Hi, I'm pretty new in network programming, and I have to understand how can I know if and UDP send fails and how ca I recover the message sent (if it is very importanat). Some code will be very useful for me. I use winsock, but I should understand also UNIX-dependent code ... bye ;)

You don't.

Generally people get around that limitation by storing messages sent until the recipient returns some sort of verification. If the verification is not recieved in a certain amount of time, the send is assumed failed, and the message resent.

Ok...

So I should verify that all the data has been sent looking (for instance) at the last bytes to see if them correctly refers to what I need?

Can I have some simple pseudo-code for starting?

Another question: someone told me that I should use TCP for chatting and for authentication, so I'm sure that the whole mssage is recived correctly ... but in this way I have to store a TCP socket for every client, losing a lot of resources then using UDP ... right?

Don't know. I just use TCP for everything. Adding an extra socket for each client is going to be far less resources than keeping a list of un-ack'd UDP messages...

Personally, I've never understood the facination with UDP around here.

What kind of program are you making? If you're only sending a couple of messages per second, you don't usually need UDP. Even if you do decide to use UDP later, you might learn more by using TCP for the early versions.