Question

我有一个struct对象。还有一种方法，其输入是有效载荷。现在我正在创建一个名为packet的mutableData，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
}

可变字节成功绑定到struct，值也在struct ICMPHeader中更新。

问题是更改struct中的值不会改变可变数据packet的值。

如果，我在创建struct之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。

在struct object icmpPtr中进行更改后，我将其更改回缓冲区指针。

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Data对象，而是更换了字节，它就像魅力一样。

根据文件：

XCode 8.1发行说明：        https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Introduction.html        Swift部分：

一个新的withUnsafeBytes（of :)函数公开了一个值在内存中   表示为UnsafeRawBufferPointer。这个例子复制了一个   异构结构为一个齐次的字节数组：
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
}
一个新的Array.withUnsafeBytes方法公开了一个数组的底层缓冲区作为UnsafeRawBufferPointer。此示例复制一个数组整数成一个字节数组：
let intArray = [1, 2, 3]
var byteBuffer = [UInt8]()

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

所以最后的实现是

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的字节到UnsafeMutableRawPointer不更新mutableData的值

1 个答案: