[thread.sema.general]

# 32 Concurrency support library [[thread]](./#thread)

## 32.8 Semaphore [[thread.sema]](thread.sema#general)

### 32.8.1 General [thread.sema.general]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10435)

Semaphores are lightweight synchronization primitives
used to constrain concurrent access to a shared resource[.](#1.sentence-1)

They are widely used to implement other synchronization primitives and,
whenever both are applicable, can be more efficient than condition variables[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L10441)

A counting semaphore is a semaphore object
that models a non-negative resource count[.](#2.sentence-1)

A binary semaphore is a semaphore object that has only two states[.](#2.sentence-2)

A binary semaphore should be more efficient than
the default implementation of a counting semaphore with a unit resource count[.](#2.sentence-3)