Class: Concurrent::Semaphore

Inherits:
SemaphoreImplementation
  • Object
show all
Defined in:
lib/concurrent-ruby/concurrent/atomic/semaphore.rb

Overview

A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each #acquire blocks if necessary until a permit is available, and then takes it. Each #release adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly. Alternatively, permits may be acquired within a block, and automatically released after the block finishes executing.

Examples:

semaphore = Concurrent::Semaphore.new(2)

t1 = Thread.new do
  semaphore.acquire
  puts "Thread 1 acquired semaphore"
end

t2 = Thread.new do
  semaphore.acquire
  puts "Thread 2 acquired semaphore"
end

t3 = Thread.new do
  semaphore.acquire
  puts "Thread 3 acquired semaphore"
end

t4 = Thread.new do
  sleep(2)
  puts "Thread 4 releasing semaphore"
  semaphore.release
end

[t1, t2, t3, t4].each(&:join)

# prints:
# Thread 3 acquired semaphore
# Thread 2 acquired semaphore
# Thread 4 releasing semaphore
# Thread 1 acquired semaphore
semaphore = Concurrent::Semaphore.new(1)

puts semaphore.available_permits
semaphore.acquire do
  puts semaphore.available_permits
end
puts semaphore.available_permits

# prints:
# 1
# 0
# 1

Instance Method Summary collapse

Constructor Details

#initialize(count) ⇒ Object

Create a new ‘Semaphore` with the initial `count`.

Parameters:

  • count (Fixnum) —

    the initial count

Raises:

  • (ArgumentError) —

    if ‘count` is not an integer



161
162
 
# File 'lib/concurrent-ruby/concurrent/atomic/semaphore.rb', line 161

class Semaphore < SemaphoreImplementation
end

Instance Method Details

#acquire(permits = 1) ⇒ nil, BasicObject

Acquires the given number of permits from this semaphore,

blocking until all are available. If a block is given,
yields to it and releases the permits afterwards.

is given, its return value is returned.

Parameters:

  • permits (Fixnum) (defaults to: 1) —

    Number of permits to acquire

Returns:

  • (nil, BasicObject) —

    Without a block, ‘nil` is returned. If a block

Raises:

  • (ArgumentError) —

    if ‘permits` is not an integer or is less than zero



161
162
 
# File 'lib/concurrent-ruby/concurrent/atomic/semaphore.rb', line 161

class Semaphore < SemaphoreImplementation
end

#available_permits ⇒ Integer

Returns the current number of permits available in this semaphore.

Returns:

  • (Integer) 


161
162
 
# File 'lib/concurrent-ruby/concurrent/atomic/semaphore.rb', line 161

class Semaphore < SemaphoreImplementation
end

#drain_permits ⇒ Integer

Acquires and returns all permits that are immediately available.

Returns:

  • (Integer) 


161
162
 
# File 'lib/concurrent-ruby/concurrent/atomic/semaphore.rb', line 161

class Semaphore < SemaphoreImplementation
end

#release(permits = 1) ⇒ nil

Releases the given number of permits, returning them to the semaphore.

Parameters:

  • permits (Fixnum) (defaults to: 1) —

    Number of permits to return to the semaphore.

Returns:

  • (nil)

Raises:

  • (ArgumentError) —

    if ‘permits` is not a number or is less than zero



161
162
 
# File 'lib/concurrent-ruby/concurrent/atomic/semaphore.rb', line 161

class Semaphore < SemaphoreImplementation
end

#try_acquire(permits = 1, timeout = nil) ⇒ true, ...

Acquires the given number of permits from this semaphore,

only if all are available at the time of invocation or within
`timeout` interval. If a block is given, yields to it if the permits
were successfully acquired, and releases them afterward, returning the
block's return value.

Parameters:

  • permits (Fixnum) (defaults to: 1) —

    the number of permits to acquire

  • timeout (Fixnum) (defaults to: nil) —

    the number of seconds to wait for the counter or ‘nil` to return immediately

Returns:

  • (true, false, nil, BasicObject) —

    ‘false` if no permits are available, `true` when acquired a permit. If a block is given, the block’s return value is returned if the permits were acquired; if not, ‘nil` is returned.

Raises:

  • (ArgumentError) —

    if ‘permits` is not an integer or is less than zero



161
162
 
# File 'lib/concurrent-ruby/concurrent/atomic/semaphore.rb', line 161

class Semaphore < SemaphoreImplementation
end