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A Shared Memory Based IPC Mechanism in Rust

Composition

  1. fdpass-rs: This is an older crate found here. However, the code only needed slight brushing up to work with latest Rust versions. Async versions of the functions have been added in case anyone wants to use them.
  2. ipc-queue: This is the underlying data structure that holds pointers to raw, anonymous files mapped to the virtual memory of the process. It uses a (non-blocking) Unix socket to send control messages which basically tell the processes where the pointers are. This crate can be safely used to provide one-way connection only. Currently, it will not throw an error if the same queue is used to communicate in both directions (and might be possible with careful programming) but it is more likely to corrupt the data. This queue needs to number of elements and size of each element.
  3. memenpsf: This crate has two structures that hierarchically wraps around the ipc-queue to provide safe, bidirectional communication and an easy to use API. This crate exports functions like recv, recv_vectored and xmit to enable users to start using the IPC without having to figure out all the internal structures.
  4. Examples: ipc-server and ipc-client are examples of how this shared memory IPC can be used to provide connectivity between two unrelated processes.

Public API (methods)

Create a new interface with name, size of buffer, control stream and type (client=0, server!=0)

new(name: String,
      cap: usize,
      stream: UnixStream,
      typ: u8
) -> Interface<T>

Send a control message over the Unix socket. msg is the read and write pointers

send_ctrl_msg(msg: [u8; 2])

Receive a control message, if any, over the Unix socket.

recv_ctrl_msg

Transmit a single packet over shared memory

xmit(buf: T)

Receive a single message over the shared memory

recv() -> Option<T>

Receive all pending messages over the shared memory

recv_vectored() -> Option<Vec<T>>

A simple run loop

run(name: String,           // name of the client
        cap: usize,         // size of the buffer
        typ: u8,            // client(0) or server(!=0)
        stream: UnixStream, // control stream
        recvr: Receiver<T>, // receive out of the loop
        sender: Sender<T>)  // send into the loop

Advantages

  • The processes don't need to be in parent-child or sibling relationship. This crate allows any two processes to communicate with each other.
  • With certain changes to the code, these crates can establish bidirectional communication network between any number of processes.

Cons

  • Despite a reasonably user-friendly API, some care still needs to be taken while programming with this. For example, element sizes and maximum number of elements have to be mentioned.
    • Interface::<[u8; 2]>::run(...) explicitly mentions that we will store 2 element u8 arrays in the queue.
    • Interface::<&[u8]>::run(...) can some times result in a segmentation fault.

Note

Currently, the example processes/threads use the run function and communicate with the interface through crossbeam channels. However, it is also possible to use the xmit(), recv() and recv_vectored() functions directly.

Current Issues

  • Both client and server run an infinite loop sending and receving data over and over again to make sure that both sides receive data properly.
  • Furthermore, the server side runs this infinite loop within the match stream loop and as a result gets stuck with a single client.

Both these issues might be resolved by refactoring the code.