Description

Concurrency primitives that may be used in a cross-process way to coordinate share memory between processes.

A small C library (libpsem) is compiled to provide portable access to semaphores (based on pyprocessing.berlios.de/). This library is then accessed using FFI to implement Ruby classes ProcessShared::Semaphore, ProcessShared::BoundedSemaphore, ProcessShared::Mutex, and ProcessShared::SharedMemory.

This is an incomplete work in progress.

License

MIT

Install

Install the gem with:

gem install process_shared

Usage

require 'process_shared'

mutex = ProcessShared::Mutex.new
mem = ProcessShared::SharedMemory.new(:int)  # extends FFI::Pointer
mem.put_int(0, 0)

pid1 = fork do
  puts "in process 1 (#{Process.pid})"
  10.times do
    sleep 0.01
    mutex.synchronize do
      value = mem.get_int(0)
      sleep 0.01
      puts "process 1 (#{Process.pid}) incrementing"
      mem.put_int(0, value + 1)
    end
  end
end

pid2 = fork do
  puts "in process 2 (#{Process.pid})"
  10.times do
    sleep 0.01
    mutex.synchronize do
      value = mem.get_int(0)
      sleep 0.01
      puts "process 2 (#{Process.pid}) decrementing"
      mem.put_int(0, value - 1)
    end
  end
end

Process.wait(pid1)
Process.wait(pid2)

puts "value should be zero: #{mem.get_int(0)}"

Transfer Objects Across Processes

# allocate a sufficient memory block
mem = ProcessShared::SharedMemory.new(1024)

# sub process can write (serialize) object to memory (with bounds checking)
pid = fork do
  mem.write_object(['a', 'b'])
end

Process.wait(pid)

# parent process can read the object back (synchronizing access
# with a Mutex left as an excercie to reader)

mem.read_object.must_equal ['a', 'b']

Todo

• Test ConditionVariable

• Implement optional override of core Thread/Mutex classes

• Extend libpsem to win32? (See Python’s processing library)

• Break out tests that use PSem.getvalue() (which isn’t supported on Mac OS X) so that the test suite will pass

• Add finalizer to Mutex? (finalizer on Semaphore objects may be enough) or a method to explicitly close and release resources?

• Test semantics of crashing processes who still hold locks, etc.

• Is SharedArray with Enumerable mixing sufficient Array-like interface?

• Remove bsem from libpsem as it is of little use and doesn’t work on Mac OS X

• Possibly implement BoundedSemaphore with arbitrary bound (in Ruby rather than relying on sem_getvalue()), but this is of little utility beyond extra error checking..