Making UnsafeMutablePointer<Int8> from String

string2cstring.swift

// Method 1
// String → UnsafePointer<Int8> → UnsafeMutablePointer<Int8>
// Note: func withCString<Result>(_ body: (UnsafePointer<Int8>) throws -> Result) rethrows -> Result
// Note: String.UTF8View doesn't include null character.

func makeCString(from str: String) -> UnsafeMutablePointer<Int8> {
    let count = str.utf8.count + 1
    let result = UnsafeMutablePointer<Int8>.allocate(capacity: count)
    str.withCString { (baseAddress) in
        // func initialize(from: UnsafePointer<Pointee>, count: Int) 
        result.initialize(from: baseAddress, count: count)
    }
    return result
}

//MARK: -- EXAMPLES

do {
    let str = "おはよう World さん　🌄  Now sun has been rising!"
    let cstr = makeCString(from: str)
    let restr = String(cString: cstr)
    print(str == restr) // true

    let str2 = "✑ 🍍 🍎 ✒"
    let cstr2 = makeCString(from: str2)
    let restr2 = String(cString: cstr2)
    print(str2 == restr2) // true
}

string2cstring2.swift

// Method 2
// UnsafeMutableBufferPointer<Int8> → UnsafeMutablePointer<Int8>
// Note: var utf8CString: ContiguousArray<CChar> { get } 
// Note: utf8CString includes the null character (a.k.a null terminator) .
// Note: CChar = Int8

func makeCString2(from str: String) -> UnsafeMutablePointer<Int8> {
    let count = str.utf8CString.count 
    let result: UnsafeMutableBufferPointer<Int8> = UnsafeMutableBufferPointer<Int8>.allocate(capacity: count)
    // func initialize<S>(from: S) -> (S.Iterator, UnsafeMutableBufferPointer<Element>.Index)
    _ = result.initialize(from: str.utf8CString)
    return result.baseAddress!
}

//MARK: -- EXAMPLE

do {
    let str = "おはよう World さん　🌄  Now sun has been rising!"
    let cstr = makeCString2(from: str)
    let restr = String(cString: cstr)
    print(str == restr) // true

    let str2 = "✑ 🍍 🍎 ✒"
    let cstr2 = makeCString2(from: str2)
    let restr2 = String(cString: cstr2)
    print(str2 == restr2) // true
}

let str = "some str"
let cStr = from.cString(using: .ascii)
// in fact, cStr the same as UnsafeMutablePointer

Thanks for your reply.

cString(using:) method is an Foundation extension, originally a method of NSString.
https://developer.apple.com/documentation/foundation/nsstring/1408489-cstring

Therefore it need to import Foundation.

Awesome! That's really help me a lot!