Converting from Swift string to const char*

Question

I'm trying to pass a const char * to an old C library converted from a Swift string in Swift.

This is the C function I'm calling:

artnet_node artnet_new(const char *ip, int verbose) { ...

how can I convert a Swift string to this const char type? It works when I pass ipAddress like this:

internal var ipAddress = "192.168.1.43"

but dit does not work when I pass it like this

internal var ipAddress:String = "192.168.1.43"

I need this in a function where I need to specify the type:

internal func setupArtnode(ip:String) -> Int{

I tried using AnyObject instead of String but that doesn't work either.

Thanks.

Answer 1

You should be able to pass a String directly to a C function expecting const char * and it will be automatically converted to a null-terminated UTF-8 string:

let string = "string"
let node = artnet_new(string, 1)

See Interacting with C APIs for more information. Here is the relevant excerpt:

When a function is declared as taking an UnsafePointer argument, it can accept any of the following:

• A String value, if Type is Int8 or UInt8. The string will automatically be converted to UTF8 in a buffer, and a pointer to that buffer is passed to the function.

Answer 2

Not sure why but this code is working. This passes a string to a C function expecting a const char* which seems to be the same as a unsafePointer.

internal func setupArtnode(ipAddress:String) -> NSInteger{
    let cString = self.ipAddress.cString(using: String.defaultCStringEncoding)!
    let newString:String = NSString(bytes: cString, length: Int(ipAddress.characters.count), encoding:String.Encoding.ascii.rawValue)! as String
    let key2Pointer = UnsafePointer<Int8>(newString)

    node = artnet_new(key2Pointer, Int32(verbose))     // VERBOSE : true(1) , false(0)
...