C++ Handbook

Header Reference: <semaphore>

Header Reference: <semaphore>

Counting and binary semaphore facilities from <semaphore>.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

  • Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
  • Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
  • Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

What header pages are for

Header reference pages are meant to answer a practical question quickly: what this header provides, when to reach for it, and which usage rules are easiest to get wrong.

  • Start here when you already know roughly which header you need but want a fast operational summary.
  • Use the example section below as a minimal pattern, then adapt it to your real container, ownership, text, or concurrency workflow.
  • Jump to broader index pages when you need exhaustive coverage rather than a header-focused summary.

Header Reference: <semaphore>

Main facilities

What this header is for

Use <semaphore> for permit-based coordination, throttling, and capacity management across threads.

Common patterns

Small worked example

#include <semaphore>

int main() {
	std::counting_semaphore<3> slots{2};
	slots.acquire();
	slots.release();
}

The key idea is permits. A semaphore answers "how many operations may proceed now?" without forcing you to expose a mutex-protected container or condition predicate directly.

Selection guide

Minimal example

#include <semaphore>

    int main() {
        std::binary_semaphore ready{0};
        ready.release();
        ready.acquire();
        return 0;
    }

What to verify before relying on this header

  • How the code will shut down, unblock, or stop safely under cancellation and failure paths.
  • Which state is shared, which synchronization primitive owns that responsibility, and which memory-ordering assumptions are actually required.
  • Whether a higher-level facility such as `std::jthread`, `std::future`, or structured task composition would reduce coordination risk.