size_t xStreamBufferSendFromISR( StreamBufferHandle_t xStreamBuffer,
const void *pvTxData,
BaseType_t *pxHigherPriorityTaskWoken );
Interrupt safe version of the API function that sends a stream of bytes to
the 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 xStreamBufferReceive()) inside a critical section and use a receive block time of 0.
Use xStreamBufferSend() to write to a stream buffer from a task. Use
xStreamBufferSendFromISR() to write to 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 to which a stream is
A pointer to the buffer that holds the bytes to be copied
into the stream buffer.
The maximum number of bytes to copy from pvTxData
into the stream buffer.
(This is an optional parameter that can be set to NULL.)
It is possible that a stream buffer will
have a task blocked on it waiting for data. Calling
xStreamBufferSendFromISR() can make data available, and so cause a task that
was waiting for data to leave the Blocked state. If calling
xStreamBufferSendFromISR() 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, xStreamBufferSendFromISR()
will set *pxHigherPriorityTaskWoken to pdTRUE. If
xStreamBufferSendFromISR() 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 example code below for an example.
The number of bytes written to the stream buffer. If a task times
out before it can write all xDataLengthBytes into the buffer it will still
write as many bytes as possible.
/* A stream buffer that has already been created. */
void vAnInterruptServiceRoutine( void )
char *pcStringToSend = "String to send";
BaseType_t xHigherPriorityTaskWoken = pdFALSE; /* Initialised to pdFALSE. */
/* Attempt to send the string to the stream buffer. */
xBytesSent = xStreamBufferSendFromISR( xStreamBuffer,
( void * ) pcStringToSend,
strlen( pcStringToSend ),
if( xBytesSent != strlen( pcStringToSend ) )
/* There was not enough free space in the stream buffer for the entire
string to be written, ut xBytesSent bytes were written. */
/* If xHigherPriorityTaskWoken was set to pdTRUE inside
xStreamBufferSendFromISR() 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 );
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