Swift3: How to create a char**? [duplicate] - c

Question

I'm trying to interact with an old C terminal app from Swift. I've successfully integrated the source code and bridged the headers from C to Swift. The code compiles and runs from Xcode 6.3 beta. I've renamed the terminal app's main entry point to:
int initialize(int argc, char **argv);
Nevertheless, I'm struggling to pass the arguments from Swift to this C function. My challenge is to convert the arguments in the right format. Typical input from Swift would look like:
let args = ["-c", "1.2.3.4", "-p", "8000"]
I've tried messing with "cStringUsingEncoding(NSUTF8StringEncoding)" and "withUnsafePointer", but no luck so far. Any help is greatly appreciated!

Answer 1

The C function
int initialize(int argc, char **argv);
is mapped to Swift as
func initialize(argc: Int32, argv: UnsafeMutablePointer<UnsafeMutablePointer<Int8>>) -> Int32
This is a possible solution:
let args = ["-c", "1.2.3.4", "-p", "8000"]
// Create [UnsafeMutablePointer<Int8>]:
var cargs = args.map { strdup($0) }
// Call C function:
let result = initialize(Int32(args.count), &cargs)
// Free the duplicated strings:
for ptr in cargs { free(ptr) }
It uses the fact that in strdup($0)
the Swift string $0 is automatically converted to a C string,
as explained in String value to UnsafePointer<UInt8> function parameter behavior.

Answer 2

Building on Martin’s answer, if you find yourself doing this a lot, you could wrap the dup/free part into a function in a similar style to String.withCString:
import Darwin
func withCStrings
<R, S: SequenceType where S.Generator.Element == String>
(strings: S, #noescape body: (UnsafeBufferPointer<UnsafeMutablePointer<Int8>>) -> R)
-> R {
let cstrings = map(strings) { strdup($0) } + [nil]
let result = cstrings.withUnsafeBufferPointer(body)
for ptr in cstrings { free(ptr) }
return result
}
let execvargs = ["/usr/bin/say"] + dropFirst(Process.arguments)
let execvresult = withCStrings(execvargs) {
execv($0[0], $0.baseAddress)
}