size_t xStreamBufferReceive( StreamBufferHandle_t xStreamBuffer,
TickType_t xTicksToWait );
Receives bytes from a stream 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 xStreamBufferSend())
inside a critical section and use a send block time of 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 xStreamBufferRead()) inside a critical section and use a receive block time of 0.
Use xStreamBufferReceive() to read from a stream buffer from a task. Use
xStreamBufferReceiveFromISR() to read from a stream buffer from an
interrupt service routine (ISR).
Stream buffer functionality is enabled by including the FreeRTOS/source/stream_buffer.c
source file in the build.
The handle of the stream buffer from which bytes are to be received.
A pointer to the buffer into which the received bytes will be
The length of the buffer pointed to by the
pvRxData parameter. This sets the maximum number of bytes to receive in one
call. xStreamBufferReceive will return as many bytes as possible up to a
maximum set by xBufferLengthBytes.
The maximum amount of time the task should remain in the
Blocked state to wait for data to become available if the stream buffer is
empty. xStreamBufferReceive() will return immediately 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. A task does not use any CPU time when it is in the
The number of bytes actually read from the stream buffer, which will
be less than xBufferLengthBytes if the call to xStreamBufferReceive() timed
out before xBufferLengthBytes were available.
void vAFunction( StreamBuffer_t xStreamBuffer )
uint8_t ucRxData[ 20 ];
const TickType_t xBlockTime = pdMS_TO_TICKS( 20 );
/* Receive up to another sizeof( ucRxData ) bytes from the stream buffer.
Wait in the Blocked state (so not using any CPU processing time) for a
maximum of 100ms for the full sizeof( ucRxData ) number of bytes to be
xReceivedBytes = xStreamBufferReceive( xStreamBuffer,
( void * ) ucRxData,
sizeof( ucRxData ),
if( xReceivedBytes > 0 )
/* A ucRxData contains another xRecievedBytes bytes of data, which can
be processed here.... */
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