Static/Shared libraries in Crystal

Write C static (or shared) libraries in Crystal. This is just a Proof of Concept that exemplifies how to expose a Crystal library to the outside world, eg: C or any language with bindings to C.

Build the library

Change CRYSTAL_ROOT to match your Crystal distribution (it's /opt/crystal by default on Linux):

$ make CRYSTAL_ROOT=/opt/crystal

You may now test that it works:

$ make test

How does it work?

We need Crystal 0.18.0+ and:

1. Some crystal code. See src/counter.cr for a basic example.

2. An API, thankfully written in Crystal to expose the Crystal internals as proper C functions with symbols that won't be mangled. See src/libcounter.cr. You'll notice that it requires a bunch of boilerplate and casting objects with Box to exchange references as void pointers with the outside world.

3. A hack to avoid the generation of a main function by Crystal, and have an alternative function that must be manually called from the outside world to initialize the GC, Crystal constants, etc. See Hack below.

4. A C header to declare the function signatures to C. See src/counter.h.

5. Use the library. See examples in test and Makefile for how to compile and link them.

Hack

In order to compile, we need to avoid loading the original main.cr from the Crystal, and replace it with. This PoC does that by hacking the CRYSTAL_PATH environment variable so it references the local src folder first. This will work with distributions of Crystal, but won't from a Git clone of Crystal, which always overwrites CRYSTAL_PATH.