.net – 为什么在尝试使用Marshal.PtrToStructure将字节数组转换为类实例时会出现访问冲突？

2019年8月3日 218次阅读

在C中,我有这个：

struct BasePacketProto
{
    unsigned short PACKET_OPCODE;
    unsigned short PACKET_MAGIC_NUMBER;
    unsigned short PACKET_REMAIN_DATA_LENGTH;
};

struct CM_Request_AcceptAccount : BasePacketProto
{
    unsigned char ACCOUNT_LOGIN[16];
    unsigned char ACCOUNT_PASSWORD[16];
};

static void SendPacket()
{
    CM_Request_AcceptAccount packet;
    packet.PACKET_OPCODE = opCM_Request_AcceptAccount;
    packet.PACKET_MAGIC_NUMBER = 123;
    packet.PACKET_REMAIN_DATA_LENGTH = sizeof(CM_Request_AcceptAccount) -
                                       sizeof(BasePacketProto);
    memcpy(packet.ACCOUNT_LOGIN, "asd", sizeof("asd") * sizeof(char));
    memcpy(packet.ACCOUNT_PASSWORD, "asd_pass", sizeof("asd_pass") * sizeof(char));

    //Send the packet to the server.
    int lLength = send(lhSocket, (const char*)&packet, sizeof(CM_Request_AcceptAccount), 0);
}

在C#,这个：

[StructLayout(LayoutKind.Sequential)]
class BasePacketProto
{
    public System.UInt16 PACKET_OPCODE;
    public System.UInt16 PACKET_MAGIC_NUMBER;
    public System.UInt16 PACKET_REMAIN_DATA_LENGTH;
}

[StructLayout(LayoutKind.Sequential)]
class CM_Request_AcceptAccount : BasePacketProto
{
    public byte[] ACCOUNT_LOGIN = new byte[16];
    public byte[] ACCOUNT_PASSWORD = new byte[16];
}

拆分数据包的类：

public class PacketProcessor
{
    static List<byte> raw_packet = new List<byte>();
    static int PACKET_HEADER_SIZE = Marshal.SizeOf(typeof(BasePacketProto));

    static public void ProcessPacketBytes(byte[] bytes, int size)
    {
        for (int i = 0; i < size; i++)
            raw_packet.Add(bytes[i]); //Adding bytes to own storage

        if (raw_packet.Count < PACKET_HEADER_SIZE) //If we don't have enough bytes
                                                   //to build base packet, we will
                                                   //return and wait for more.
            return;

        //This packet building works fine!
        BasePacketProto bpp =
            ConvertBytesTo<BasePacketProto>(raw_packet.GetRange(
                0, PACKET_HEADER_SIZE).ToArray());

        if (raw_packet.Count >= (PACKET_HEADER_SIZE +
                                 bpp.PACKET_REMAIN_DATA_LENGTH)) //If we have enough
                                           bytes in storage to restore child packet.
        {
            switch ((ClientPacketOpcodes)bpp.PACKET_OPCODE)
            {
                case ClientPacketOpcodes.opCM_Request_AcceptAccount:
                    //But this one fails
                    bpp = ConvertBytesTo<CM_Request_AcceptAccount>(raw_packet.GetRange(
                        0, PACKET_HEADER_SIZE + bpp.PACKET_REMAIN_DATA_LENGTH).ToArray());

                    PacketHandler.Handle_opCM_Request_AcceptAccount((CM_Request_AcceptAccount)bpp);
                    break;

                default:
                    break;
            }

            raw_packet.RemoveRange(0, PACKET_HEADER_SIZE + bpp.PACKET_REMAIN_DATA_LENGTH);
        }
    }

    static T ConvertBytesTo<T>(byte[] data)
    {
        unsafe
        {
            fixed(byte *ptr = data)
            {
                //I am getting an access violation here when trying to
                //build child packet :(
                return (T)Marshal.PtrToStructure(new IntPtr(ptr), typeof(T));
            }
        }
    }
}

接收线程中的某个地方：

while (clientStream.CanRead)
{
    byte[] temp_buff = new byte[1024];
    int received = 0;

    while ((received = clientStream.Read(temp_buff, 0, 1024)) > 0) //Here we receive 38 bytes
    {
        //Passing it to the packet splitter.
        PacketProcessor.ProcessPacketBytes(temp_buff, received);
    }
}

客户端向服务器发送数据包的结果：

System.AccessViolationException was not handled
Attempt to read or write to protected memory. Most likely it points that other memory is corrupted.

为什么它无法将38个字节转换为CM_Request_AcceptAccount？我该怎么做才能让它发挥作用？

最佳答案 当你声明一个类似的结构时

struct CM_Request_AcceptAccount : BasePacketProto
{
    unsigned char ACCOUNT_LOGIN[16];
    unsigned char ACCOUNT_PASSWORD[16];
};

在C中,数组是’内联’和’固定’长度,换句话说,每个字符对结构的’大小’贡献16个字节.

但在C#中,您重新声明了相同的结构：

[StructLayout(LayoutKind.Sequential)]
class CM_Request_AcceptAccount : BasePacketProto
{
    public byte[] ACCOUNT_LOGIN = new byte[16];
    public byte[] ACCOUNT_PASSWORD = new byte[16];
}

在这里,您不提供前16个字节属于ACCOUNT_LOGIN数组而下一个16属于ACCOUNT_PASSWORD的信息.

这条线
    byte [] ACCOUNT_LOGIN =新字节[16]

不告诉编组员什么.它只会导致CLR在代码中创建CM_Request_AcceptAccount实例时在堆上分配一个16字节的数组.

为了正确编组结构,请将C#声明更改为：

[StructLayout(LayoutKind.Sequential)]
class CM_Request_AcceptAccount : BasePacketProto
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=16)]
    public byte[] ACCOUNT_LOGIN;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=16)]
    public byte[] ACCOUNT_PASSWORD;
}

附加信息：由于您的char数组用于保存C风格的字符串,您也可以使用

[MarshalAs(UnmanagedType.ByValTStr, SizeConst=16)]
public string ACCOUNT_LOGIN;

这里要记住的一点是,编组器会在C代码中设置的16个字符中使用空终止字符.

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