Passing swift structure to c function

I wants to pass structure to the c function .
I have 3 files .

  1. lib.dylib -> file contains function definition
  2. lib.h -> file contains function declaration
  3. structdefination.h -> file contains structure declaration

I am using Xcode for creating project . After creating project when i have add two file to the project i.e lib.dylib and lib.h .
function declaration for c function in lib.h file was like


extern Info create(Ref p0, char* p1, Config* p2, char** p3);

After clicking on Generated Interface in Xcode for lib.h . Function declaration for same function is


public func create(_ p0: Int32, _ p1: UnsafeMutablePointer<Int8>!, _ p2: UnsafeMutablePointer<Int32>!, _ p3: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!) -> Int32

When i add third file ( i.e structdefination.h) to the project . After adding the file when i click on Generated interface . Function declaration for same function changes . may due to including structure declaration file in project .
New function declaration:


public func create(_ p0: Ref, _ p1: UnsafeMutablePointer<Int8>!, _ p2: UnsafeMutablePointer<Config>!, _ p3: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!) -> Info

Type alias created for c function :


typealias create = @convention(c)(Int32,UnsafeMutablePointer<Int8>?,UnsafeMutableRawPointer?,UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>?)-> -> Int32

Swift code using for calling c function :

let dynammicFileHandle = dlopen("lib.dylib",RTLD_LOCAL|RTLD_NOW)
            if(dynammicFileHandle != nil){
                let sym = dlsym(dynammicFileHandle, "create")
                if(sym != nil){
                    let create = unsafeBitCast(sym, to: create.self)
                    let error : NSString = ""
                    var ptrToerror = UnsafeMutablePointer<CChar>(mutating: error.utf8String)
                    let ptrToName = UnsafeMutablePointer<CChar>(mutating: Name.utf8String)
                    var config = Config()// How can i create c structure in swift
                    let ptrConfig = UnsafeMutableRawPointer(&config)
                    //let Result = create(Ref,ptrTotName, ptrToConfig , &ptrToerror)//How can i pass structure 
            }else{
                print("Handle not found ")
            }

Tired :
I have tried to define c structure in swift and create typealias as ((structure ,Data)->(structurename))

But when program executes shows error

Can't unsafeBitCast between types of different sizes

Question

  1. How can i declare c structure in swift and pass same structure pointer to the c function using swift 5 ?

Note : I am creating SPM Package

How can i declare c structure in swift and pass same structure pointer
to the c function using swift 5 ?

There’s currently no way to declare a struct in Swift that’s guaranteed to follow C layout conventions. If you want to share a struct between Swift and C, you have to declare the struct in C and then import it into Swift.

Doing this sharing in app code is simple (use a bridging header) but it seems that won’t work for you:

I am creating SPM Package

Check out this thread, which has a suggestion for how to do this in an SPM package.

Share and Enjoy

Quinn “The Eskimo!” @ DTS @ Apple

1 Like

@eskimo Thanks for sharing :slight_smile:

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