NSMutableData 到 UnsafeMutableRawPointer 的字节不更新 mutableData 的值

Question

我有一个结构对象。和一个方法，其输入是有效载荷。现在我正在创建一个名为 packet 的可变数据，它的可变字节指的是 ICMPHeader 结构。

struct ICMPHeader {
    var type:UInt8
    var code:UInt8
    var checksum:UInt16
    var identifier:UInt16
    var sequenceNumber:UInt16
};


func createPacket(payload:NSData) -> NSData(){
    var packet:NSMutableData?
    var icmpPtr:ICMPHeader = ICMPHeader(type: 0, code: 0, checksum: 0, identifier: 0, sequenceNumber: 0)
    packet = NSMutableData(length: Int(MemoryLayout<ICMPHeader>.size + payload.length))

    if packet != nil {

       icmpPtr = packet!.mutableBytes.assumingMemoryBound(to: ICMPHeader.self).pointee

       icmpPtr.type = type
       icmpPtr.code = 0
       icmpPtr.checksum = 0
       icmpPtr.identifier = CFSwapInt16BigToHost(identifier)
       icmpPtr.sequenceNumber = CFSwapInt16HostToBig(identifier)
       memcpy(&icmpPtr + 1, payload.bytes, payload.length)

       if (requiresChecksum) {
           icmpPtr.checksum = in_cksum(packet!.bytes, bufferLen: packet!.length);
       }

   }
   return packet
}

可变字节已成功绑定到结构，并且结构 ICMPHeader 中的值也已更新。

问题是改变结构中的值并没有改变可变数据的值packet。

如果我在创建结构后尝试重新创建数据包，那么它就会崩溃。

package = NSMutableData(bytes: unsafeBitCast(icmpPtr, to: UnsafeMutableRawPointer.self), length: Int(MemoryLayout<ICMPHeader>.size + payload.length))

Answer 1

我在 XCode 8.1 Swift 发行说明中找到了答案。

对结构对象进行更改后 icmpPtr 我已将其改回缓冲区指针。

var byteBuffer = [UInt8]()
withUnsafeBytes(of: &icmpPtr) {
    (bytes: UnsafeRawBufferPointer) in byteBuffer += bytes
}
package.replaceBytes(in: NSMakeRange(0, byteBuffer.count), withBytes: byteBuffer)

因此，我没有创建新的数据对象，而是替换了字节，效果非常好。

根据文档：

XCode 8.1 Release Notes: https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Introduction.html Swift Section:

A new withUnsafeBytes(of:) function exposes a value's in-memory representation as an UnsafeRawBufferPointer. This example copies a heterogenous struct into a homogeneous array of bytes:
struct Header {
 var x: Int
 var y: Float
}

var header = Header(x: 0, y: 0.0)
var byteBuffer = [UInt8]()

withUnsafeBytes(of: &header) {
    (bytes: UnsafeRawBufferPointer) in byteBuffer += bytes
}
A new Array.withUnsafeBytes method exposes an array's underlying buffer as an UnsafeRawBufferPointer. This example copies an array of integers into an array of bytes:
let intArray = [1, 2, 3]
var byteBuffer = [UInt8]()

intArray.withUnsafeBytes {
    (bytes: UnsafeRawBufferPointer) in byteBuffer += bytes
}

所以最终实现是

struct ICMPHeader {
    var type:UInt8
    var code:UInt8
    var checksum:UInt16
    var identifier:UInt16
    var sequenceNumber:UInt16
};


func createPacket(payload:NSData) -> NSData(){
    var packet:NSMutableData?
    var icmpPtr:ICMPHeader = ICMPHeader(type: 0, code: 0, checksum: 0, identifier: 0, sequenceNumber: 0)
    packet = NSMutableData(length: Int(MemoryLayout<ICMPHeader>.size + payload.length))

    if packet != nil {

       icmpPtr = packet!.mutableBytes.assumingMemoryBound(to: ICMPHeader.self).pointee

       icmpPtr.type = type
       icmpPtr.code = 0
       icmpPtr.checksum = 0
       icmpPtr.identifier = CFSwapInt16BigToHost(identifier)
       icmpPtr.sequenceNumber = CFSwapInt16HostToBig(identifier)
       memcpy(&icmpPtr + 1, payload.bytes, payload.length)

       if (requiresChecksum) {
           icmpPtr.checksum = in_cksum(packet!.bytes, bufferLen: packet!.length);
       }

       var byteBuffer = [UInt8]()
       withUnsafeBytes(of: &icmpPtr) {
         (bytes: UnsafeRawBufferPointer) in byteBuffer += bytes
       }
       packet.replaceBytes(in: NSMakeRange(0, byteBuffer.count), withBytes: byteBuffer)

   }
   return packet
}

NSMutableData 到 UnsafeMutableRawPointer 的字节不更新 mutableData 的值

NSMutableData's bytes to UnsafeMutableRawPointer not updating the value of mutableData

unsafe-pointers

icmp

ios

swift3

unsafemutablepointer