| .. _module-pw_stream: |
| |
| --------- |
| pw_stream |
| --------- |
| |
| ``pw_stream`` provides a foundational interface for streaming data from one part |
| of a system to another. In the simplest use cases, this is basically a memcpy |
| behind a reusable interface that can be passed around the system. On the other |
| hand, the flexibility of this interface means a ``pw_stream`` could terminate is |
| something more complex, like a UART stream or flash memory. |
| |
| Overview |
| ======== |
| At the most basic level, ``pw_stream``'s interfaces provide very simple handles |
| to enabling streaming data from one location in a system to an endpoint. |
| |
| Example: |
| |
| .. code-block:: cpp |
| |
| void DumpSensorData(pw::stream::Writer& writer) { |
| static char temp[64]; |
| ImuSample imu_sample; |
| imu.GetSample(&info); |
| size_t bytes_written = imu_sample.AsCsv(temp, sizeof(temp)); |
| writer.Write(temp, bytes_written); |
| } |
| |
| In this example, ``DumpSensorData()`` only cares that it has access to a |
| ``Writer`` that it can use to stream data to using ``Writer::Write()``. The |
| ``Writer`` itself can be backed by anything that can act as a data "sink." |
| |
| |
| pw::stream::Writer |
| ------------------ |
| This is the foundational stream ``Writer`` abstract class. Any class that wishes |
| to implement the ``Writer`` interface **must** provide a ``DoWrite()`` |
| implementation. Note that ``Write()`` itself is **not** virtual, and should not |
| be overridden. |
| |
| Buffering |
| ^^^^^^^^^ |
| If any buffering occurs in a ``Writer`` and data must be flushed before it is |
| fully committed to the sink, a ``Writer`` implementation is resposible for any |
| ``Flush()`` capability. |
| |
| pw::stream::Reader |
| ------------------ |
| This is the foundational stream ``Reader`` abstract class. Any class that wishes |
| to implement the ``Reader`` interface **must** provide a ``DoRead()`` |
| implementation. Note that ``Read()`` itself is **not** virtual, and should not |
| be overridden. |
| |
| pw::stream::MemoryWriter |
| ------------------------ |
| The ``MemoryWriter`` class implements the ``Writer`` interface by backing the |
| data destination with an **externally-provided** memory buffer. |
| ``MemoryWriterBuffer`` extends ``MemoryWriter`` to internally provide a memory |
| buffer. |
| |
| pw::stream::MemoryReader |
| ------------------------ |
| The ``MemoryReader`` class implements the ``Reader`` interface by backing the |
| data source with an **externally-provided** memory buffer. |
| |
| pw::stream::NullWriter |
| ------------------------ |
| The ``NullWriter`` class implements the ``Writer`` interface by dropping all |
| requested data writes, similar to ``/dev/null``. |
| |
| Why use pw_stream? |
| ================== |
| |
| Standard API |
| ------------ |
| ``pw_stream`` provides a standard way for classes to express that they have the |
| ability to write data. Writing to one sink versus another sink is a matter of |
| just passing a reference to the appropriate ``Writer``. |
| |
| As an example, imagine dumping sensor data. If written against a random HAL |
| or one-off class, there's porting work required to write to a different sink |
| (imagine writing over UART vs dumping to flash memory). Building a "dumping" |
| implementation against the ``Writer`` interface prevents a dependency from |
| forming on an artisainal API that would require porting work. |
| |
| Similarly, after building a ``Writer`` implementation for a Sink that data |
| could be dumped to, that same ``Writer`` can be reused for other contexts that |
| already write data to the ``pw::stream::Writer`` interface. |
| |
| Before: |
| |
| .. code-block:: cpp |
| |
| // Not reusable, depends on `Uart`. |
| void DumpSensorData(Uart& uart) { |
| static char temp[64]; |
| ImuSample imu_sample; |
| imu.GetSample(&info); |
| size_t bytes_written = imu_sample.AsCsv(temp, sizeof(temp)); |
| uart.Transmit(temp, bytes_written, /*timeout_ms=*/ 200); |
| } |
| |
| After: |
| |
| .. code-block:: cpp |
| |
| // Reusable; no more Uart dependency! |
| void DumpSensorData(Writer& writer) { |
| static char temp[64]; |
| ImuSample imu_sample; |
| imu.GetSample(&info); |
| size_t bytes_written = imu_sample.AsCsv(temp, sizeof(temp)); |
| writer.Write(temp, bytes_written); |
| } |
| |
| Reduce intermediate buffers |
| --------------------------- |
| Often functions that write larger blobs of data request a buffer is passed as |
| the destination that data should be written to. This *requires* a buffer is |
| allocated, even if the data only exists in that buffer for a very short period |
| of time before it's written somewhere else. |
| |
| In situations where data read from somewhere will immediately be written |
| somewhere else, a ``Writer`` interface can cut out the middleman buffer. |
| |
| Before: |
| |
| .. code-block:: cpp |
| |
| // Requires an intermediate buffer to write the data as CSV. |
| void DumpSensorData(Uart* uart) { |
| char temp[64]; |
| ImuSample imu_sample; |
| imu.GetSample(&info); |
| size_t bytes_written = imu_sample.AsCsv(temp, sizeof(temp)); |
| uart.Transmit(temp, bytes_written, /*timeout_ms=*/ 200); |
| } |
| |
| After: |
| |
| .. code-block:: cpp |
| |
| // Both DumpSensorData() and RawSample::AsCsv() use a Writer, eliminating the |
| // need for an intermediate buffer. |
| void DumpSensorData(Writer* writer) { |
| RawSample imu_sample; |
| imu.GetSample(&info); |
| imu_sample.AsCsv(writer); |
| } |
| |
| Prevent buffer overflow |
| ----------------------- |
| When copying data from one buffer to another, there must be checks to ensure the |
| copy does not overflow the destination buffer. As this sort of logic is |
| duplicated throughout a codebase, there's more opportunities for bound-checking |
| bugs to sneak in. ``Writers`` manage this logic internally rather than pushing |
| the bounds checking to the code that is moving or writing the data. |
| |
| Similarly, since only the ``Writer`` has access to any underlying buffers, it's |
| harder for functions that share a ``Writer`` to accidentally clobber data |
| written by others using the same buffer. |
| |
| Before: |
| |
| .. code-block:: cpp |
| |
| Status BuildPacket(Id dest, span<const std::byte> payload, |
| span<std::byte> dest) { |
| Header header; |
| if (dest.size_bytes() + payload.size_bytes() < sizeof(Header)) { |
| return Status::RESOURCE_EXHAUSTED; |
| } |
| header.dest = dest; |
| header.src = DeviceId(); |
| header.payload_size = payload.size_bytes(); |
| |
| memcpy(dest.data(), &header, sizeof(header)); |
| // Forgetting this line would clobber buffer contents. Also, using |
| // a temporary span instead could leave `dest` to be misused elsewhere in |
| // the function. |
| dest = dest.subspan(sizeof(header)); |
| memcpy(dest.data(), payload.data(), payload.size_bytes()); |
| } |
| |
| After: |
| |
| .. code-block:: cpp |
| |
| Status BuildPacket(Id dest, span<const std::byte> payload, Writer& writer) { |
| Header header; |
| header.dest = dest; |
| header.src = DeviceId(); |
| header.payload_size = payload.size_bytes(); |
| |
| writer.Write(header); |
| return writer.Write(payload); |
| } |
| |
| Why NOT pw_stream? |
| ================== |
| pw_stream provides a virtual interface. This inherently has more overhead than |
| a regular function call. In extremely performance-sensitive contexts, a virtual |
| interface might not provide enough utility to justify the performance cost. |
| |
| Dependencies |
| ============ |
| * ``pw_assert`` module |
| * ``pw_preprocessor`` module |
| * ``pw_status`` module |
| * ``pw_span`` module |