Generate a library and load it

[Fulgurance]

Hi guys, I have a question (for Linux).

How can I build a crystal library I wrote as a library (.so), and then load it from another crystal program ?

Because I start seriously to think to split my project in libraries, because I am starting to write too an init system (in Crystal as well)

[kojix2]

To the best of my knowledge, the best documentation on this topic is here.
Please read Asterite’s comments and the replies to them.

Embeddable / Interoperable with ruby

Let’s take this code: def foo(x) if x.is_a?(Int32) 1 else 2 end end foo(1 || 'a') foo('a' || 1) We can compile it like this: crystal build --prelude=empty --emit llvm-ir foo.cr That will give us the LLVM IR for that file, and specifically for what the generated foo function is. How does the generated code check if x.is_a?(Int32)? The answer is that Crystal gives each type a unique ID: an integer. Here’s the LLVM IR code for the generated foo function: define internal i32 @"…

[kojix2]

Creating a C-ABI Shared Library in Crystal

crystal build sum.cr --single-module --link-flags="-shared" -o libsum.so

fun crystal_init : Void
  GC.init

  LibCrystalMain.__crystal_main(0, Pointer(Pointer(UInt8)).null)
end

fun sum(a : Int32, b : Int32): Int32
  puts "Calculate sum"
  a + b
end

This only works when there is only one shared library; with two or more, there will be a segmentation fault.

One approach would be to use a single libgc across all libraries while thoroughly mangling other symbols, but this is highly complex. Another idea—related to Ruby’s “external type definitions”—involves generating a type and ID translation table at compile time, though Crystal’s open classes and abstract types complicate things. Finally, there is a possibility that a dedicated shared library format for Crystal, rather than relying on the C ABI, could be devised (though exporting to WebAssembly might become more prevalent).

Since this issue is often discussed, I have written what I have heard so far and what can be inferred from it. Comments from those who know a little more about the issue are expected.

[ysbaddaden]

This is currently impossible.

It’s technically possible to create a library from crystal code, but every compilation of a Crystal program will generate the same symbols that will conflict with each other.

We don’t have the means to create a public external API while hiding the internal private symbols.