ardupilot/libraries/AP_HAL/Semaphores.h

#pragma once

#include "AP_HAL_Namespace.h"

#include <AP_Common/AP_Common.h>

#define HAL_SEMAPHORE_BLOCK_FOREVER 0

class AP_HAL::Semaphore {
public:

    Semaphore() {}

    // do not allow copying
    CLASS_NO_COPY(Semaphore);

    virtual bool take(uint32_t timeout_ms) WARN_IF_UNUSED = 0 ;
    virtual bool take_nonblocking() WARN_IF_UNUSED = 0;

    // a variant that blocks forever
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wunused-result"
    virtual void take_blocking() { take(HAL_SEMAPHORE_BLOCK_FOREVER); };
    #pragma GCC diagnostic pop
    
    virtual bool give() = 0;
    virtual ~Semaphore(void) {}
};

/*
  a method to make semaphores less error prone. The WITH_SEMAPHORE()
  macro will block forever for a semaphore, and will automatically
  release the semaphore when it goes out of scope

  Note that we have two types of semaphores. A normal semaphore can
  only be taken once. A recursive semaphore allows for the thread
  holding the semaphore to take it again. It must be released the same
  number of times it is taken.

  The WITH_SEMAPHORE() macro can be used with either type of semaphore
 */

class WithSemaphore {
public:
    WithSemaphore(AP_HAL::Semaphore *mtx, uint32_t line);
    WithSemaphore(AP_HAL::Semaphore &mtx, uint32_t line);

    ~WithSemaphore();
private:
    AP_HAL::Semaphore &_mtx;
};

// From: https://stackoverflow.com/questions/19666142/why-is-a-level-of-indirection-needed-for-this-concatenation-macro
#define WITH_SEMAPHORE( sem ) JOIN( sem, __LINE__, __COUNTER__ )
#define JOIN( sem, line, counter ) _DO_JOIN( sem, line, counter )
#define _DO_JOIN( sem, line, counter ) WithSemaphore _getsem ## counter(sem, line)