sys_semaphore.h File Reference

Cross-process named semaphore backed by the host kernel. More...

#include <memory>
#include <string>
#include "./macros.h"

Include dependency graph for sys_semaphore.h:

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Classes
class	vlink::SysSemaphore
	Counting semaphore that can be shared across processes through an OS name. More...

Namespaces
	vlink

Detailed Description

Cross-process named semaphore backed by the host kernel.

vlink::SysSemaphore wraps the platform IPC primitive that is closest to a counting semaphore: a POSIX named semaphore on Linux-like targets, a named Win32 semaphore on Windows, and a dispatch semaphore on macOS. Cross-process synchronisation works on every backend that exposes a kernel name; the macOS dispatch backend is process-local because Apple deprecated the named POSIX API.

Comparison with the in-process vlink::Semaphore:

Aspect	Semaphore (in-process)	SysSemaphore (kernel)
Scope	Single process	Cross-process via OS name
Backing primitive	std::condition_variable	sem_open / CreateSemaphore
Naming	None (object handle only)	UTF-8 name (POSIX requires "/x")
Signal-safe release	No	Yes on POSIX
Persistence	Bound to process lifetime	Persists in kernel namespace until unlinked

Lifecycle:

1. Construct with the initial count for the creator process.
2. Call attach(name) to create the kernel object or open an existing one.
3. Use acquire and release for P/V.
4. Call detach(force) explicitly, or rely on the destructor (force=false).

Note

• The initial count is honoured only on first creation. Subsequent attach() calls on a pre-existing kernel object inherit its current value.
• macOS ignores name and produces an in-process dispatch semaphore.

Example

// Process A (server)
vlink::SysSemaphore sem(0);
sem.attach("/vlink_ready");
do_init();
sem.release();
 
// Process B (client)
vlink::SysSemaphore sem;
sem.attach("/vlink_ready");
sem.acquire();