1.. _module-pw_multibuf: 2 3=========== 4pw_multibuf 5=========== 6.. pigweed-module:: 7 :name: pw_multibuf 8 9Sending or receiving messages via RPC, transfer, or sockets often requires a 10series of intermediate buffers, each requiring their own copy of the data. 11``pw_multibuf`` allows data to be written *once*, eliminating the memory, CPU 12and latency overhead of copying. 13 14----------------- 15How does it work? 16----------------- 17``pw_multibuf`` uses several techniques to minimize copying of data: 18 19- **Header and Footer Reservation**: Lower-level components can reserve space 20 within a buffer for headers and/or footers. This allows headers and footers 21 to be added to user-provided data without moving users' data. 22- **Native Scatter/Gather and Fragmentation Support**: Buffers can refer to 23 multiple separate chunks of memory. Messages can be built up from 24 discontiguous allocations, and users' data can be fragmented across multiple 25 packets. 26- **Divisible Memory Regions**: Incoming buffers can be divided without a copy, 27 allowing incoming data to be freely demultiplexed. 28 29------------------------------- 30What kinds of data is this for? 31------------------------------- 32``pw_multibuf`` is best used in code that wants to read, write, or pass along 33data which are one of the following: 34 35- **Large**: ``pw_multibuf`` is designed to allow breaking up data into 36 multiple chunks. It also supports asynchronous allocation for when there may 37 not be sufficient space for incoming data. 38- **Communications-Oriented**: Data which is being received or sent across 39 sockets, various packets, or shared-memory protocols can benefit from the 40 fragmentation, multiplexing, and header/footer-reservation properties of 41 ``pw_multibuf``. 42- **Copy-Averse**: ``pw_multibuf`` is structured to allow users to pass around 43 and mutate buffers without copying or moving data in-memory. This can be 44 especially useful when working in systems that are latency-sensitive, 45 need to pass large amounts of data, or when memory usage is constrained. 46 47------------- 48API Reference 49------------- 50Most users of ``pw_multibuf`` will start by allocating a ``MultiBuf`` using 51a ``MultiBufAllocator`` class, such as the ``SimpleAllocator``. 52 53``MultiBuf`` s consist of a number of ``Chunk`` s of contiguous memory. 54These ``Chunk`` s can be grown, shrunk, modified, or extracted from the 55``MultiBuf``. ``MultiBuf`` exposes an ``std::byte`` iterator interface as well 56as a ``Chunk`` iterator available through the ``Chunks()`` method. 57 58An RAII-style ``OwnedChunk`` is also provided, and manages the lifetime of 59``Chunk`` s which are not currently stored inside of a ``MultiBuf``. 60 61.. doxygenclass:: pw::multibuf::Chunk 62 :members: 63 64.. doxygenclass:: pw::multibuf::OwnedChunk 65 :members: 66 67.. doxygenclass:: pw::multibuf::MultiBuf 68 :members: 69 70.. doxygenclass:: pw::multibuf::MultiBufAllocator 71 :members: 72 73.. doxygenclass:: pw::multibuf::SimpleAllocator 74 :members: 75 76.. doxygenclass:: pw::multibuf::Stream 77 :members: 78 79Test-only features 80================== 81.. doxygenclass:: pw::multibuf::test::SimpleAllocatorForTest 82 :members: 83 84--------------------------- 85Allocator Implementors' API 86--------------------------- 87Some users will need to directly implement the ``MultiBufAllocator`` interface 88in order to provide allocation out of a particular region, provide particular 89allocation policy, fix Chunks to some size (such as MTU size - header for 90socket implementations), or specify other custom behavior. 91 92These users will also need to understand and implement the following APIs: 93 94.. doxygenclass:: pw::multibuf::ChunkRegionTracker 95 :members: 96 97A simple implementation of a ``ChunkRegionTracker`` is provided, called 98``HeaderChunkRegionTracker``. It stores its ``Chunk`` and region metadata in a 99``Allocator`` allocation alongside the data. The allocation process is 100synchronous, making this class suitable for testing. The allocated region or 101``Chunk`` must not outlive the provided allocator. 102 103.. doxygenclass:: pw::multibuf::HeaderChunkRegionTracker 104 :members: 105 106Another ``ChunkRegionTracker`` specialization is the lightweight 107``SingleChunkRegionTracker``, which does not rely on ``Allocator`` and uses the 108provided memory view to create a single chunk. This is useful when a single 109``Chunk`` is sufficient at no extra overhead. However, the user needs to own 110the provided memory and know when a new ``Chunk`` can be requested. 111 112.. doxygenclass:: pw::multibuf::SingleChunkRegionTracker 113 :members: 114