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[gemv]: https://rubygems.org/gems/process_shared
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process_shared
==============

Cross-process concurrency primitives that may be used to coordinate
shared memory between processes.

```ruby
require 'process_shared'

mutex = ProcessShared::Mutex.new
cond = ProcessShared::ConditionVariable.new

mem = ProcessShared::SharedMemory.new(:int32, 2)  # extends FFI::Pointer

pid1 = fork do
  nums = mutex.synchronize do
    cond.wait(mutex)
    mem.get_array_of_int(0, 2)
  end
  puts "process #{Process.pid} received #{nums}"
end

pid2 = fork do
  nums = [12345, 67890]
  mutex.synchronize do
    puts "process #{Process.pid} sending #{nums}"
    mem.put_array_of_int(0, nums)
    cond.signal
  end
end

Process.waitall
```

[API Documentation](http://www.rubydoc.info/github/pmahoney/process_shared/master)

FFI is used to access POSIX semaphore on Linux or Mach semaphores on
Mac. Atop these semaphores are implemented `ProcessShared::Semaphore`,
`ProcessShared::Mutex`. POSIX shared memory is used to implement
`ProcessShared::SharedMemory`.

On Linux, POSIX semaphores support `sem_timedwait()` which can wait on
a semaphore but stop waiting after a timeout.

Mac OS X's implementation of POSIX semaphores does not support
timeouts. But, the Mach layer in Mac OS X has its own semaphores that
do support timeouts. Thus, process_shared implements a moderate subset
of the Mach API, which is quite a bit different from POSIX. Namely,
semaphores created in one process are not available in child processes
created via `fork()`. Mach does provide the means to copy capabilities
between tasks (Mach equivalent to processes). In a giant hack, **on OS
X, `process_shared` overrides Ruby's `fork`** methods so that
semaphores are copied from parent to child to emulate the POSIX
behavior.

This is an incomplete work in progress.

License
-------

MIT

Install
-------

Install the gem with:

    gem install process_shared

Usage
-----

```ruby
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
---------------------------------

```ruby
# 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 to win32?  (See Python's processing library)
* 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?