shmtool

shmtool

A command line utility and library for interacting with System V-style shared memory segments, written in Golang.

NOTE: This is not a portable, cross-platform shared memory library. It works with Linux (known) and various Unix-based systems (untested). It notably will NOT work on Windows.

If you need a cross-platform and/or Windows-compatible library, check out:

• https://github.com/hidez8891/shm
• https://github.com/edsrzf/mmap-go

Overview

Shared memory is an inter-process communication mechanism that allows for multiple, independent processes to access and modify the same portion of system memory for the purpose of sharing data between them. This library implements a Golang wrapper around the original implementation of this which is present on almost all *NIX systems that implement portions of the UNIX System V feature set.

The use of the calls implemented by this library has largely been supplanted by POSIX shared memory and the mmap() call, but there are some use cases that still require this particular approach to shared memory management. One notable example is the MIT-SHM X Server extension which expects SysV shared memory semantics.

Basic Usage

package main

import (
  "github.com/ghetzel/shmtool/shm"
  "io"
  "os"
)

func main() {
  // create a 28MiB shared memory segment that other processes can write to
  if segment, err := shm.Create(1024 * 1024 * 28); err == nil {
    // Mark the segment for destruction when the program exits
    //
    // NOTE: The memory segment will only be destroyed when all processes that have attached
    //       to it have detached, which can happen explicitly (here via segment.Detach(ptr)),
    //       or implicitly when the process exits.
    //
    // NOTE: Memory is not overwritten / zeroed out when destroyed.  If you have sensitive data in
    //       this memory segment, you must overwrite it yourself before detaching.
    //
    defer segment.Destroy()

    // Call the Attach() function on the created segment to get the memory address
    // where data can be read or written.  You must do this even if you don't use the address
    // directly as this is what makes the shared memory segment "part of" this process's memory
    // space, and thus allowing you to read from/write to it.
    if segmentAddress, err := segment.Attach(); err == nil {
      defer segment.Detach(segmentAddress)

      // Write the contents of standard input to the shared memory area.
      if _, err := io.Copy(segment, os.Stdin); err != nil {
        panic(err.Error())
      }

      // Do something, maybe tell another process to start and read from this segment (which
      // is communicated by giving the other process the address in segmentAddress).
      //

      // Read the contents of the shared memory area, which may (or may not) have been modified by
      // another program.
      if _, err := io.Copy(os.Stdout, segment); err != nil {
        panic(err.Error())
      }
    } else {
      panic(err.Error())
    }
  } else {
    panic(err.Error())
  }
}

See Also

• System V interprocess communication mechanisms
• Use of the shmget() system call