[swift-users] Communicating with dynamically loaded swift library

[Daniel Dunbar]

The way that I have done this in the past is pass a protocol as an unsafe pointer to an exposed entry point:
```swift
            let entryPoint = dlsym(handle, “initializePlugin”)
            guard entryPoint != nil else {
                fatalError("missing plugin entry point: \(pluginPath)")
            }
            typealias PluginInitializationFunc = @convention(c) (UnsafeRawPointer) -> ()
            let f = unsafeBitCast(entryPoint, to: PluginInitializationFunc.self)
            f(Unmanaged.passUnretained(self).toOpaque())
```

and then in the plugin convert back to the appropriate type:

```
@_cdecl("initializePlugin")
public func initializePlugin(_ ptr: UnsafeRawPointer) {
    let manager = Unmanaged<PluginManager>.fromOpaque(ptr).takeUnretainedValue()
```

HTH,
 - Daniel

> On Oct 4, 2017, at 11:02 AM, Ján Kosa via swift-users <swift-users at swift.org> wrote:
> 
> Hello folks,
> 
> I have been toying with dynamic libraries, trying to implement plugin functionality. I was able to get to the point where I can call simple function in loaded library, but I am having troubles starting more sophisticated communication channel.
> 
> There are 3 projects
> - PluginConsumer is an app that loads plugin libraries 
> - MyPlugin is a plugin implementation, output is dynamic library that PluginConsumer loads
> - PluginInterface is common interface that both MyPlugin and PluginConsumer use, so that they know how to communicate
> 
> My first idea was to have PluginInterface be a simple SPM project with single file where the bare-bones PluginInterface class would be:
> 
> 
> open class PluginInterface {
> 
>     open func sayHi()
> 
> }
> 
> 
> 
> Package.swift file:
> 
> 
> 
> // swift-tools-version:4.0
> 
> import PackageDescription
> 
> let package = Package(
> 
>     name: "PluginInterface",
> 
>     products: [ .library(name: "PluginInterface", type: .dynamic, targets: ["PluginInterface"]) ],
> 
>     targets: [ .target(name: "PluginInterface") ]
> 
> 
> )
> 
> 
> 
> 
> 
> UserPlugin is also very simple project containing only one file:
> 
> 
> 
> public func getPlugin() -> AnyObject {
> 
>     return MyPlugin()
> 
> 
> }
> 
> 
> 
> class MyPlugin: PluginInterface {
> 
>     override func sayHi() {
> 
>         print("Hi from my plugin")
> 
>     }
> 
> }
> 
> Package.swift:
> 
> 
> 
> // swift-tools-version:4.0
> 
> import PackageDescription
> 
> let package = Package(
> 
>     name: "MyPlugin",
> 
>     products: [ .library(name: "MyPlugin", type: .dynamic, targets: ["MyPlugin"]) ],
> 
>     dependencies: [ .package(url: "url_to_PluginInterface", from: "0.0.0"), ],
> 
>     targets: [
> 
>         .target(name: "PluginInterface", dependencies: ["PluginInterface"]),
> 
>         .target(name: "MyPlugin", dependencies: ["PluginInterface"]),
> 
>     ]
> 
> 
> )
> 
> 
> 
> The PluginConsumer is bit more complicated, but here is relevant part (lib loading and function calling):
> 
> 
> 
> typealias InitFunction = @convention(c) () -> AnyObject
> 
> 
> 
> let openRes = dlopen(pathToLib, RTLD_NOW|RTLD_LOCAL)
> 
> if openRes != nil {
> 
>     defer {
> 
>         dlclose(openRes)
> 
>     }
> 
>     let symbolName = "mangled_symbol_name"
> 
>     let sym = dlsym(openRes, symbolName)
> 
> 
> 
>     if sym != nil {
> 
>         let f: InitFunction = unsafeBitCast(sym, to: InitFunction.self)
> 
>         let plugin = f() as? PluginInterface
> 
>     }
> 
> 
> }
> 
> Package.swift file:
> 
> // swift-tools-version:4.0
> 
> import PackageDescription
> 
> let package = Package(
> 
>     name: "PluginConsumer",
> 
>     dependencies: [ .package(url: "path_to_plugin_interface", from: "0.0.0") ],
> 
>     targets: [ .target(name: "PluginConsumer", dependencies: ["PluginConsumer"]) ]
> 
> 
> )
> 
> 
> 
> This all compiles nicely, MyPlugin project creates dylib file that executable created by PluginConsumer can load, but the problem is with following line:
> 
> let plugin = f() as? PluginInterface
> 
> Type of the plugin is MyPlugin, but from the consumer's view, it doesn't inherit from PluginInterface so I can't call sayHi() method. I assume this is because there is no relation between PluginInterface class that compiler uses for MyPlugin project one that it uses for PluginConsumer project. After library is loaded, they are two completely different classes that happen to share same name. Is my assumption correct and how do I go about fixing it?
> 
> I had an idea I could make PluginInterface emit dynamic library that would be dynamically linked by both MyPlugin and PluginConsumer, thus making them share same PluginInterface class, but I can't figure out how to do that (or if it's right way of doing this).
> 
> 
> 
> Any help appreciated :)
> 
> Lope
> 
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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