I heard that FIFOs are named pipes. And they have exactly the same semantics. On the other hand, I think Unix domain socket is quite similar to pipe (although I've never made use of it). So I wonder if they all refer to the same implementation in Linux kernel. Any idea?
Are FIFO, pipe & Unix domain socket the same thing in Linux kernel?
pipe;linux kernel;socket;ipc;fifo
UNIX domain sockets and FIFO may share some part of their implementation but they are conceptually very different. FIFO functions at a very low level. One process writes bytes into the pipe and another one reads from it. A UNIX domain socket has the same behaviour than a TCP/IP socket. A socket is bidirectionnal and can be used by a lot of processes simultaneously. A process can accept many connections on the same socket and attend several clients simultaneously. The kernel delivers a new file descriptor each time connect(2) or accept(2) is called on the socket. The packets will always go to the right process. On a FIFO, this would be impossible. For bidirectional comunication, you need two FIFOs, and you need a pair of FIFOs for each of your clients. There is no way of writing or reading in a selective way, because they are a much more primitive way to communicate.Anonymous pipes and FIFOs are very similar. The difference is that anonymous pipes don't exist as files on the filesystem so no process can open(2) it. They are used by processes that share them by another method. If a process opens a FIFOs and then fork(2) for example, its son will inherit its file descriptors and among them, the pipe.The UNIX domain sockets, anonymous pipes and FIFOs are similar in the fact they use shared memory segments. The details of implementation may vary from one system to another but the idea is always the same: attach the same portion of memory in two distinct processes memory mapping to have them sharing data (edit: that would one obvious way to implement it but that is not how it is actually done in Linux, which simply uses the kernel memory for the buffers, see answer by @tjb63 below). The kernel then handles the system calls and abstracts the mechanism. 