如何通过引用将 UnsafePointer<UInt8> 传递给 C API？

Question

我写了一些 Swift 2.2 代码来与 OpenSSL C API 交互，现在我正试图将它转换为 Swift 3.

在Swift 2

let octets = pkcs7_d_data(pkcs7_d_sign(receiptPKCS7).memory.contents)
var ptr = UnsafePointer<UInt8>(octets.memory.data)

// now pass pointer by reference
ASN1_get_object(&ptr, &length, &type, &xclass, end - ptr)

在 Swift 3 中，我不得不进行一些更改

// use guard so i dont have to constantly unwrap these values

guard let octets = pkcs7_d_data(pkcs7_d_sign(receiptPKCS7).pointee.contents),
      var ptr = UnsafePointer<UInt8>(octets.pointee.data) else {
    return nil
}

ASN1_get_object(&ptr, &length, &type, &xclass, end - ptr)
//              ^^^ this is now a compiler error

不幸的是，由于这个错误，我无法再通过引用将 ptr 传递给 ASN1_get_object：

Cannot pass immutable value as inout argument: Implicit conversion from UnsafePointer<UInt8> to UnsafePointer<UInt8>? requires a temporary

然后剩下的错误被截断（没有展开箭头）。

我尝试过的：

将 ptr 赋值更改为 UnsafePointer<UInt8>(octets.pointee.data)? 但后来我被告知我现有的赋值已经产生一个可选的
将 UnsafePointer 更改为 UnsafeMutablePointer

这里需要更改什么？

Answer 1

问题似乎是 octets.pointee.data 是一个 UnsafeMutablePointer<UInt8>!，但 ASN1_get_object 期望 UnsafePointer<UInt8>?.

的地址

以下代码可以编译，但我无法对其进行测试：

if
    let octets = pkcs7_d_data(pkcs7_d_sign(receiptPKCS7).pointee.contents),
    let data = octets.pointee.data {

    var ptr: UnsafePointer? = UnsafePointer(data) // (*)
    var length = 0
    var tag: Int32 = 0
    var xclass: Int32 = 0

    ASN1_get_object(&ptr, &length, &tag, &xclass, Int(octets.pointee.length))
}

(*) 是使其编译的指针转换。

如何通过引用将 UnsafePointer<UInt8> 传递给 C API？

How to pass an UnsafePointer<UInt8> to a C API by reference?

openssl

pass-by-reference

unsafe-pointers

swift

swift3