dynamic_libraries.swift

/**
 
 This is the simplest way I've found to create a dynamic library in swift, then call a function from it in another swift program.
 
 It doesn't use any package manager nonsense, just a compiled library and executable.

 **/
 
///// Step 1: Create your dylib/dll
 
// libby.swift
// This is the code for the library you want to compile.
 
// Need to declare with @_cdecl so that this function gets an objective-c/c legible signature.
// That way dlsym() can find it when you load the 
@_cdecl("external_func")
public func external_func ()
{
	print ("a func, a very palpable func!")
}

///// Step 2: Compile it

/**

  In the terminal, run:
  
  swiftc -emit-library libby.swift -o libby.dylib
  
  (tested with Swift v5.3.2)
  
 **/
 
///// Step 3: Create an executable that calls it:

// main.swift

import Darwin

// Open the dynamic library
let handle = dlopen("./libby.dylib", RTLD_NOW)
if let s = dlerror() { print(String(cString: s)) } 
if handle == nil {  exit(0) }

// Get the function's symbol
let symbol = dlsym(handle, "external_func")
if let t = dlerror() { print(String(cString: t)); exit(0) } 
if symbol == nil {  exit(0) }

// Cast the unsafe pointer symbol to a swift callable func
typealias func_alias = @convention(c) ()->()
let function = unsafeBitCast(symbol, to:func_alias.self)

// Call it!
function()

// Clean up
dlclose(handle)


 ///// Step 4: Compile your executable
 
 /**
 
 In the terminal, run:
 
 swiftc main.swift
 
 **/
 
///// Step 5: Run your program

/**
 In the terminal, run:
 
 ./main
 
 Output:
  a func, a very palpable func!
  
 And you're done!
**/