Quality RTOS & Embedded Software


[RTOS Stream Buffer API]


 StreamBufferHandle_t xStreamBufferCreate( size_t xBufferSizeBytes,
                                           size_t xTriggerLevelBytes );

Creates a new stream buffer using dynamically allocated memory. See xStreamBufferCreateStatic() 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 xStreamBufferCreate() to be available.

Stream buffer functionality is enabled by including the FreeRTOS/source/stream_buffer.c source file in the build.

xBufferSizeBytes   The total number of bytes the stream buffer will be able to hold at any one time.
xTriggerLevelBytes   The number of bytes that must be in the stream buffer before a task that is blocked on the stream buffer to wait for data is moved out of the blocked state. For example, if a task is blocked on a read of an empty stream buffer that has a trigger level of 1 then the task will be unblocked when a single byte is written to the buffer or the task’s block time expires. As another example, if a task is blocked on a read of an empty stream buffer that has a trigger level of 10 then the task will not be unblocked until the stream buffer contains at least 10 bytes or the task’s block time expires. If a reading task’s block time expires before the trigger level is reached then the task will still receive however many bytes are actually available. Setting a trigger level of 0 will result in a trigger level of 1 being used. It is not valid to specify a trigger level that is greater than the buffer size.
If NULL is returned, then the stream buffer cannot be created because there is insufficient heap memory available for FreeRTOS to allocate the stream buffer data structures and storage area. A non-NULL value being returned indicates that the stream buffer has been created successfully – the returned value should be stored as the handle to the created stream buffer.

Example usage:

void vAFunction( void )
StreamBufferHandle_t xStreamBuffer;
const size_t xStreamBufferSizeBytes = 100, xTriggerLevel = 10;

    /* Create a stream buffer that can hold 100 bytes.  The memory used to hold
    both the stream buffer structure and the data in the stream buffer is
    allocated dynamically. */
    xStreamBuffer = xStreamBufferCreate( xStreamBufferSizeBytes, xTriggerLevel );

    if( xStreamBuffer == NULL )
        /* There was not enough heap memory space available to create the
        stream buffer. */
        /* The stream buffer was created successfully and can now be used. */
