通过Indy UDP正确发送和接收结构 [英] Correctly sending and receiving a struct through Indy UDP

问题描述

考虑使用Borland 2007和Indy UDP服务器和客户端的以下最小程序:

Consider the following minimal program using Borland 2007 and Indy UDP server and client:

struct DATA_PACKAGE
{
    int t;
    int x;
    int y;
};

void __fastcall TForm1::Button1Click(TObject *Sender)
{
   DATA_PACKAGE a;
   a.t = 3;
   a.x = 2;
   a.y = 1;
   Form1->Memo1->Lines->Add("sent " + IntToStr(sizeof(DATA_PACKAGE)));
   Form1->UDPClient1->SendBuffer(server,port,RawToBytes(&a, sizeof(DATA_PACKAGE)));
}

void __fastcall TForm1::UDPServer1UDPRead(TObject *Sender, TBytes AData,
      TIdSocketHandle *ABinding)
{
    DATA_PACKAGE r;
    Form1->Memo1->Lines->Add("received " + IntToStr(sizeof(AData)));
    BytesToRaw(AData, &r, sizeof(AData));
    Form1->Memo1->Lines->Add(IntToStr(r.t) + " " + IntToStr(r.x) + " " + IntToStr(r.y));
}

输出:

sent 12
received 4
3 4717901 0 

首先，为什么它发送12个字节，但只接收4个字节?

First of all, why is it sending 12, but only receiving 4 bytes?

第二，x和y发生了什么?

Secondly what happends to x and y ?

当我将t，x，y的数据类型更改为short时，我得到:

When I change the datatype of t,x,y to short, I get:

sent 6 
received 4 
3 2 0

环顾四周，我发现指针指出了该结构的包装(或可能是字节序?)很重要，但是我找不到一个明确的指南来正确设计它.

looking around I found pointers that the packing (and possibly the endianness?) of the struct is of importance, I could however not find a clear guide how design it properly.

推荐答案

TBytes是动态字节数组，由RTL作为指针实现，这就是sizeof(AData)返回4的原因.请勿使用sizeof(AData)，请改用AData.Length属性:

TBytes is a dynamic array of bytes, it is implemented by the RTL as a pointer, which is why sizeof(AData) is returning 4. Do not use sizeof(AData), use the AData.Length property instead:

void __fastcall TForm1::UDPServer1UDPRead(TObject *Sender, TBytes AData,
      TIdSocketHandle *ABinding)
{
    DATA_PACKAGE r;
    Form1->Memo1->Lines->Add("received " + IntToStr(AData.Length));
    BytesToRaw(AData, &r, AData.Length);
    Form1->Memo1->Lines->Add(IntToStr(r.t) + " " + IntToStr(r.x) + " " + IntToStr(r.y));
}

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