Swift wrappers for C functions taking char** arguments

CStringArray.swift

// Usage

let argv = CStringArray(["ls", "/"])
posix_spawnp(nil, argv.pointers[0], nil, nil, argv.pointers, nil)

// Is this really the best way to extend the lifetime of C-style strings? The lifetime
// of those passed to the String.withCString closure are only guaranteed valid during
// that call. Tried cheating this by returning the same C string from the closure but it
// gets dealloc'd almost immediately after the closure returns. This isn't terrible when
// dealing with a small number of constant C strings since you can nest closures. But
// this breaks down when it's dynamic, e.g. creating the char** argv array for an exec
// call.
class CString {
    private let _len: Int
    let buffer: UnsafeMutablePointer<Int8>
    
    init(_ string: String) {
        (_len, buffer) = string.withCString {
            let len = Int(strlen($0) + 1)
            let dst = strcpy(UnsafeMutablePointer<Int8>.alloc(len), $0)
            return (len, dst)
        }
    }
    
    deinit {
        buffer.dealloc(_len)
    }
}

// An array of C-style strings (e.g. char**) for easier interop.
class CStringArray {
    // Have to keep the owning CString's alive so that the pointers
    // in our buffer aren't dealloc'd out from under us.
    private let _strings: [CString]
    let pointers: [UnsafeMutablePointer<Int8>]

    init(_ strings: [String]) {
        _strings = strings.map { CString($0) }
        pointers = _strings.map { $0.buffer }
        // NULL-terminate our string pointer buffer since things like
        // exec*() and posix_spawn() require this.
        pointers.append(nil)
    }
}

Maybe we could hang on to the closures somehow? Something like:

var mm = [ (UnsafePointer) : UnsafePointer ]()
let a = { cstr in ..., mm[a] = cstr; }
str1.withCString(a)
str2.withCString(a)

Just an idea, didn't try. Though I guess the pointer is not necessarily 'retained' by the closure, but free'd externally after the closure invocation. Hm, maybe still worth a try ;-)

Hey -- thanks, this was very helpful! I think you could get rid of the len state if you wanted to by changing the deinit to:

    deinit {
        buffer.dealloc(Int(strlen(buffer) + 1))
    }