what is the protocol parameter in winsock's socket function for?

The winsock function socket expects as third parameter the protocol what usually is IPROTO_TCP for socket type SOCK_STREAM and IPROTO_UDP for socket type SOCK_DGRAM. When I pass a 0 value as the protocol parameter, TCP and UDP work as expected.

    SOCKET s = socket(AF_INET, SOCK_DGRAM, 0)

    // s is a valid socket

What is the IPROTO_IP protocol parameter value meant to? If it's only intented to be used with SOCK_RAW, why is there this kind of redundancy?

socket(AF_INET, SOCK_STREAM, IPROTO_TCP);
socket(AF_INET, SOCK_DGRAM, IPROTO_UDP);

What actually does the protocol parameter specify? when I can just use another value, it looks like that it's unimportant.

I want to send UDP packets (including broadcasts) from a PC with more than one netword card to a specific ethernet segment. While the IP routing normally select the network card (and source address) I would like specify the adapter(s) and think about raw sockets or any other means to achieve this goal. Probably this IPPROTO_IP may help in this case.

I think the documentation for socket (which can be found here: http://msdn.microsoft.com/en-us/library/ms740506(VS.85).aspx) is pretty clear on what the value is for and why passing 0 is fine if you don't care.

A situation where you might want to pass something different is if you wanted to set up a socket for an unusual connection type; such as bluetooth, or if you wanted to create a PGM reliable multicast socket, etc.

Your second question is unrelated to raw sockets or the protocol parameters. What you need to do is simply bind your socket to the address of the local interface that you want to use; so rather than binding to INADDR_ANY and allowing the stack to decide for you, you tell it which interface to use.