Use C function in Swift

Question

I want to use a C function in Swift, which has the following method definition:

int startTest(char *test1, char* test2)

If I call this method from my Swift code like this

startTest("test1", "test2")

I get the following error message:

'String' is not convertible to 'UnsafeMutablePointer<Int8>'

If I change my method definition to:

int startTest(const char *test1, const char* test2)

and call that method like this:

var test1 = "test1" var test2 = "test2" startTest(&test1, &test2)

I get

'String' is not identical to 'Int8'

So my question is: how can I use the C function? (it is part of a library, so changing the method call could be problematic).

Thanks in advance!

Well you have a function accepting two chars but you're calling it with two strings. — Andrew
– Andrew, Commented Sep 2, 2014 at 20:26
Yes, but how can I cast a String to a char*? Thats exactly my problem, because the C function does not know String type, it can only work with char* — Chris R.
– Chris R., Commented Sep 2, 2014 at 20:30
This question and answer might be more googlable if there was something in the title about char * and/or const char *. Perhaps consider editing? — David Conrad
– David Conrad, Commented Sep 2, 2014 at 20:34

Martin R · Accepted Answer · 2014-09-02 20:31:02Z

In the case of

int startTest(const char *test1, const char* test2);

you can call the function from Swift simply as

let result = startTest(test1, test2)

(without the address-of operators). The Swift strings are converted automatically to C Strings for the function call

In the case of

int startTest(char *test1, char* test2);

you need to call the function with a (variable) Int8 buffer, because the Swift compiler must assume that the strings might be modified from the C function.

Example:

var cString1 = test1.cStringUsingEncoding(NSUTF8StringEncoding)! var cString2 = test2.cStringUsingEncoding(NSUTF8StringEncoding)! let result = startTest(&cString1, &cString2)

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