Unix sockets: when to use bind() function?

Question

I've not a clear idea about when I have to use the bind() function. I guess it should be used whenever I need to receive data (i.e. recv() or recvfrom() functions) whether I'm using TCP or UDP, but somebody told me this is not the case.

Can anyone clarify a bit?

EDIT I've read the answers but actually I'm not so clear. Let's take an example where I have an UDP client which sends the data to the server and then has to get a response. I have to use bind here, right?

Answer 1

This answer is a little bit long-winded, but I think it will help.

When we do computer networking, we're really just doing inter-process communication. Lets say on your own computer you had two programs that wanted to talk to each other. You might use pipe to send the data from one program to another. When you say ls | grep pdf you are taking the output of ls and feeding it into grep. In this way, you have unidirectional communication between the two separate programs ls and grep.

When you do this, someone needs to keep track of the Process ID (PID) of each process. That PID is a unique identifier for each process and it helps us track who the "source" and "destination" processes are for the data we want to transfer.

So now lets say you have data from a webserver than you want to transfer to a browser. Well, this is the same scenario as above - interprocess communication between two programs, the "server" and "browser".

Except this time those two programs are on different computers. The mechanism for interprocess communication across two computers are called "sockets".

So great. You take some data, lob it over the wire, and the other computer receives it. Except that computer doesn't know what to do with that data. Remember we said we need a PID to know which processes are communicating? The same is true in networking. When your computer receives HTML data, how does it know to send it to "firefox" rather than "pidgin"?

Well when you transmit network data, you specify that it goes on a specific "port". Port 80 is usually used for web, port 25 for telnet, port 443 for HTTPS, etc.

And that "port" is bound to a specific process ID on the machine. This is why we have ports. This is why we use bind(). In order to tell the sender which process should receive our data.

This should explain the answers people have posted. If you are a sender, you don't care what the outgoing port is, so you usually don't use bind() to specify that port. If you are a receiver, well, everyone else has to know where to look for you. So you bind() your program to port 80 and then tell everyone to make sure to transmit data there.

To answer your hw question, yes, your probably want to use bind() for your server. But the clients don't need to use bind() - they just need to make sure they transmit data to whatever port you've chosen.