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.

Please add clarification on what doesn't work- compilation or execution, and any associated error messages. — sbooth
– sbooth, Commented Dec 24, 2016 at 3:23

sbooth · Accepted Answer · 2016-12-23 22:49:51Z

11

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.

edited Dec 23, 2016 at 22:49

answered Dec 23, 2016 at 22:42

sbooth

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2 Comments

Steven B. Over a year ago

for some reason the function wont take the argument and takes the default value, i updated the question with some more info

Steven B. Over a year ago

your quote got me to use UnsafePointer<Int8> which seemed to work, I added my working code. thanks!!

Steven B. · Accepted Answer · 2016-12-28 11:16:49Z

4

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)
...

answered Dec 28, 2016 at 11:16

Steven B.

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Comments

Steve · Accepted Answer · 2018-04-20 08:59:44Z

2

Simple way for Swift 3

var ipAddress: String = "192.168.1.43"
var verbose: Int = 1
artnet_node artnet_new((ipAddress as NSString).utf8String, verbose)

answered Apr 20, 2018 at 8:59

Steve

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1 Comment

vine'th Over a year ago

While using sqlite3 API sqlite3_bind_text() with swift, I was seeing memory corruption and weird results if the String was directly passed. (ipAddress as NSString).utf8String fixed this.

Code Different · Accepted Answer · 2016-12-23 22:26:54Z

1

You didn't specify what your Swift array contains. In any case, you need to convert your Swift array to an array of Int8:

let str = "Hello world"
let cArray = str.cString(using: .utf8)

artnet_new(cArray, 1)

edited Dec 23, 2016 at 22:26

answered Dec 23, 2016 at 22:23

Code Different

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1 Comment

Steven B. Over a year ago

I'm sorry i mistyped, I'm trying to change a single string to a const char :s, edited the question

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