Re: about Winsock send() and recv()

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>
Thu, 14 Feb 2008 10:50:34 -0600
Tim Roberts wrote:

"" <> wrote:


I'm very new on C and Winsock programming (i did some VB with winsock
I've started with a little app. It works pretty well using an ECHO
server: i send a message, and i receive the same. Cool.
But, when i connect with a SMTP, POP or any other service, i receive
a first message, and then it hangs up.
int sendm(int socket){
FD_SET writeable;
FD_SET(socket, &writeable);
if(select(socket, 0, &writeable, 0, 0) != SOCKET_ERROR){
scanf("%s\r\n", request);
snd_b = strlen(request);
snd_b = send(socket, (char *)request, (snd_b + 1), 0);
return snd_b;

The problem is your use of the dangerous "scanf" function. Because
of the way you have called this, the "request" variable will not have
a CR or LF at the end -- scanf will remove it. TCP services like
SMTP and POP always expect lines to end with a end-of-line sequence
(usually \r\n, although every server I've encountered accepts a lone

So, it's just like you telnetted into your SMTP server, and typed
without pressing return at the end. You'd hang.

Use "fgets" instead of "scanf".

Better yet, accept any type of line ending when you read the request
locally, and tag on the correct protocol-dependent ending (here "\r\n") when
sending to the socket.

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