Semaphore

LibXR::Semaphore provides counting semaphores to coordinate access to shared resources between multiple tasks or ISRs. It supports blocking waits (Wait) in thread context and safe releases (PostFromCallback) in interrupt context. Current implementations cover a Linux futex-backed host path, FreeRTOS Counting Semaphore, ThreadX TX_SEMAPHORE, and none / webasm polling-style variants built around a scalar count plus Timer::RefreshTimerInIdle() while waiting.

Design Highlights

Goal	Description
Cross-platform	Abstracts over Linux futex wait/wake, SemaphoreHandle_t, TX_SEMAPHORE, and similar primitives.
ISR-friendly	Provides PostFromCallback(bool in_isr) to safely release from ISR/DMA callbacks with task switching.
Timeout support	Wait(timeout_ms) supports millisecond-level timeout.
Lightweight	Depends on C++20 and optionally on RTOS headers.
Observability	Value() returns the current count, useful for debugging and performance monitoring.

Core Interface

class Semaphore {
public:
  Semaphore(uint32_t init_count = 0);
  ~Semaphore();

  void     Post();                    // Release from thread context
  void     PostFromCallback(bool in_isr);// Release from ISR/callback context
  ErrorCode Wait(uint32_t timeout=UINT32_MAX); // Blocking wait
  size_t   Value();                   // Current count
};

Error Codes

• ErrorCode::OK       Operation successful
• ErrorCode::TIMEOUT  Wait timed out
• ErrorCode::FAILED The underlying wait failed (Linux, ThreadX)

⚠️ Note: Wait() must not be called from an ISR.