Convert 'UnsafePointer<CChar>' in Swift [duplicate] - ios

Question

I have a Swift program that does interop with a C library. This C library returns a structure with a char[] array inside, like this:
struct record
{
char name[8];
};
The definition is correctly imported into Swift. However, the field is interpreted as a tuple of 8 Int8 elements (typed (Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8)), which I have no idea how to transform into a String with Swift.
There is no String initializer that accepts an Int8 tuple, and it doesn't seem possible to get a pointer to the first element of the tuple (since types can be heterogenous, that's not really surprising).
Right now, my best idea is to create a tiny C function that accepts a pointer to the structure itself and return name as a char* pointer instead of an array, and go with that.
Is there, however, are pure Swift way to do it?

Answer 1

The C array char name[8] is imported to Swift as a tuple:
(Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8)
The address of name is the same as the address of name[0], and
Swift preserves the memory layout of structures imported from C, as
confirmed by Apple engineer Joe Groff:
... You can leave the struct defined in C and import it into Swift. Swift will respect C's layout.
As a consequence, we can pass the address of record.name,
converted to an UInt8 pointer, to
the String initializer. The following code has been updated for Swift 4.2 and later:
let record = someFunctionReturningAStructRecord()
let name = withUnsafePointer(to: record.name) {
$0.withMemoryRebound(to: UInt8.self, capacity: MemoryLayout.size(ofValue: $0)) {
String(cString: $0)
}
}
NOTE: It is assumed that the bytes in name[] are a valid NUL-terminated UTF-8 sequence.
For older versions of Swift:
// Swift 2:
var record = someFunctionReturningAStructRecord()
let name = withUnsafePointer(&record.name) {
String.fromCString(UnsafePointer($0))!
}
// Swift 3:
var record = someFunctionReturningAStructRecord()
let name = withUnsafePointer(to: &record.name) {
$0.withMemoryRebound(to: UInt8.self, capacity: MemoryLayout.size(ofValue: record.name)) {
String(cString: $0)
}
}

Answer 2

You can actually collect a tuple into an array by using Swift's variadic parameter syntax:
let record = getRecord()
let (int8s: Int8...) = myRecord // int8s is an [Int8]
let uint8s = int8s.map { UInt8($0) }
let string = String(bytes: uint8s, encoding: NSASCIIStringEncoding)
// myString == Optional("12345678")

Answer 3

I'm interested in working this out for my own purposes as well, so I added a new function:
func asciiCArrayToSwiftString(cString:Int8...) -> String
{
var swiftString = String() // The Swift String to be Returned is Intialized to an Empty String
var workingCharacter:UnicodeScalar = UnicodeScalar(UInt8(cString[0]))
var count:Int = cString.count
for var i:Int = 0; i < count; i++
{
workingCharacter = UnicodeScalar(UInt8(cString[i])) // Convert the Int8 Character to a Unicode Scalar
swiftString.append(workingCharacter) // Append the Unicode Scalar
}
return swiftString // Return the Swift String
}
I call this function with:
let t:Int8 = Int8(116)
let e:Int8 = Int8(101)
let s:Int8 = Int8(115)
let testCString = (t, e, s, t)
let testSwiftString = wispStringConverter.asciiCArrayToSwiftString(testCString.0, testCString.1, testCString.2, testCString.3)
println("testSwiftString = \(testSwiftString)")
the resulting output is:
testSwiftString = test

Answer 4

I have just experienced a similar issue using Swift 3. (3.0.2). I was attempting to convert an Array of CChar, [CChar] to a String in Swift. It turns out Swift 3 has a String initializer which will take a cString.
Example:
let a = "abc".cString(using: .utf8) // type of a is [CChar]
let b = String(cString: a!, encoding: .utf8) // type of b is String
print("a = \(a)")
print("b = \(b)")
results in
a = Optional([97, 98, 99, 0])
b = Optional("abc")
Note that the cString function on String results in an Optional. It must be force unwrapped when used in the String.init function creating b. And b is also Optional... meaning both could end up being nil, so error checking should also be used.

Answer 5

Try this:
func asciiCStringToSwiftString(cString:UnsafePointer<UInt8>, maxLength:Int) -> String
{
var swiftString = String() // The Swift String to be Returned is Intialized to an Empty String
var workingCharacter:UnicodeScalar = UnicodeScalar(cString[0])
var count:Int = 0 // An Index Into the C String Array Starting With the First Character
while cString[count] != 0 // While We Haven't reached the End of the String
{
workingCharacter = UnicodeScalar(cString[count]) // Convert the ASCII Character to a Unicode Scalar
swiftString.append(workingCharacter) // Append the Unicode Scalar Version of the ASCII Character
count++ // Increment the Index to Look at the Next ASCII Character
if count > maxLength // Set a Limit In Case the C string was Not NULL Terminated
{
if printDebugLogs == true
{
swiftString="Reached String Length Limit in Converting ASCII C String To Swift String"
}
return swiftString
}
}
return swiftString // Return the Swift String
}

Answer 6

Here's a solution I came up with which uses reflection to actually convert the tuple into an [Int8] (see Any way to iterate a tuple in swift?), and then converts it to a string using fromCString...() methods.
func arrayForTuple<T,E>(tuple:T) -> [E] {
let reflection = reflect(tuple)
var arr : [E] = []
for i in 0..<reflection.count {
if let value = reflection[i].1.value as? E {
arr.append(value)
}
}
return arr
}
public extension String {
public static func fromTuple<T>(tuple:T) -> String? {
var charArray = arrayForTuple(tuple) as [Int8]
var nameString = String.fromCString(UnsafePointer<CChar>(charArray))
if nameString == nil {
nameString = String.fromCStringRepairingIllFormedUTF8(UnsafePointer<CChar>(charArray)).0
}
return nameString
}
}

Answer 7

Swift 3. Only uses reflection. This version stops building the string when it encounters a null byte. Tested.
func TupleOfInt8sToString( _ tupleOfInt8s:Any ) -> String? {
var result:String? = nil
let mirror = Mirror(reflecting: tupleOfInt8s)
for child in mirror.children {
guard let characterValue = child.value as? Int8, characterValue != 0 else {
break
}
if result == nil {
result = String()
}
result?.append(Character(UnicodeScalar(UInt8(characterValue))))
}
return result
}

Answer 8

Details
Xcode 11.2.1 (11B500), Swift 5.1
Solution
extension String {
init?(fromTuple value: Any) {
guard let string = Tuple(value).toString() else { return nil }
self = string
}
init?(cString: UnsafeMutablePointer<Int8>?) {
guard let cString = cString else { return nil }
self = String(cString: cString)
}
init?(cString: UnsafeMutablePointer<CUnsignedChar>?) {
guard let cString = cString else { return nil }
self = String(cString: cString)
}
init? (cString: Any) {
if let pointer = cString as? UnsafeMutablePointer<CChar> {
self = String(cString: pointer)
return
}
if let pointer = cString as? UnsafeMutablePointer<CUnsignedChar> {
self = String(cString: pointer)
return
}
if let string = String(fromTuple: cString) {
self = string
return
}
return nil
}
}
// https://stackoverflow.com/a/58869882/4488252
struct Tuple<T> {
let original: T
private let array: [Mirror.Child]
init(_ value: T) {
self.original = value
array = Array(Mirror(reflecting: original).children)
}
func compactMap<V>(_ transform: (Mirror.Child) -> V?) -> [V] { array.compactMap(transform) }
func toString() -> String? {
let chars = compactMap { (_, value) -> String? in
var scalar: Unicode.Scalar!
switch value {
case is CUnsignedChar: scalar = .init(value as! CUnsignedChar)
case is CChar: scalar = .init(UInt8(value as! CChar))
default: break
}
guard let _scalar = scalar else { return nil }
return String(_scalar)
}
if chars.isEmpty && !array.isEmpty { return nil }
return chars.joined()
}
}
Usage (full sample)
Code in C language (Header.h)
#ifndef Header_h
#define Header_h
#ifdef __cplusplus
extern "C" {
#endif
char c_str1[] = "Hello world!";
char c_str2[50] = "Hello world!";
char *c_str3 = c_str2;
typedef unsigned char UTF8CHAR;
UTF8CHAR c_str4[] = {72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 32, 0};
UTF8CHAR *c_str5 = c_str4;
UTF8CHAR c_str6[] = {'H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!', '\0'};
UTF8CHAR *c_str7 = 0;
UTF8CHAR *c_str8 = "";
#define UI BYTE
#ifdef __cplusplus
}
#endif
#endif /* Header_h */
...-Bridging-Header.h
#include "Header.h"
Swift code
func test() {
printInfo(c_str1)
printInfo(c_str2)
printInfo(c_str3)
printInfo(c_str4)
printInfo(c_str5)
printInfo(c_str6)
printInfo(c_str7)
printInfo(c_str8)
print(String(fromTuple: c_str1) as Any)
print(String(fromTuple: c_str2) as Any)
print(String(cString: c_str3) as Any)
print(String(fromTuple: c_str4) as Any)
print(String(cString: c_str5) as Any)
print(String(fromTuple: c_str6) as Any)
print(String(fromTuple: c_str7) as Any)
print(String(cString: c_str8) as Any)
}
var counter = 1;
func printInfo(_ value: Any?) {
print("name: str_\(counter)")
counter += 1
guard let value = value else { return }
print("type: \(type(of: value))")
print("value: \(value)")
print("swift string: \(String(cString: value))")
print("\n-----------------")
}
Output
name: str_1
type: (Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8)
value: (72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 33, 0)
swift string: Optional("Hello world!\0")
-----------------
name: str_2
type: (Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8)
value: (72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 33, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
swift string: Optional("Hello world!\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
-----------------
name: str_3
type: UnsafeMutablePointer<Int8>
value: 0x000000010e8c5d40
swift string: Optional("Hello world!")
-----------------
name: str_4
type: (UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8)
value: (72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 32, 0)
swift string: Optional("Hello world \0")
-----------------
name: str_5
type: UnsafeMutablePointer<UInt8>
value: 0x000000010e8c5d80
swift string: Optional("Hello world ")
-----------------
name: str_6
type: (UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8)
value: (72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 33, 0)
swift string: Optional("Hello world!\0")
-----------------
name: str_7
name: str_8
type: UnsafeMutablePointer<UInt8>
value: 0x000000010e8c0ae0
swift string: Optional("")
-----------------
Optional("Hello world!\0")
Optional("Hello world!\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
Optional("Hello world!")
Optional("Hello world \0")
Optional("Hello world ")
Optional("Hello world!\0")
Optional("")
Optional("")

Answer 9

There have been multiple answers already on this topic, but not a single one is a simple one line nor do they address non null terminated ones.
Assuming the String is NULL terminated:
struct record {
char name[8];
};
//Might by unsafe, depends
String(cString: &record.name.0)
//Safe
String(cString: unsafeBitCast(UnsafePointer(&record.name), to: UnsafePointer<Int8>.self))
For Strings that aren't NULL terminated:
//Might by unsafe, depends
String(cString: &record.name.0).prefix(MemoryLayout.size(ofValue: record.name))
//Safe
String(bytesNoCopy: UnsafeMutableRawPointer(mutating: &record.name), length: MemoryLayout.size(ofValue: record.name), encoding: .utf8, freeWhenDone: false)
––––
Regarding #MartinR concern about passing just one byte, you could also pass a pointer to the entire variable too, but personally, I've never experienced swift just passing one byte, so it should be safe.