TCP client/server data exchange

JudyL_MD

bigdenny200 wrote:

I am just using one recv(), so you mean that it is possible that I just get: "hell" and nothing else? Note, I am calling recv() only once.

Exactly. There is no guarantee that a single recv call on one end of a connection will get all the data sent with a single send call on the other end. Looking at the reply you sent to Mark, you need to keep calling recv until you get your "end of packet" marker ;. Handle that packet somewhere somewhen and repeat the process. Judy

Mark Salsbery

bigdenny200 wrote:

Not sure if it is very efficient

I would imagine it is not :) At some point, the TCP protocol has to decide it's waited long enough for 512 bytes and just return what it's received so far. I'm not sure what that timeout value is (or how it decides) but it's certainly less efficient than knowing the exact count to recv. Also, make sure you follow Judy's advice... it is theoretically possible for recv() to receive one byte at a time! Your recv() logic MUST be in a loop, or allow for partial receives some other way. Another reason knowing the exact length to receive is good :) Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Jorgen Sigvardsson

You are best off by buffering the incoming data, and parse from it. I recommend a ring buffer, preferable one which can hold the biggest possible command (otherwise you won't be able to parse a complete sentence). Whatever you do, don't read one byte at a time. It is horrendously inefficient.

-- Kein Mitleid Für Die Mehrheit

bigdenny200

Dear Mark, I got a bit confused. Let me ask you one thing. In previous replies you mention, "a fixed-length "packet" scheme". For example I want to send text: "hiu","bye","guy" seperately. Ans assume I call them using three respectivesend calls. The server knows it should get 3 bytes at a time. So I have

while ( i = recv(buff, 3,..) )
{...}

My question is, does this way guarantee me that first time buffer will contain "hiu", second time "bye", etc. ? Also, what do you mean by: "a 4-byte int - don't forget byte order issues!" In your previous reply :) ? As I can see I am not of appropriate knowledge :( :) Thanks

Jorgen Sigvardsson

JudyL_FL wrote:

There is no guarantee that a single recv call on one end of a connection will get all the data sent with a single send call on the other end.

It is possible with fixed sized messages, and blocking sockets. Although, I don't think the message sizes are fixed, because he mentions parsing messages, separated with a ;

-- Kein Mitleid Für Die Mehrheit

bigdenny200

Thats what I am doing, but as Mark suggested, the TCP timeout can stop receiving the data, if the buffer is too big? Did I misundertand something ?

bigdenny200

I could fix their lengths Jorgen, i.e. if I have "bye" I could remake it to "bye**" and remove two stars later. here block size would be 5. Is that what you mean ? Thanks

Mark Salsbery

bigdenny200 wrote:

you mention, "a fixed-length "packet" scheme"

By that, I meant, for example, a structure (which is fixed length) - since the sender sends a structure, the receiver knows to receive sizeof(thestructure) bytes. This is inefficient if the structure isn't always fully used, since there will be needless extra bytes sent across the network.

bigdenny200 wrote:

The server knows it should get 3 bytes at a time. So I have while ( i = recv(buff, 3,..) ) {...}

The problem here (that Judy was referring to) is that a recv() call is successful even if only ONE byte is received. Even though the sender sent 3 bytes, that doesn't mean the receiver receives 3 bytes. TCP guarantees you'll eventually get the 3 bytes, in order, but there's no guarantee they will come in one call to recv(). Remember, TCP only knows bytes. Here's an example, with some error checking, of a way to receive a known number of bytes into a BYTE buffer...

int BytesRemaining = # of expected bytes;
BYTE *pCurBuf = start address of buffer to recv bytes into;

while (BytesRemaining > 0)
{
int CurBytesReceived = ::recv(hSocket, (char*)pCurBuf, BytesRemaining, 0);

if (CurBytesReceived == 0)
{
// connection was gracefully closed

break;
        }
        else if (nCurBytesReceived == SOCKET_ERROR)
        {
            // Some error occurred!

int rc = WSAGetLastError();

if (rc != WSAEWOULDBLOCK)
            {
                break;
            }
        }
        else
        {
            // Some bytes were successfully received

&nb

Mark Salsbery

Joergen Sigvardsson wrote:

It is possible with fixed sized messages

Not guaranteed. Even with fixed sized messages, there's NO guarantee the TCP protocol won't deliver it in different size chunks. Only a datagram protocol guarantees this. :) Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

bigdenny200

Dear Mark, thank you for your (very) helpful replies. I will try to run the code tomorrow, and see what I get. Thanks for the other hint as well, but believe me I just prey this thing to work only for Windows :):)

Mark Salsbery

bigdenny200 wrote:

I just prey

I would pray instead :)

Mark Salsbery Microsoft MVP - Visual C++ :java:

Jorgen Sigvardsson

With a buffer I mean that you should read in fixed sized chunks. Say 512 bytes per chunk (or less, if your sockets are non-blocking, in which you could end up with 0 bytes, should no bytes have been delivered to the reader). After you read data from the socket, parse/continue parsing the buffer. Once you find the packet you are looking for, remove it from the buffer, and keep the rest of the buffer (might be a partial packet).

Jorgen Sigvardsson

Yes, but looking at it that way (with text based commands and all), it's smarter to take the buffered approach. That way you don't lock yourself down in case you need to change the protocol.

JudyL_MD

Right you are on both accounts - one recv will do if you know the packet size and the OP is not using a fixed size packet

JudyL_MD

He is right if it is a blocking socket. If you ask for x bytes in the recv function, it will either deliver x bytes or return a timeout indication. Judy

Mark Salsbery

What's a blocking socket? ;P Thanks Judy!! Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

JudyL_MD

Some mythical beast that I've heard about and know the legends about but have never encountered in real life. :cool:

Mark Salsbery

Well Judy, I had to test this before I respectfully disagreed with you :) I respectfully disagree... A blocking Recv() WILL return before all bytes requested are received. Here's the test I did - note I try to recv 512 bytes but only send 256...

//----------------------------------------------
// Listener/ receiver thread (started in CMYTestDlg::OnOK() below)
//----------------------------------------------
UINT __cdecl TestThreadProc( LPVOID pParam )
{
    SOCKET ListenSocket;
    ListenSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ListenSocket == INVALID_SOCKET)
    {
        return 1;
    }

sockaddr_in service;
    service.sin_family = AF_INET;
    service.sin_addr.s_addr = inet_addr("127.0.0.1");
    service.sin_port = htons(2460);

if (bind( ListenSocket, (SOCKADDR*) &service, sizeof(service)) == SOCKET_ERROR)
    {
        closesocket(ListenSocket);
        return 1;
    }

if (listen( ListenSocket, 1 ) == SOCKET_ERROR)
    {
        closesocket(ListenSocket);
        return 1;
    }

SOCKET AcceptSocket;

AcceptSocket = accept( ListenSocket, NULL, NULL );
    if (AcceptSocket == INVALID_SOCKET)
    {
        closesocket(ListenSocket);
        return 1;
    }

BYTE *pBuf = new BYTE[512];
BYTE *pCurBuf = pBuf;
int BytesRemaining = 512;

while (BytesRemaining > 0)
{
int CurBytesReceived = ::recv(AcceptSocket, (char*)pCurBuf, BytesRemaining, 0);

if (CurBytesReceived == 0) //<-- put a breakpoint here - 256 bytes will be received
{
// connection was gracefully closed

break;
}
e

JudyL_MD

Can you try calling WSAGetLastError after you receive the partial bytes? I'm curious if it is set to show the timeout. According to the documentation, recv should either read all or timeout. The docs do say: Windows Sockets 2 does not define any standard blocking time-out mechanism for this function I think we're hitting this case - the receive has timed out and who knows how the function is reporting the timeout. I think we can safely say the following: For a blocking socket, you need to check the result from the recv and if you don't get the requested number of bytes, you have hit the timeout condition and need to handle it accordingly. Judy

Mark Salsbery

JudyL_FL wrote:

Can you try calling WSAGetLastError after you receive the partial bytes?

Sure :) WSAGetLastError() returns 0; A long time ago, while writing some of my first communication code, I got bit by this (using pipes actually, but it made me look at my sockets code) and it taught me to read the documentation thoroughly (which I wasn't doing apparently :)). That's when I found this: "For connection-oriented sockets (type SOCK_STREAM for example), calling recv will return as much data as is currently available—up to the size of the buffer specified." FWIW, This also applies to pipes. So, any amount of bytes returned by recv() indicates success. This can be seen especially when receiving amounts larger than the socket buffer, where the protocol MUST return a buffer full of data to make room to receive more. This seems to be the number one thing overlooked by beginner socket programmers. I studied MS's implementation of TCP a while back, and there is a timeout involved internally, but it's not an error of any kind. I believe by default it's 200ms (that's the number that comes to mind) before a recv() returns what it has currently buffered. That's what makes it inefficient to always request more bytes than you're expecting. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java: