Convert a Swift Array of String to a to a C string array pointer - swift

Question

I'm on Swift 3, and I need to interact with an C API, which accepts a NULL-terminated list of strings, for example
const char *cmd[] = {"name1", "value1", NULL};
command(cmd);
In Swift, the API was imported like
func command(_ args: UnsafeMutablePointer<UnsafePointer<Int8>?>!)
After trying hundreds of times using type casting or unsafeAddress(of:) I still cannot get this work. Even though I pass a valid pointer that passed compilation, it crashes at runtime saying invalid memory access (at strlen function). Or maybe it's something about ARC?
let array = ["name1", "value1", nil]
// ???
// args: UnsafeMutablePointer<UnsafePointer<Int8>?>
command(args)

Answer 1

You can proceed similarly as in How to pass an array of Swift strings to a C function taking a char ** parameter. It is a bit different because of the different
const-ness of the argument array, and because there is a terminating
nil (which must not be passed to strdup()).
This is how it should work:
let array: [String?] = ["name1", "name2", nil]
// Create [UnsafePointer<Int8>]:
var cargs = array.map { $0.flatMap { UnsafePointer<Int8>(strdup($0)) } }
// Call C function:
let result = command(&cargs)
// Free the duplicated strings:
for ptr in cargs { free(UnsafeMutablePointer(mutating: ptr)) }

Answer 2

This class provides a pointer that works with char** and automatically deallocates the memory, even though it's a struct (using a little trick with a mapped data with deallocator).
public struct CStringArray {
public let pointer: UnsafeMutablePointer<UnsafeMutablePointer<CChar>?>
public let count: Int
private var data: Data
public init(_ array: [String]) {
let count = array.count
// Allocate memory to hold the CStrings and a terminating nil
let pointer = UnsafeMutablePointer<UnsafeMutablePointer<CChar>?>.allocate(capacity: count + 1)
pointer.initialize(repeating: nil, count: count + 1) // Implicit terminating nil at the end of the array
// Populate the allocated memory with pointers to CStrings
var e = 0
array.forEach {
pointer[e] = strdup($0)
e += 1
}
// This uses the deallocator available on the data structure as a solution to the fact that structs do not have `deinit`
self.data = Data(bytesNoCopy: pointer, count: MemoryLayout<UnsafeMutablePointer<CChar>>.size * count, deallocator: .custom({_,_ in
for i in 0...count - 1 {
free(pointer[i])
}
pointer.deallocate()
}))
self.pointer = pointer
self.count = array.count
}
public subscript(index: Data.Index) -> UnsafeMutablePointer<CChar>? {
get {
precondition(index >= 0 && index < count, "Index out of range")
return pointer[index]
}
}
public subscript(index: Data.Index) -> String? {
get {
precondition(index >= 0 && index < count, "Index out of range")
if let pointee = pointer[index] {
return String(cString: pointee)
}
return nil
}
}
}