From 7030d64a95138482c2537f3a1280c94ce36a3c10 Mon Sep 17 00:00:00 2001 From: hathach Date: Thu, 17 May 2018 15:55:41 +0700 Subject: rename vendor to lib --- lib/FreeRTOS/Source/include/message_buffer.h | 780 +++++++++++++++++++++++++++ 1 file changed, 780 insertions(+) create mode 100644 lib/FreeRTOS/Source/include/message_buffer.h (limited to 'lib/FreeRTOS/Source/include/message_buffer.h') diff --git a/lib/FreeRTOS/Source/include/message_buffer.h b/lib/FreeRTOS/Source/include/message_buffer.h new file mode 100644 index 000000000..cdd050c21 --- /dev/null +++ b/lib/FreeRTOS/Source/include/message_buffer.h @@ -0,0 +1,780 @@ +/* + * FreeRTOS Kernel V10.0.0 + * Copyright (C) 2017 Amazon.com, Inc. or its affiliates. All Rights Reserved. + * + * 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. If you wish to use our Amazon + * FreeRTOS name, please do so in a fair use way that does not cause confusion. + * + * 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. + * + * http://www.FreeRTOS.org + * http://aws.amazon.com/freertos + * + * 1 tab == 4 spaces! + */ + + +/* + * Message buffers build functionality on top of FreeRTOS stream buffers. + * Whereas stream buffers are used to send a continuous stream of data from one + * task or interrupt to another, message buffers are used to send variable + * length discrete messages from one task or interrupt to another. Their + * implementation is light weight, making them particularly suited for interrupt + * to task and core to core communication scenarios. + * + * ***NOTE***: Uniquely among FreeRTOS objects, the stream buffer + * implementation (so also the message buffer implementation, as message buffers + * are built on top of stream buffers) assumes there is only one task or + * interrupt that will write to the buffer (the writer), and only one task or + * interrupt that will read from the buffer (the reader). It is safe for the + * writer and reader to be different tasks or interrupts, but, unlike other + * FreeRTOS objects, it is not safe to have multiple different writers or + * multiple different readers. If there are to be multiple different writers + * then the application writer must place each call to a writing API function + * (such as xMessageBufferSend()) inside a critical section and set the send + * block time to 0. Likewise, if there are to be multiple different readers + * then the application writer must place each call to a reading API function + * (such as xMessageBufferRead()) inside a critical section and set the receive + * timeout to 0. + * + * Message buffers hold variable length messages. To enable that, when a + * message is written to the message buffer an additional sizeof( size_t ) bytes + * are also written to store the message's length (that happens internally, with + * the API function). sizeof( size_t ) is typically 4 bytes on a 32-bit + * architecture, so writing a 10 byte message to a message buffer on a 32-bit + * architecture will actually reduce the available space in the message buffer + * by 14 bytes (10 byte are used by the message, and 4 bytes to hold the length + * of the message). + */ + +#ifndef FREERTOS_MESSAGE_BUFFER_H +#define FREERTOS_MESSAGE_BUFFER_H + +/* Message buffers are built onto of stream buffers. */ +#include "stream_buffer.h" + +#if defined( __cplusplus ) +extern "C" { +#endif + +/** + * Type by which message buffers are referenced. For example, a call to + * xMessageBufferCreate() returns an MessageBufferHandle_t variable that can + * then be used as a parameter to xMessageBufferSend(), xMessageBufferReceive(), + * etc. + */ +typedef void * MessageBufferHandle_t; + +/*-----------------------------------------------------------*/ + +/** + * message_buffer.h + * +
+MessageBufferHandle_t xMessageBufferCreate( size_t xBufferSizeBytes );
+
+ * + * Creates a new message buffer using dynamically allocated memory. See + * xMessageBufferCreateStatic() for a version that uses statically allocated + * memory (memory that is allocated at compile time). + * + * configSUPPORT_DYNAMIC_ALLOCATION must be set to 1 or left undefined in + * FreeRTOSConfig.h for xMessageBufferCreate() to be available. + * + * @param xBufferSizeBytes The total number of bytes (not messages) the message + * buffer will be able to hold at any one time. When a message is written to + * the message buffer an additional sizeof( size_t ) bytes are also written to + * store the message's length. sizeof( size_t ) is typically 4 bytes on a + * 32-bit architecture, so on most 32-bit architectures a 10 byte message will + * take up 14 bytes of message buffer space. + * + * @return If NULL is returned, then the message buffer cannot be created + * because there is insufficient heap memory available for FreeRTOS to allocate + * the message buffer data structures and storage area. A non-NULL value being + * returned indicates that the message buffer has been created successfully - + * the returned value should be stored as the handle to the created message + * buffer. + * + * Example use: +
+
+void vAFunction( void )
+{
+MessageBufferHandle_t xMessageBuffer;
+const size_t xMessageBufferSizeBytes = 100;
+
+    // Create a message buffer that can hold 100 bytes.  The memory used to hold
+    // both the message buffer structure and the messages themselves is allocated
+    // dynamically.  Each message added to the buffer consumes an additional 4
+    // bytes which are used to hold the lengh of the message.
+    xMessageBuffer = xMessageBufferCreate( xMessageBufferSizeBytes );
+
+    if( xMessageBuffer == NULL )
+    {
+        // There was not enough heap memory space available to create the
+        // message buffer.
+    }
+    else
+    {
+        // The message buffer was created successfully and can now be used.
+    }
+
+
+ * \defgroup xMessageBufferCreate xMessageBufferCreate + * \ingroup MessageBufferManagement + */ +#define xMessageBufferCreate( xBufferSizeBytes ) ( MessageBufferHandle_t ) xStreamBufferGenericCreate( xBufferSizeBytes, ( size_t ) 0, pdTRUE ) + +/** + * message_buffer.h + * +
+MessageBufferHandle_t xMessageBufferCreateStatic( size_t xBufferSizeBytes,
+                                                  uint8_t *pucMessageBufferStorageArea,
+                                                  StaticMessageBuffer_t *pxStaticMessageBuffer );
+
+ * Creates a new message buffer using statically allocated memory. See + * xMessageBufferCreate() for a version that uses dynamically allocated memory. + * + * @param xBufferSizeBytes The size, in bytes, of the buffer pointed to by the + * pucMessageBufferStorageArea parameter. When a message is written to the + * message buffer an additional sizeof( size_t ) bytes are also written to store + * the message's length. sizeof( size_t ) is typically 4 bytes on a 32-bit + * architecture, so on most 32-bit architecture a 10 byte message will take up + * 14 bytes of message buffer space. The maximum number of bytes that can be + * stored in the message buffer is actually (xBufferSizeBytes - 1). + * + * @param pucMessageBufferStorageArea Must point to a uint8_t array that is at + * least xBufferSizeBytes + 1 big. This is the array to which messages are + * copied when they are written to the message buffer. + * + * @param pxStaticMessageBuffer Must point to a variable of type + * StaticMessageBuffer_t, which will be used to hold the message buffer's data + * structure. + * + * @return If the message buffer is created successfully then a handle to the + * created message buffer is returned. If either pucMessageBufferStorageArea or + * pxStaticmessageBuffer are NULL then NULL is returned. + * + * Example use: +
+
+// Used to dimension the array used to hold the messages.  The available space
+// will actually be one less than this, so 999.
+#define STORAGE_SIZE_BYTES 1000
+
+// Defines the memory that will actually hold the messages within the message
+// buffer.
+static uint8_t ucStorageBuffer[ STORAGE_SIZE_BYTES ];
+
+// The variable used to hold the message buffer structure.
+StaticMessageBuffer_t xMessageBufferStruct;
+
+void MyFunction( void )
+{
+MessageBufferHandle_t xMessageBuffer;
+
+    xMessageBuffer = xMessageBufferCreateStatic( sizeof( ucBufferStorage ),
+                                                 ucBufferStorage,
+                                                 &xMessageBufferStruct );
+
+    // As neither the pucMessageBufferStorageArea or pxStaticMessageBuffer
+    // parameters were NULL, xMessageBuffer will not be NULL, and can be used to
+    // reference the created message buffer in other message buffer API calls.
+
+    // Other code that uses the message buffer can go here.
+}
+
+
+ * \defgroup xMessageBufferCreateStatic xMessageBufferCreateStatic + * \ingroup MessageBufferManagement + */ +#define xMessageBufferCreateStatic( xBufferSizeBytes, pucMessageBufferStorageArea, pxStaticMessageBuffer ) ( MessageBufferHandle_t ) xStreamBufferGenericCreateStatic( xBufferSizeBytes, 0, pdTRUE, pucMessageBufferStorageArea, pxStaticMessageBuffer ) + +/** + * message_buffer.h + * +
+size_t xMessageBufferSend( MessageBufferHandle_t xMessageBuffer,
+                           const void *pvTxData,
+                           size_t xDataLengthBytes,
+                           TickType_t xTicksToWait );
+
+ *
+ * Sends a discrete message to the message buffer.  The message can be any
+ * length that fits within the buffer's free space, and is copied into the
+ * buffer.
+ *
+ * ***NOTE***:  Uniquely among FreeRTOS objects, the stream buffer
+ * implementation (so also the message buffer implementation, as message buffers
+ * are built on top of stream buffers) assumes there is only one task or
+ * interrupt that will write to the buffer (the writer), and only one task or
+ * interrupt that will read from the buffer (the reader).  It is safe for the
+ * writer and reader to be different tasks or interrupts, but, unlike other
+ * FreeRTOS objects, it is not safe to have multiple different writers or
+ * multiple different readers.  If there are to be multiple different writers
+ * then the application writer must place each call to a writing API function
+ * (such as xMessageBufferSend()) inside a critical section and set the send
+ * block time to 0.  Likewise, if there are to be multiple different readers
+ * then the application writer must place each call to a reading API function
+ * (such as xMessageBufferRead()) inside a critical section and set the receive
+ * block time to 0.
+ *
+ * Use xMessageBufferSend() to write to a message buffer from a task.  Use
+ * xMessageBufferSendFromISR() to write to a message buffer from an interrupt
+ * service routine (ISR).
+ *
+ * @param xMessageBuffer The handle of the message buffer to which a message is
+ * being sent.
+ *
+ * @param pvTxData A pointer to the message that is to be copied into the
+ * message buffer.
+ *
+ * @param xDataLengthBytes The length of the message.  That is, the number of
+ * bytes to copy from pvTxData into the message buffer.  When a message is
+ * written to the message buffer an additional sizeof( size_t ) bytes are also
+ * written to store the message's length.  sizeof( size_t ) is typically 4 bytes
+ * on a 32-bit architecture, so on most 32-bit architecture setting
+ * xDataLengthBytes to 20 will reduce the free space in the message buffer by 24
+ * bytes (20 bytes of message data and 4 bytes to hold the message length).
+ *
+ * @param xTicksToWait The maximum amount of time the calling task should remain
+ * in the Blocked state to wait for enough space to become available in the
+ * message buffer, should the message buffer have insufficient space when
+ * xMessageBufferSend() is called.  The calling task will never block if
+ * xTicksToWait is zero.  The block time is specified in tick periods, so the
+ * absolute time it represents is dependent on the tick frequency.  The macro
+ * pdMS_TO_TICKS() can be used to convert a time specified in milliseconds into
+ * a time specified in ticks.  Setting xTicksToWait to portMAX_DELAY will cause
+ * the task to wait indefinitely (without timing out), provided
+ * INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h.  Tasks do not use any
+ * CPU time when they are in the Blocked state.
+ *
+ * @return The number of bytes written to the message buffer.  If the call to
+ * xMessageBufferSend() times out before there was enough space to write the
+ * message into the message buffer then zero is returned.  If the call did not
+ * time out then xDataLengthBytes is returned.
+ *
+ * Example use:
+
+void vAFunction( MessageBufferHandle_t xMessageBuffer )
+{
+size_t xBytesSent;
+uint8_t ucArrayToSend[] = { 0, 1, 2, 3 };
+char *pcStringToSend = "String to send";
+const TickType_t x100ms = pdMS_TO_TICKS( 100 );
+
+    // Send an array to the message buffer, blocking for a maximum of 100ms to
+    // wait for enough space to be available in the message buffer.
+    xBytesSent = xMessageBufferSend( xMessageBuffer, ( void * ) ucArrayToSend, sizeof( ucArrayToSend ), x100ms );
+
+    if( xBytesSent != sizeof( ucArrayToSend ) )
+    {
+        // The call to xMessageBufferSend() times out before there was enough
+        // space in the buffer for the data to be written.
+    }
+
+    // Send the string to the message buffer.  Return immediately if there is
+    // not enough space in the buffer.
+    xBytesSent = xMessageBufferSend( xMessageBuffer, ( void * ) pcStringToSend, strlen( pcStringToSend ), 0 );
+
+    if( xBytesSent != strlen( pcStringToSend ) )
+    {
+        // The string could not be added to the message buffer because there was
+        // not enough free space in the buffer.
+    }
+}
+

+ * \defgroup xMessageBufferSend xMessageBufferSend
+ * \ingroup MessageBufferManagement
+ */
+#define xMessageBufferSend( xMessageBuffer, pvTxData, xDataLengthBytes, xTicksToWait ) xStreamBufferSend( ( StreamBufferHandle_t ) xMessageBuffer, pvTxData, xDataLengthBytes, xTicksToWait )
+
+/**
+ * message_buffer.h
+ *
+
+size_t xMessageBufferSendFromISR( MessageBufferHandle_t xMessageBuffer,
+                                  const void *pvTxData,
+                                  size_t xDataLengthBytes,
+                                  BaseType_t *pxHigherPriorityTaskWoken );
+
+ *
+ * Interrupt safe version of the API function that sends a discrete message to
+ * the message buffer.  The message can be any length that fits within the
+ * buffer's free space, and is copied into the buffer.
+ *
+ * ***NOTE***:  Uniquely among FreeRTOS objects, the stream buffer
+ * implementation (so also the message buffer implementation, as message buffers
+ * are built on top of stream buffers) assumes there is only one task or
+ * interrupt that will write to the buffer (the writer), and only one task or
+ * interrupt that will read from the buffer (the reader).  It is safe for the
+ * writer and reader to be different tasks or interrupts, but, unlike other
+ * FreeRTOS objects, it is not safe to have multiple different writers or
+ * multiple different readers.  If there are to be multiple different writers
+ * then the application writer must place each call to a writing API function
+ * (such as xMessageBufferSend()) inside a critical section and set the send
+ * block time to 0.  Likewise, if there are to be multiple different readers
+ * then the application writer must place each call to a reading API function
+ * (such as xMessageBufferRead()) inside a critical section and set the receive
+ * block time to 0.
+ *
+ * Use xMessageBufferSend() to write to a message buffer from a task.  Use
+ * xMessageBufferSendFromISR() to write to a message buffer from an interrupt
+ * service routine (ISR).
+ *
+ * @param xMessageBuffer The handle of the message buffer to which a message is
+ * being sent.
+ *
+ * @param pvTxData A pointer to the message that is to be copied into the
+ * message buffer.
+ *
+ * @param xDataLengthBytes The length of the message.  That is, the number of
+ * bytes to copy from pvTxData into the message buffer.  When a message is
+ * written to the message buffer an additional sizeof( size_t ) bytes are also
+ * written to store the message's length.  sizeof( size_t ) is typically 4 bytes
+ * on a 32-bit architecture, so on most 32-bit architecture setting
+ * xDataLengthBytes to 20 will reduce the free space in the message buffer by 24
+ * bytes (20 bytes of message data and 4 bytes to hold the message length).
+ *
+ * @param pxHigherPriorityTaskWoken  It is possible that a message buffer will
+ * have a task blocked on it waiting for data.  Calling
+ * xMessageBufferSendFromISR() can make data available, and so cause a task that
+ * was waiting for data to leave the Blocked state.  If calling
+ * xMessageBufferSendFromISR() causes a task to leave the Blocked state, and the
+ * unblocked task has a priority higher than the currently executing task (the
+ * task that was interrupted), then, internally, xMessageBufferSendFromISR()
+ * will set *pxHigherPriorityTaskWoken to pdTRUE.  If
+ * xMessageBufferSendFromISR() sets this value to pdTRUE, then normally a
+ * context switch should be performed before the interrupt is exited.  This will
+ * ensure that the interrupt returns directly to the highest priority Ready
+ * state task.  *pxHigherPriorityTaskWoken should be set to pdFALSE before it
+ * is passed into the function.  See the code example below for an example.
+ *
+ * @return The number of bytes actually written to the message buffer.  If the
+ * message buffer didn't have enough free space for the message to be stored
+ * then 0 is returned, otherwise xDataLengthBytes is returned.
+ *
+ * Example use:
+
+// A message buffer that has already been created.
+MessageBufferHandle_t xMessageBuffer;
+
+void vAnInterruptServiceRoutine( void )
+{
+size_t xBytesSent;
+char *pcStringToSend = "String to send";
+BaseType_t xHigherPriorityTaskWoken = pdFALSE; // Initialised to pdFALSE.
+
+    // Attempt to send the string to the message buffer.
+    xBytesSent = xMessageBufferSendFromISR( xMessageBuffer,
+                                            ( void * ) pcStringToSend,
+                                            strlen( pcStringToSend ),
+                                            &xHigherPriorityTaskWoken );
+
+    if( xBytesSent != strlen( pcStringToSend ) )
+    {
+        // The string could not be added to the message buffer because there was
+        // not enough free space in the buffer.
+    }
+
+    // If xHigherPriorityTaskWoken was set to pdTRUE inside
+    // xMessageBufferSendFromISR() then a task that has a priority above the
+    // priority of the currently executing task was unblocked and a context
+    // switch should be performed to ensure the ISR returns to the unblocked
+    // task.  In most FreeRTOS ports this is done by simply passing
+    // xHigherPriorityTaskWoken into taskYIELD_FROM_ISR(), which will test the
+    // variables value, and perform the context switch if necessary.  Check the
+    // documentation for the port in use for port specific instructions.
+    taskYIELD_FROM_ISR( xHigherPriorityTaskWoken );
+}
+

+ * \defgroup xMessageBufferSendFromISR xMessageBufferSendFromISR
+ * \ingroup MessageBufferManagement
+ */
+#define xMessageBufferSendFromISR( xMessageBuffer, pvTxData, xDataLengthBytes, pxHigherPriorityTaskWoken ) xStreamBufferSendFromISR( ( StreamBufferHandle_t ) xMessageBuffer, pvTxData, xDataLengthBytes, pxHigherPriorityTaskWoken )
+
+/**
+ * message_buffer.h
+ *
+
+size_t xMessageBufferReceive( MessageBufferHandle_t xMessageBuffer,
+                              void *pvRxData,
+                              size_t xBufferLengthBytes,
+                              TickType_t xTicksToWait );
+

+ *
+ * Receives a discrete message from a message buffer.  Messages can be of
+ * variable length and are copied out of the buffer.
+ *
+ * ***NOTE***:  Uniquely among FreeRTOS objects, the stream buffer
+ * implementation (so also the message buffer implementation, as message buffers
+ * are built on top of stream buffers) assumes there is only one task or
+ * interrupt that will write to the buffer (the writer), and only one task or
+ * interrupt that will read from the buffer (the reader).  It is safe for the
+ * writer and reader to be different tasks or interrupts, but, unlike other
+ * FreeRTOS objects, it is not safe to have multiple different writers or
+ * multiple different readers.  If there are to be multiple different writers
+ * then the application writer must place each call to a writing API function
+ * (such as xMessageBufferSend()) inside a critical section and set the send
+ * block time to 0.  Likewise, if there are to be multiple different readers
+ * then the application writer must place each call to a reading API function
+ * (such as xMessageBufferRead()) inside a critical section and set the receive
+ * block time to 0.
+ *
+ * Use xMessageBufferReceive() to read from a message buffer from a task.  Use
+ * xMessageBufferReceiveFromISR() to read from a message buffer from an
+ * interrupt service routine (ISR).
+ *
+ * @param xMessageBuffer The handle of the message buffer from which a message
+ * is being received.
+ *
+ * @param pvRxData A pointer to the buffer into which the received message is
+ * to be copied.
+ *
+ * @param xBufferLengthBytes The length of the buffer pointed to by the pvRxData
+ * parameter.  This sets the maximum length of the message that can be received.
+ * If xBufferLengthBytes is too small to hold the next message then the message
+ * will be left in the message buffer and 0 will be returned.
+ *
+ * @param xTicksToWait The maximum amount of time the task should remain in the
+ * Blocked state to wait for a message, should the message buffer be empty.
+ * xMessageBufferReceive() will return immediately if xTicksToWait is zero and
+ * the message buffer is empty.  The block time is specified in tick periods, so
+ * the absolute time it represents is dependent on the tick frequency.  The
+ * macro pdMS_TO_TICKS() can be used to convert a time specified in milliseconds
+ * into a time specified in ticks.  Setting xTicksToWait to portMAX_DELAY will
+ * cause the task to wait indefinitely (without timing out), provided
+ * INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h.  Tasks do not use any
+ * CPU time when they are in the Blocked state.
+ *
+ * @return The length, in bytes, of the message read from the message buffer, if
+ * any.  If xMessageBufferReceive() times out before a message became available
+ * then zero is returned.  If the length of the message is greater than
+ * xBufferLengthBytes then the message will be left in the message buffer and
+ * zero is returned.
+ *
+ * Example use:
+
+void vAFunction( MessageBuffer_t xMessageBuffer )
+{
+uint8_t ucRxData[ 20 ];
+size_t xReceivedBytes;
+const TickType_t xBlockTime = pdMS_TO_TICKS( 20 );
+
+    // Receive the next message from the message buffer.  Wait in the Blocked
+    // state (so not using any CPU processing time) for a maximum of 100ms for
+    // a message to become available.
+    xReceivedBytes = xMessageBufferReceive( xMessageBuffer,
+                                            ( void * ) ucRxData,
+                                            sizeof( ucRxData ),
+                                            xBlockTime );
+
+    if( xReceivedBytes > 0 )
+    {
+        // A ucRxData contains a message that is xReceivedBytes long.  Process
+        // the message here....
+    }
+}
+

+ * \defgroup xMessageBufferReceive xMessageBufferReceive
+ * \ingroup MessageBufferManagement
+ */
+#define xMessageBufferReceive( xMessageBuffer, pvRxData, xBufferLengthBytes, xTicksToWait ) xStreamBufferReceive( ( StreamBufferHandle_t ) xMessageBuffer, pvRxData, xBufferLengthBytes, xTicksToWait )
+
+
+/**
+ * message_buffer.h
+ *
+
+size_t xMessageBufferReceiveFromISR( MessageBufferHandle_t xMessageBuffer,
+                                     void *pvRxData,
+                                     size_t xBufferLengthBytes,
+                                     BaseType_t *pxHigherPriorityTaskWoken );
+

+ *
+ * An interrupt safe version of the API function that receives a discrete
+ * message from a message buffer.  Messages can be of variable length and are
+ * copied out of the buffer.
+ *
+ * ***NOTE***:  Uniquely among FreeRTOS objects, the stream buffer
+ * implementation (so also the message buffer implementation, as message buffers
+ * are built on top of stream buffers) assumes there is only one task or
+ * interrupt that will write to the buffer (the writer), and only one task or
+ * interrupt that will read from the buffer (the reader).  It is safe for the
+ * writer and reader to be different tasks or interrupts, but, unlike other
+ * FreeRTOS objects, it is not safe to have multiple different writers or
+ * multiple different readers.  If there are to be multiple different writers
+ * then the application writer must place each call to a writing API function
+ * (such as xMessageBufferSend()) inside a critical section and set the send
+ * block time to 0.  Likewise, if there are to be multiple different readers
+ * then the application writer must place each call to a reading API function
+ * (such as xMessageBufferRead()) inside a critical section and set the receive
+ * block time to 0.
+ *
+ * Use xMessageBufferReceive() to read from a message buffer from a task.  Use
+ * xMessageBufferReceiveFromISR() to read from a message buffer from an
+ * interrupt service routine (ISR).
+ *
+ * @param xMessageBuffer The handle of the message buffer from which a message
+ * is being received.
+ *
+ * @param pvRxData A pointer to the buffer into which the received message is
+ * to be copied.
+ *
+ * @param xBufferLengthBytes The length of the buffer pointed to by the pvRxData
+ * parameter.  This sets the maximum length of the message that can be received.
+ * If xBufferLengthBytes is too small to hold the next message then the message
+ * will be left in the message buffer and 0 will be returned.
+ *
+ * @param pxHigherPriorityTaskWoken  It is possible that a message buffer will
+ * have a task blocked on it waiting for space to become available.  Calling
+ * xMessageBufferReceiveFromISR() can make space available, and so cause a task
+ * that is waiting for space to leave the Blocked state.  If calling
+ * xMessageBufferReceiveFromISR() causes a task to leave the Blocked state, and
+ * the unblocked task has a priority higher than the currently executing task
+ * (the task that was interrupted), then, internally,
+ * xMessageBufferReceiveFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE.
+ * If xMessageBufferReceiveFromISR() sets this value to pdTRUE, then normally a
+ * context switch should be performed before the interrupt is exited.  That will
+ * ensure the interrupt returns directly to the highest priority Ready state
+ * task.  *pxHigherPriorityTaskWoken should be set to pdFALSE before it is
+ * passed into the function.  See the code example below for an example.
+ *
+ * @return The length, in bytes, of the message read from the message buffer, if
+ * any.
+ *
+ * Example use:
+
+// A message buffer that has already been created.
+MessageBuffer_t xMessageBuffer;
+
+void vAnInterruptServiceRoutine( void )
+{
+uint8_t ucRxData[ 20 ];
+size_t xReceivedBytes;
+BaseType_t xHigherPriorityTaskWoken = pdFALSE;  // Initialised to pdFALSE.
+
+    // Receive the next message from the message buffer.
+    xReceivedBytes = xMessageBufferReceiveFromISR( xMessageBuffer,
+                                                  ( void * ) ucRxData,
+                                                  sizeof( ucRxData ),
+                                                  &xHigherPriorityTaskWoken );
+
+    if( xReceivedBytes > 0 )
+    {
+        // A ucRxData contains a message that is xReceivedBytes long.  Process
+        // the message here....
+    }
+
+    // If xHigherPriorityTaskWoken was set to pdTRUE inside
+    // xMessageBufferReceiveFromISR() then a task that has a priority above the
+    // priority of the currently executing task was unblocked and a context
+    // switch should be performed to ensure the ISR returns to the unblocked
+    // task.  In most FreeRTOS ports this is done by simply passing
+    // xHigherPriorityTaskWoken into taskYIELD_FROM_ISR(), which will test the
+    // variables value, and perform the context switch if necessary.  Check the
+    // documentation for the port in use for port specific instructions.
+    taskYIELD_FROM_ISR( xHigherPriorityTaskWoken );
+}
+

+ * \defgroup xMessageBufferReceiveFromISR xMessageBufferReceiveFromISR
+ * \ingroup MessageBufferManagement
+ */
+#define xMessageBufferReceiveFromISR( xMessageBuffer, pvRxData, xBufferLengthBytes, pxHigherPriorityTaskWoken ) xStreamBufferReceiveFromISR( ( StreamBufferHandle_t ) xMessageBuffer, pvRxData, xBufferLengthBytes, pxHigherPriorityTaskWoken )
+
+/**
+ * message_buffer.h
+ *
+
+void vMessageBufferDelete( MessageBufferHandle_t xMessageBuffer );
+

+ *
+ * Deletes a message buffer that was previously created using a call to
+ * xMessageBufferCreate() or xMessageBufferCreateStatic().  If the message
+ * buffer was created using dynamic memory (that is, by xMessageBufferCreate()),
+ * then the allocated memory is freed.
+ *
+ * A message buffer handle must not be used after the message buffer has been
+ * deleted.
+ *
+ * @param xMessageBuffer The handle of the message buffer to be deleted.
+ *
+ */
+#define vMessageBufferDelete( xMessageBuffer ) vStreamBufferDelete( ( StreamBufferHandle_t ) xMessageBuffer )
+
+/**
+ * message_buffer.h
+
+BaseType_t xMessageBufferIsFull( MessageBufferHandle_t xMessageBuffer ) );
+

+ *
+ * Tests to see if a message buffer is full.  A message buffer is full if it
+ * cannot accept any more messages, of any size, until space is made available
+ * by a message being removed from the message buffer.
+ *
+ * @param xMessageBuffer The handle of the message buffer being queried.
+ *
+ * @return If the message buffer referenced by xMessageBuffer is full then
+ * pdTRUE is returned.  Otherwise pdFALSE is returned.
+ */
+#define xMessageBufferIsFull( xMessageBuffer ) xStreamBufferIsFull( ( StreamBufferHandle_t ) xMessageBuffer )
+
+/**
+ * message_buffer.h
+
+BaseType_t xMessageBufferIsEmpty( MessageBufferHandle_t xMessageBuffer ) );
+

+ *
+ * Tests to see if a message buffer is empty (does not contain any messages).
+ *
+ * @param xMessageBuffer The handle of the message buffer being queried.
+ *
+ * @return If the message buffer referenced by xMessageBuffer is empty then
+ * pdTRUE is returned.  Otherwise pdFALSE is returned.
+ *
+ */
+#define xMessageBufferIsEmpty( xMessageBuffer ) xStreamBufferIsEmpty( ( StreamBufferHandle_t ) xMessageBuffer )
+
+/**
+ * message_buffer.h
+
+BaseType_t xMessageBufferReset( MessageBufferHandle_t xMessageBuffer );
+

+ *
+ * Resets a message buffer to its initial empty state, discarding any message it
+ * contained.
+ *
+ * A message buffer can only be reset if there are no tasks blocked on it.
+ *
+ * @param xMessageBuffer The handle of the message buffer being reset.
+ *
+ * @return If the message buffer was reset then pdPASS is returned.  If the
+ * message buffer could not be reset because either there was a task blocked on
+ * the message queue to wait for space to become available, or to wait for a
+ * a message to be available, then pdFAIL is returned.
+ *
+ * \defgroup xMessageBufferReset xMessageBufferReset
+ * \ingroup MessageBufferManagement
+ */
+#define xMessageBufferReset( xMessageBuffer ) xStreamBufferReset( ( StreamBufferHandle_t ) xMessageBuffer )
+
+
+/**
+ * message_buffer.h
+
+size_t xMessageBufferSpaceAvailable( MessageBufferHandle_t xMessageBuffer ) );
+

+ * Returns the number of bytes of free space in the message buffer.
+ *
+ * @param xMessageBuffer The handle of the message buffer being queried.
+ *
+ * @return The number of bytes that can be written to the message buffer before
+ * the message buffer would be full.  When a message is written to the message
+ * buffer an additional sizeof( size_t ) bytes are also written to store the
+ * message's length.  sizeof( size_t ) is typically 4 bytes on a 32-bit
+ * architecture, so if xMessageBufferSpacesAvailable() returns 10, then the size
+ * of the largest message that can be written to the message buffer is 6 bytes.
+ *
+ * \defgroup xMessageBufferSpaceAvailable xMessageBufferSpaceAvailable
+ * \ingroup MessageBufferManagement
+ */
+#define xMessageBufferSpaceAvailable( xMessageBuffer ) xStreamBufferSpacesAvailable( ( StreamBufferHandle_t ) xMessageBuffer )
+
+/**
+ * message_buffer.h
+ *
+
+BaseType_t xMessageBufferSendCompletedFromISR( MessageBufferHandle_t xStreamBuffer, BaseType_t *pxHigherPriorityTaskWoken );
+

+ *
+ * For advanced users only.
+ *
+ * The sbSEND_COMPLETED() macro is called from within the FreeRTOS APIs when
+ * data is sent to a message buffer or stream buffer.  If there was a task that
+ * was blocked on the message or stream buffer waiting for data to arrive then
+ * the sbSEND_COMPLETED() macro sends a notification to the task to remove it
+ * from the Blocked state.  xMessageBufferSendCompletedFromISR() does the same
+ * thing.  It is provided to enable application writers to implement their own
+ * version of sbSEND_COMPLETED(), and MUST NOT BE USED AT ANY OTHER TIME.
+ *
+ * See the example implemented in FreeRTOS/Demo/Minimal/MessageBufferAMP.c for
+ * additional information.
+ *
+ * @param xStreamBuffer The handle of the stream buffer to which data was
+ * written.
+ *
+ * @param pxHigherPriorityTaskWoken *pxHigherPriorityTaskWoken should be
+ * initialised to pdFALSE before it is passed into
+ * xMessageBufferSendCompletedFromISR().  If calling
+ * xMessageBufferSendCompletedFromISR() removes a task from the Blocked state,
+ * and the task has a priority above the priority of the currently running task,
+ * then *pxHigherPriorityTaskWoken will get set to pdTRUE indicating that a
+ * context switch should be performed before exiting the ISR.
+ *
+ * @return If a task was removed from the Blocked state then pdTRUE is returned.
+ * Otherwise pdFALSE is returned.
+ *
+ * \defgroup xMessageBufferSendCompletedFromISR xMessageBufferSendCompletedFromISR
+ * \ingroup StreamBufferManagement
+ */
+#define xMessageBufferSendCompletedFromISR( xMessageBuffer, pxHigherPriorityTaskWoken ) xStreamBufferSendCompletedFromISR( ( StreamBufferHandle_t ) xMessageBuffer, pxHigherPriorityTaskWoken )
+
+/**
+ * message_buffer.h
+ *
+
+BaseType_t xMessageBufferReceiveCompletedFromISR( MessageBufferHandle_t xStreamBuffer, BaseType_t *pxHigherPriorityTaskWoken );
+

+ *
+ * For advanced users only.
+ *
+ * The sbRECEIVE_COMPLETED() macro is called from within the FreeRTOS APIs when
+ * data is read out of a message buffer or stream buffer.  If there was a task
+ * that was blocked on the message or stream buffer waiting for data to arrive
+ * then the sbRECEIVE_COMPLETED() macro sends a notification to the task to
+ * remove it from the Blocked state.  xMessageBufferReceiveCompletedFromISR()
+ * does the same thing.  It is provided to enable application writers to
+ * implement their own version of sbRECEIVE_COMPLETED(), and MUST NOT BE USED AT
+ * ANY OTHER TIME.
+ *
+ * See the example implemented in FreeRTOS/Demo/Minimal/MessageBufferAMP.c for
+ * additional information.
+ *
+ * @param xStreamBuffer The handle of the stream buffer from which data was
+ * read.
+ *
+ * @param pxHigherPriorityTaskWoken *pxHigherPriorityTaskWoken should be
+ * initialised to pdFALSE before it is passed into
+ * xMessageBufferReceiveCompletedFromISR().  If calling
+ * xMessageBufferReceiveCompletedFromISR() removes a task from the Blocked state,
+ * and the task has a priority above the priority of the currently running task,
+ * then *pxHigherPriorityTaskWoken will get set to pdTRUE indicating that a
+ * context switch should be performed before exiting the ISR.
+ *
+ * @return If a task was removed from the Blocked state then pdTRUE is returned.
+ * Otherwise pdFALSE is returned.
+ *
+ * \defgroup xMessageBufferReceiveCompletedFromISR xMessageBufferReceiveCompletedFromISR
+ * \ingroup StreamBufferManagement
+ */
+#define xMessageBufferReceiveCompletedFromISR( xMessageBuffer, pxHigherPriorityTaskWoken ) xStreamBufferReceiveCompletedFromISR( ( StreamBufferHandle_t ) xMessageBuffer, pxHigherPriorityTaskWoken )
+
+#if defined( __cplusplus )
+} /* extern "C" */
+#endif
+
+#endif	/* !defined( FREERTOS_MESSAGE_BUFFER_H ) */
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