Passing a Swift string to C

Question

I am pretty new to Swift, and I don't have much exposure to C.

I am trying to write a function in C that will get a Swift string that I can then do something with. The problem is that I'm not 100% sure what the type should be in Swift to make C like what it sees.

So far, I have found several examples on Stack that seem like good starting points, but some examples seem dated for the current version of Swift.

I first started by using this example to get C and Swift talking to one another: Swift call C call Swift? I then took that and tried updating the Swift function to return a string of some kind. I understand that it needs to be a UTF-8 return type, but I'm not sure how to go about sending things properly. I've looked at How to pass a Swift string to a c function?, How to convert String to UnsafePointer<UInt8> and length, and How to convert string to unicode(UTF-8) string in Swift?, but none of them really work for a solution. Or I'm just typing it in incorrectly. So far, the closest I can get to returning something is as follows.

In Swift, my ViewController is:

import UIKit

class ViewController: UIViewController {

    @_silgen_name("mySwiftFunc") // give the function a C name
    public func mySwiftFunc(number: Int) -> [CChar]
    {
        print("Hello from Swift: \(number)")

        let address: String = "hello there";
        let newString = address.cString(using: String.Encoding.utf8)
        return newString!
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        blah()

    }
}

And in C, the header is like:

#ifndef cfile_h
#define cfile_h

#include <stdio.h>

const char * mySwiftFunc(int);
int blah(void);

#endif /* cfile_h */

And the source is like:

#include "cfile.h"

int blah() {

    const char * retVal = mySwiftFunc(42); // call swift function
    printf("Hello from C: %s", retVal);

    return 0;
}

There is a bridging header file that just has #include "cfile.h". Obviously, there is still a lot of remnants from the first example, and these will be cleaned up later.

What needs to change to make this work? Right now, the console spits out

Hello from Swift: 42
Hello from C: (B\214

Answer 1

The Swift equivalent of const char * is UnsafePointer<CChar>?, so that's the correct return value. Then you have to think about memory management. One options is do allocate memory for the C string in the Swift function, and leave it to the caller to release the memory eventually:

public func mySwiftFunc(number: Int) -> UnsafePointer<CChar>? {
    print("Hello from Swift: \(number)")

    let address = "hello there"
    let newString = strdup(address)
    return UnsafePointer(newString)
}

passes a Swift string to strdup() so that a (temporary) C string representation is created automatically. This C string is then duplicated. The calling C function has to release that memory when it is no longer needed:

int blah() {
    const char *retVal = mySwiftFunc(42);
    printf("Hello from C: %s\n", retVal);
    free((char *)retVal);
    return 0;
}

⚠️ BUT: Please note that there are more problems in your code:

• mySwiftFunc() is an instance method of a class, and therefore has an implicit self argument, which is ignored by the calling C function. That might work by chance, or cause strange failures.
• @_silgen_name should not be used outside of the Swift standard library, see this discusssion in the Swift forum.
• A slightly better alternative is @_cdecl but even that is not officially supported. @_cdecl can only be used with global functions.
• Generally, calling Swift functions directly from C is not officially supported, see this discussion in the Swift forum for the reasons and possible alternatives.