I was thinking about how Redis supports loading .so files at runtime and wondering if there’s a way to do something similar in Crystal.
If it requires them being written in Crystal, that’s probably okay, but since Crystal can interface with C pretty nicely I imagine if it’s possible at all it can be done both ways.
You can use libdl to load shared libraries at runtime. It is usually available as part of libc and Crystal’s stdlib C bindings have definitions for the most common functions.
Note: This doesn’t work with binaries statically linked against musl libc because the library is not available in that mode.
With loading done, there are two main ways to actually use a library.
For full flexibility you can use libffi to define bindings at runtime. That’s what we’re using for Crystal’s interpreter implementation, for example. So there is a FFI implementation available in the compiler code (crystal/src/compiler/crystal/ffi at master · crystal-lang/crystal · GitHub), but it’s closely tied to the interpreter’s needs and won’t work generically. It can serve as a basis for a more generic implementation.
If you know the symbols exposed in the library at compile time, you can skip FFI and make function calls directly with a hard coded call interface. Example: Call a function in a shared library - Rosetta Code
This works well for loading optional dependencies, for example. When available, the lib get’s loaded and used, and if not, the program still works without it.
This is fantastic! Thanks!