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Beyond Network Sockets

Up until this point, we first learned how to use the Berkeley Sockets API, then we learned about the client-server model, based on this API. So now we know that sockets offer a ubiquitous interface for inter-process communication, which is great. A program written in Python can easily send data to another one written in C, D, Java, Haskell, you name it. However, in the section dedicated to networking, we saw that it takes a whole stack of protocols to send this message from one process to the other. As you might imagine, this is much slower even than local I/O using files.

So far we've only used sockets for local communication, but in practice it is a bit counterproductive to use network sockets for local IPC due to their high latency. Wouldn't it be great if we had a way to use the sockets API for local IPC without having to deal with this increased latency? Well, there is a way and it's called UNIX sockets.

UNIX Sockets​

UNIX sockets are created using the socket() syscall and are bound TO A FILE instead of an IP and port using bind(). You may already see a similarity with named pipes. Just like them, UNIX sockets don't work by writing data to the file (that would be slow), but instead the kernel retains the data they send internally so that send() and recv() can read it from the kernel's storage. You can use read() and write() to read/write data from/to both network and UNIX sockets as well, by the way. The differences between using send()/recv() or write()/read() are rather subtle and are described in this Stack Overflow thread.

UNIX sockets are a feature of POSIX-compliant operating systems (e.g. Linux, macOS) and are not available on non-POSIX operating systems, such as Microsoft Windows. However, there are third-party libraries providing similar features to UNIX sockets in non-POSIX systems, but they might not have the same performance and reliability.

Practice: Receive from UNIX Socket​

Navigate to support/receive-challenges/receive_unix_socket.c. Don't write any code yet. Let's compare UNIX sockets with network sockets first:

  • To create them, give socket() the PF_UNIX/AF_UNIX instead of PF_INET/AF_INET. The latter are used to create network sockets. You can use PF_* or AF_* interchangeably. They mean "protocol families" and "address families", respectively, but they are essentially the same thing. But apart from this, UNIX sockets can be set to work in both TCP (SOCK_STREAM) and UDP (SOCK_DGRAM) modes.

  • The bind() call now takes a path as argument, as specified in the UNIX socket man page.

And this is it. Everything else is the same.

Now fill in the TODOs in receive_unix_socket.c. Use the example at the bottom of the UNIX socket man page if you get stuck at bind(). When you're done, compile the code and then run send_unix_socket. If you did this task correctly, receive_unix_socket should display the flag.