Andres Andres - 3 months ago 10
Swift Question

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

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

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)
}
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