如何获得UnsafeMutableRawPointer的值？

Question

我正在尝试获取UnsafeMutableRawPointer指向的地址，但我无法这样做。我也是Swift的新手，所以我可能会遗漏某些东西或完全错误。我希望将原始值转换为CChar。

Answer 1

对路人的注意：我的大部分答案都没有意义，因为它没有回答上面的初始问题，而是回答chat中出现的问题。 OP。

花了我几个小时，但是现在我学会了一些装配，我可以回答一些问题。

1. CChar字面意思是C Char。它代表char类型C。它是typealias到Int8。这是一个字节。您不能像指针类型一样使用它，因为它们是8个字节（在64位计算机上）。

2. 您不需要所有这些UnsafeMutableRawPointer样板，并且当然不需要访问其原始值。您可以直接在需要指针的位置传递数组。

     

   当一个函数声明为接受UnsafePointer参数时，它可以接受以下任何一个：...

        

     
   • 一个[Type]值，作为指向数组开头的指针传递。
     

   来自Interacting with C APIs - pointers的

   。

3. 您遇到的问题是0x8(%rdi)的变异似乎没有反映在Swift方面。这里的问题是你写的是8字节的偏移量，但是print(a.load(as: void_star.self))正在读取第一个字节。你正在读一个你从未修改过的字节。

我做了一些进一步的探索。这是我冒险中的战利品：

exampleSwift.swift：

@_silgen_name("incrementByValue")
    func incrementByValue(_: Int64)

@_silgen_name("incrementByReference")
    func incrementByReference(_: inout Int64)

@_silgen_name("return1234")
    func return1234() -> Int64

@_silgen_name("incrementElements")
    func incrementElements(of _: UnsafeRawPointer, count _: Int)

var a: Int64 = 100
print("\"a\" before \"incrementByValue(a)\": \(a)")
incrementByValue(a)
print("\"a\" after \"incrementByValue(a)\": \(a)\n")

var b: Int64 = 200
print("\"b\" before \"incrementByValue(b)\": \(b)")
incrementByReference(&b)
print("\"b\" after \"incrementByValue(b)\": \(b)\n")

print("return1234() returned: \(return1234())\n")

var array: [Int64] = Array(0...5)

print("\"array\" before incrementElements(of: array, count: array.count): \n\t\(array)")
incrementElements(of: array, count: array.count)
print("\"array\" after incrementElements(of: array, count: array.count): \n\t\(array)\n")

exampleASM.s：

.text

.globl _incrementByValue
.globl _incrementByReference
.globl _return1234
.globl _incrementElements

// A test routine that demonstrates operating on a value
_incrementByValue:
    // %rdi contains a copy of the argument passed in.
    // Changes here won't be reflected back in Swift
    incq %rdi
    ret

// A test routine that demonstrates operating on a reference
_incrementByReference:
    // %rdi contains a reference tp the argument passed in.
    // Changes to the reference itself won't be reflected back in Swift,
    // but changes to the referenced memory will.
    incq (%rdi)
    ret

// A test routine that demonstrates the use of %rax for returning a value
_return1234:
    movq $1234, %rax    // return value is in rax
    ret

//A test routine that demonstrates operating on an array
_incrementElements:
    // %rdi: Pointer to first of n Int64 elements
    // %rsi: the array count, n

    movq    %rsi, %rcx  // Set loop counter (%rcx) to n
    aLoop:
        incq    (%rdi)      // increment value pointer to by %rdi
        add     $8, %rdi    // advance pointer by 8 bytes
        loop    aLoop       // loop back to aLoop if rcx > 0

    ret

编译，链接并运行：

llvm-g++ -c exampleASM.s &&
swiftc -c exampleSwift.swift &&
ld exampleASM.o exampleSwift.o -o exampleBinary -force_load /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/arc/libarclite_macosx.a -framework CoreFoundation -macosx_version_min 10.12.0 -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -lobjc -lSystem -arch x86_64 -L /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx -rpath /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx -no_objc_category_merging &&
./exampleBinary

输出：

"a" before "incrementByValue(a)": 100
"a" after "incrementByValue(a)": 100

"b" before "incrementByValue(b)": 200
"b" after "incrementByValue(b)": 201

return1234() returned: 1234

"array" before incrementElements(of: array, count: array.count): 
    [0, 1, 2, 3, 4, 5]
"array" after incrementElements(of: array, count: array.count): 
    [1, 2, 3, 4, 5, 6]

Answer 2

将MutableRawPointer转换为object，我们使用fromOpaque api

/// Unsafely turns an opaque C pointer into an unmanaged class reference.
///
/// This operation does not change reference counts.
///
///     let str: CFString = Unmanaged.fromOpaque(ptr).takeUnretainedValue()
///
/// - Parameter value: An opaque C pointer.
/// - Returns: An unmanaged class reference to `value`.
public static func fromOpaque(_ value: UnsafeRawPointer) -> Unmanaged<Instance>

示例：

var info:UnsafeMutableRawPointer = ....
let obj = Unmanaged<$AnyObject>.fromOpaque(info).takeUnretainedValue()