Winsock, weird sideeffect

Magos · 05-02-2005, 11:13 AM

I got this odd error where if I send some data from a client to a server the message is received but I also get a number of empty messages, the same amount as the nr of letters in the message (in the example below, the string "Hello" gets 5 empty messages.
It's a non-blocking system, the client sends simply using send() and the server's recieve-part looks like this:

Code:

case FD_READ:
{
	CHAR Char;
	INT Result;
	std::string String;
	std::vector<CHAR> Buffer;

	while(TRUE)
	{
		Result = recv(W, &Char, 1, 0);
				
		if(Result == SOCKET_ERROR)
		{
			if(WSAGetLastError() == WSAEWOULDBLOCK) break;
			ErrorMessage = "recv() failed!";
			return FALSE;
		}

		if(Result < 1) break;

		Buffer.push_back(Char);
	}

	Buffer.push_back('\0');
	String = "MSG: ";
	String += &Buffer[0];
	AddMessage(String);

	return TRUE;
}

As an example, if the client sends the string "Hello" to the server it would receive these messages:

Code:

MSG: Hello
MSG:
MSG:
MSG:
MSG:
MSG:

Prolly some stupid error, but I just can't find it...

**Salem** · 05-02-2005, 11:30 AM

It seems to me you add a string even if your while loop exits on the first pass.

Magos · 05-02-2005, 11:47 AM

I could add:

Code:

if(String != "") AddMessage(String);

or some other test to prevent this, but that doesn't remove the fact that the FD_READ message is sent 1000 times for a 1000-byte message, which is insane...

**Salem** · 05-02-2005, 12:25 PM

Are you saying that you read a char, then the loop breaks out because of this
if(WSAGetLastError() == WSAEWOULDBLOCK) break;

Quantum1024 · 05-02-2005, 12:25 PM

Quote:

Originally Posted by Magos

that doesn't remove the fact that the FD_READ message is sent 1000 times for a 1000-byte message, which is insane...

That's because you read it char at a time and FD_READ is sent to indicate that data is still available.

Magos · 05-02-2005, 12:37 PM

Quote:

That's because you read it char at a time and FD_READ is sent to indicate that data is still available.

That seems like a logical reason for my "error". Would it be better to read it it chunks (say 1024 bytes) and then check by the return value how much of the buffer is valid and use that?

**Salem** · 05-02-2005, 12:40 PM

Is there some specific API you can use to find the amount of data available at the moment?

At the moment, you can only guarantee with 'data available' that there is at least one byte.

If you try and read 10 say, and there is only 9 available, then I think you're going to get "WOULDBLOCK" back straight away.

legend · 05-02-2005, 12:48 PM

you can (I think) query the recieve buffer with something like this:

needed = recv(W, &Char, 0, MSG_PEEK);

Quantum1024 · 05-02-2005, 01:13 PM

MSG_PEEK fills the buffer just the same the only difference is that the data isn't removed from wnsock's internal bufers so another recv call would return the same data. I don't recomend using MSG_PEEK EVER!
To get the amount of data available use ioctlsocket with the FIONREAD flag.
The 3rd argument to recv isn't the amount of data you want to receive it is the length of the buffer where the data will be placed so if there is less data then the buffer length recv will still return with no error, the amount received is the return value.

legend · 05-02-2005, 01:46 PM

I was basing my advice on this from MSDN:

"MSG_PEEK: Peeks at the incoming data. The data is copied into the buffer, but is not removed from the input queue. The function subsequently returns the amount of data that can be read in a single call to the recv."

but on closer inspection:

"...which may not be the same as the total amount of data queued on the socket".

so that probably wouldn't be such a good idea after all.

05-02-2005, 11:13 AM	#1
Magos Confused Join Date: Sep 2001 Location: Sweden Posts: 3,122	Winsock, weird sideeffect I got this odd error where if I send some data from a client to a server the message is received but I also get a number of empty messages, the same amount as the nr of letters in the message (in the example below, the string "Hello" gets 5 empty messages. It's a non-blocking system, the client sends simply using send() and the server's recieve-part looks like this: Code: case FD_READ: { CHAR Char; INT Result; std::string String; std::vector<CHAR> Buffer; while(TRUE) { Result = recv(W, &Char, 1, 0); if(Result == SOCKET_ERROR) { if(WSAGetLastError() == WSAEWOULDBLOCK) break; ErrorMessage = "recv() failed!"; return FALSE; } if(Result < 1) break; Buffer.push_back(Char); } Buffer.push_back('\0'); String = "MSG: "; String += &Buffer[0]; AddMessage(String); return TRUE; } As an example, if the client sends the string "Hello" to the server it would receive these messages: Code: MSG: Hello MSG: MSG: MSG: MSG: MSG: Prolly some stupid error, but I just can't find it... __________________ MagosX.com Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Magos is offline

05-02-2005, 11:30 AM	#2
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	It seems to me you add a string even if your while loop exits on the first pass. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
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05-02-2005, 11:47 AM	#3
Magos Confused Join Date: Sep 2001 Location: Sweden Posts: 3,122	I could add: Code: if(String != "") AddMessage(String); or some other test to prevent this, but that doesn't remove the fact that the FD_READ message is sent 1000 times for a 1000-byte message, which is insane... __________________ MagosX.com Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Magos is offline

05-02-2005, 12:25 PM	#4
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	Are you saying that you read a char, then the loop breaks out because of this if(WSAGetLastError() == WSAEWOULDBLOCK) break; __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

05-02-2005, 12:40 PM	#7
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	Is there some specific API you can use to find the amount of data available at the moment? At the moment, you can only guarantee with 'data available' that there is at least one byte. If you try and read 10 say, and there is only 9 available, then I think you're going to get "WOULDBLOCK" back straight away. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

05-02-2005, 12:48 PM	#8
legend Registered User Join Date: May 2005 Posts: 24	you can (I think) query the recieve buffer with something like this: needed = recv(W, &Char, 0, MSG_PEEK);
legend is offline

05-02-2005, 01:13 PM	#9
Quantum1024 Registered User Join Date: Jan 2005 Posts: 847	MSG_PEEK fills the buffer just the same the only difference is that the data isn't removed from wnsock's internal bufers so another recv call would return the same data. I don't recomend using MSG_PEEK EVER! To get the amount of data available use ioctlsocket with the FIONREAD flag. The 3rd argument to recv isn't the amount of data you want to receive it is the length of the buffer where the data will be placed so if there is less data then the buffer length recv will still return with no error, the amount received is the return value.
Quantum1024 is offline

05-02-2005, 01:46 PM	#10
legend Registered User Join Date: May 2005 Posts: 24	I was basing my advice on this from MSDN: "MSG_PEEK: Peeks at the incoming data. The data is copied into the buffer, but is not removed from the input queue. The function subsequently returns the amount of data that can be read in a single call to the recv." but on closer inspection: "...which may not be the same as the total amount of data queued on the socket". so that probably wouldn't be such a good idea after all.
legend is offline

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