Comparing Unix/Linux IPC

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Unix IPC

Here are the big seven:

  1. Pipe

    Useful only among processes related as parent/child. Call pipe(2) and fork(2). Unidirectional.

  2. FIFO, or named pipe

    Two unrelated processes can use FIFO unlike plain pipe. Call mkfifo(3). Unidirectional.

  3. Socket and Unix Domain Socket

    Bidirectional. Meant for network communication, but can be used locally too. Can be used for different protocol. There’s no message boundary for TCP. Call socket(2).

  4. Message Queue

    OS maintains discrete message. See sys/msg.h.

  5. Signal

    Signal sends an integer to another process. Doesn’t mesh well with multi-threads. Call kill(2).

  6. Semaphore

    A synchronization mechanism for multi processes or threads, similar to a queue of people waiting for bathroom. See sys/sem.h.

  7. Shared memory

    Do your own concurrency control. Call shmget(2).

Message Boundary issue

One determining factor when choosing one method over the other is the message boundary issue. You may expect “messages” to be discrete from each other, but it’s not for byte streams like TCP or Pipe.

Consider a pair of echo client and server. The client sends string, the server receives it and sends it right back. Suppose the client sends “Hello”, “Hello”, and “How about an answer?”.

With byte stream protocols, the server can receive as “Hell”, “oHelloHow”, and ” about an answer?”; or more realistically “HelloHelloHow about an answer?”. The server has no clue where the message boundary is.

An age old trick is to limit the message length to CHAR_MAX or UINT_MAX and agree to send the message length first in char or uint. So, if you are at the receiving side, you have to read the message length first. This also implies that only one thread should be doing the message reading at a time.

With discrete protocols like UDP or message queues, you don’t have to worry about this issue, but programmatically byte streams are easier to deal with because they behave like files and stdin/out.

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