Receiving order of socket

Question

I am using socket to send data from local machine to remote in TCP, stream mode. The code in the local side is :

// ----------- Local
send(sd, pData, iSize, 0);  // send data

The size of the data is about 1Mb, so socket might divide it to several packets. While I am recieving the data on remote side, I have to recieve the data separately, and then combine them together. The code in the remote side is :

// ----------- Remote : Receiving data
int         iSizeThis(0);// size of a single separated data
static int  iSizeAcc(0);//size of the total data I have already got.
static int  iDataSize(0);// size of the original data.

// Get size
if (iDataSize <= 0)
{
    if ( (iSizeThis = recv(cli_sd, (char*)&iDataSize, 4, MSG_PEEK)) == 0) {
        ....
    } else if (iSizeThis == SOCKET_ERROR) {
        ....
    } else {
        // Allocates memory
        if (iDataSize > 0)
            pData = realloc(pData, iDataSize);
    }

} else if (iSizeAcc < iDataSize){

    // Get data.
    // The size of the data is about 1Mb, so socket will divide it to several packets.
    // I have to recieve the data separately, and then combine them together.

    iSizeThis = recv(cli_sd, ((char*)pData) + iSizeAcc, iDataSize - iSizeAcc, 0);

    iSizeAcc += iSizeThis;

    //{// If I uncomment this block, the recieving order will be reversed. Why?????
    //  static int i(0);
    //  std::ostringstream oss;
    //  oss << i++ << "\n\n";
    //  oss << "iSizeThis : " << iSizeThis << "\n";
    //  oss << "iSizeAcc : " << iSizeAcc << "\n";
    //  oss << "iDataSize : " << iDataSize << "\n";
    //  ::MessageBoxA(this->GetSafeHwnd(), oss.str().c_str(), "---", 0);
    //}

    // If all the fragment are combined into pData, the save it to a file.
    if (iSizeAcc >= iDataSize){
        // Save to file
        FILE * pFile;
        pFile = fopen ("CCC.dat","wb");
        if (pFile != NULL){
            fwrite ( ((char*)pData)+4 , 1 , iDataSize-4 , pFile );
            fclose (pFile);
        }

        iSizeAcc = 0;
        iDataSize = 0;
    }
}

The odd thing is. If I uncomment the message block on remote side, the recieving order will be reversed. Thus, the result of the remote data is not in a correct order.

Why? (And how could I get the correct order of each fragment?)

Thanks in advance.

Answer 1

While the MessageBoxA function is executing, it pumps messages to your window. Whether or not your thread was expecting them, MessageBoxA dispatched them to you.

Answer 2

Calling MessageBoxA (a blocking, modal dialog) in a receive loop is a fundamentally flawed idea. If you want to see the values, run it in a debugger, print them to a dialog (e.g. a text field), output them to the console or dump them to a file.