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KernelinsideStream buffers

Updated Jul 2025

xStreamBufferSetTriggerLevel()

[RTOS Stream Buffer API]

stream_buffer.h

1BaseType_t xStreamBufferSetTriggerLevel( StreamBufferHandle_t xStreamBuffer,
2                                         size_t xTriggerLevel );

A stream buffer's trigger level is 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.

A trigger level is set when the stream buffer is created, and can be modified using xStreamBufferSetTriggerLevel().

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

Parameters:

  • xStreamBuffer

    The handle of the stream buffer being updated.

  • xTriggerLevel

    The new trigger level for the stream buffer.

Returns:

If xTriggerLevel was less than or equal to the stream buffer's length then the trigger level will be updated and pdTRUE is returned. Otherwise pdFALSE is returned.