FreeRTOS implementation of etl::mutex (#202)

Signed-off-by: Phil Wise <phil@phil-wise.com>

include/etl/mutex.h

@@ -34,6 +34,9 @@ SOFTWARE.
 #if ETL_CPP11_SUPPORTED == 1 && !defined(ETL_NO_STL)
   #include "mutex/mutex_std.h"
   #define ETL_HAS_MUTEX 1
+#elif defined(ETL_TARGET_OS_FREERTOS)
+  #include "mutex/mutex_freertos.h"
+  #define ETL_HAS_MUTEX 1
 #elif defined(ETL_COMPILER_ARM5) || defined(ETL_COMPILER_ARM6) || defined(ETL_COMPILER_ARM7) || defined(ETL_COMPILER_ARM8)
   #include "mutex/mutex_arm.h"
   #define ETL_HAS_MUTEX 1

include/etl/mutex/mutex_freertos.h

@@ -0,0 +1,79 @@
+/******************************************************************************
+The MIT License(MIT)
+
+Embedded Template Library.
+https://github.com/ETLCPP/etl
+https://www.etlcpp.com
+
+Copyright(c) 2020 Phil Wise phil@phil-wise.com
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files(the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions :
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+******************************************************************************/
+
+#ifndef ETL_MUTEX_FREERTOS_INCLUDED
+#define ETL_MUTEX_FREERTOS_INCLUDED
+
+#include "../platform.h"
+
+#include "FreeRTOS.h"
+#include <semphr.h>
+
+namespace etl
+{
+//***************************************************************************
+///\ingroup mutex
+///\brief This mutex class is implemented using FreeRTOS's mutexes
+//***************************************************************************
+class mutex
+{
+ public:
+
+
+  mutex()
+  {
+    access = xSemaphoreCreateMutexStatic(&mutex_allocation);
+  }
+
+  void lock()
+  {
+    xSemaphoreTake(access, portMAX_DELAY); // portMAX_DELAY=block forever
+  }
+
+  bool try_lock()
+  {
+    return xSemaphoreTake(access, 0) == pdTRUE;
+  }
+
+  void unlock()
+  {
+    xSemaphoreGive(access);
+  }
+
+ private:
+  // Non-copyable
+  mutex(const mutex&);
+  mutex& operator=(const mutex&);
+  // Memory to hold the mutex
+  StaticSemaphore_t mutex_allocation;
+  // The mutex handle itself
+  SemaphoreHandle_t access;
+};
+}
+
+#endif