Swift: How to call a C function loaded from a dylib

Question

Is there a way to call a C function loaded from a dylib from Swift?

This is my dylib file:

cppdemofile.cpp

#include "cppdemofile.h"

int add(int a, int b) {
    return a + b;
}

cppdemofile.h

#ifndef __CppDemoLibrary__cppdemofile__
#define __CppDemoLibrary__cppdemofile__

#pragma GCC visibility push(default)

extern "C" int add(int a, int b);

#pragma GCC visibility pop

#endif

compile to dylib and check:

nm -gU libCppDemoLibrary.dylib
0000000000000f80 T _add

... copy libCppDemoLibrary.dylib to ~/lib...

Swift program:

@IBAction func buttonClick(sender: NSButton) {
    let handle = dlopen("libCppDemoLibrary.dylib", RTLD_NOW)
    if (handle != nil) {
        var sym = dlsym(handle, "add")
        if (sym != nil) {
            let pointer = UnsafeMutablePointer<(CInt, CInt) -> CInt>(sym)

            // When debugging, I'm reaching up to this point...
            // but now, how do I call the 'add' function here???
            // var result = ???

            // label.stringValue = "Total: " + String(result)
        }
    }
}

How do I call the add function? Is it ok to use a dylib? Should I instead add these sources to my swift project?

Answer 1

Calling the add function from Swift is possible because you defined it to have C linkage with extern "C".

Making the library a Swift module (as suggested by jtbandes in above comments) might be the better solution, but here is how you can use the function pointer return by dlsym() from Swift:

First add

typedef int(*addFunc)(int, int);

to the bridging header file, or alternatively define

typealias addFunc = @convention(c) (CInt, CInt) -> CInt

in Swift. Then the following works:

let handle = dlopen(path, RTLD_NOW)
if (handle != nil) {
    var sym = dlsym(handle, "add")
    if (sym != nil) {
        let f = unsafeBitCast(sym, addFunc.self)
        let result = f(12, 45)
        print(result)
    }
    dlclose(handle)
}

Of course this will crash if addFunc does not match the actual signature of the loaded function.

---

Update for Swift 3:

if let handle = dlopen(path, RTLD_NOW) {
    if let sym = dlsym(handle, "add") {
        let f = unsafeBitCast(sym, to: addFunc.self)
        let result = f(12, 45)
        print(result)
    }
    dlclose(handle)
}