex interface file. On the other hand, the advantages are the support for multiple languages and time savings when using simple functions.

Let me show a brief demo of a function that calculates the power of a number

int pow(int n) {
  return n * n;
}

To generate this glue code, we have to write an input interface file containing the desired functions to be exported. The first section is called the preamble and provides declarations to get the code to compile. The second declaration contains all the functions that you will be included in the wrapper code.

%module example
%{
  extern int pow(int n);
%}

extern int pow(int n);

To use this module in Python:

# Will create example.py and example_wrap.c
➜ swig -python example.i
➜ gcc -shared -fPIC example.c example_wrap.c \ 
  -I/usr/include/python2.7 -o _example.so
➜ python2
>>>> import example
>>>> example.power(3)
9

To use this module in Ruby:

# Content of extconf.rb
require 'mkmf'
create_makefile('example')

# In shell
➜ swig -ruby example.i
# This command will create the Makefile
➜ ruby extconf.rb
➜ make
➜ irb
irb(main):001:0> require './example'
irb(main):001:0> Example.power(2)
=> 9

Nice! We saw that with the special SWIG interface file, we can create the binding code easily into the supported languages and SWIG will handle all the particularities of each language binding. Check the SWIG interface file of the library if you are curious about it.

SWIG is pretty useful but has one big downside. It is not possible to use it to target web browsers. To run our C code in them, we would have to rewrite the library in Javascript or use it with a plugin (Flash, NaCl).

However, there is a better alternative. With the Emscripten project, we can compile C and C++ code into Javascript that runs in the browsers without any external plugins.

Basically, this image show how this source-to-source compilation works:

First, Emscripten calls clang to compile your C files and generate the LLVM bitcode from them. Then, the resulting bitcode is fed to the Emscripten LLVM backend called Fastcomp, which is translated to Javascript code. All these operations are wrapped into the emcc command, so all this process is transparent to the user.

The resulting Javascript is in the asm.js format, which is, in short, a strict subset of Javascript that is optimized for performance. For more information about it, check this post by John Resig.

The demo game was adapted from javascript-tetris and reuses its visual components, only replacing the Tetris game logic.

If the programming language you are looking for is not yet supported by SWIG (Rust, Go or Nim), you will have to write the binding code by hand. ='(