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Add a new section under Implementation Details explaining the CRC32 variants available in NuttX, including the difference between the native crc32() and the new Linux/zlib-compatible crc32_ieee(). Signed-off-by: hanzhijian <hanzhijian@zepp.com>
90 lines
2.8 KiB
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90 lines
2.8 KiB
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===
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CRC
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===
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Overview
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========
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NuttX provides CRC (Cyclic Redundancy Check) implementations for CRC8, CRC16,
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and CRC32 in ``libs/libc/misc/``. Each family offers multiple polynomial
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variants, following a consistent naming convention.
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CRC32 Variants
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==============
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NuttX ships several CRC32 variants that differ in polynomial, init value,
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and final XOR. The two most commonly used are ``crc32()`` and ``crc32_ieee()``.
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crc32() — NuttX Native
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-----------------------
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The original NuttX CRC32 implementation using the standard ISO 3309 / ITU-T
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V.42 polynomial (0xEDB88320 reflected). It uses **init value 0** with
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**no final XOR**:
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.. code-block:: c
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uint32_t crc32(FAR const uint8_t *src, size_t len);
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uint32_t crc32part(FAR const uint8_t *src, size_t len, uint32_t crc32val);
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This is the default CRC32 used by internal NuttX subsystems (bbsram, sbram,
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etc.) and is maintained for backward compatibility.
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crc32_ieee() — Linux/zlib Compatible
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-------------------------------------
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An IEEE-standard CRC32 compatible with Linux ``crc32()`` (from zlib). It uses
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the same polynomial but with **init value 0xFFFFFFFF** and **final XOR
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0xFFFFFFFF**:
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.. code-block:: c
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uint32_t crc32_ieee(FAR const uint8_t *src, size_t len);
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uint32_t crc32_ieeepart(FAR const uint8_t *src, size_t len, uint32_t crc32val);
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Use this variant when interoperating with Linux systems, e.g., for CRC
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checksums in network protocols, file transfers, or UART communication layers.
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Comparison
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----------
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============== ============ ========== ==========
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Function Init Value Final XOR Polynomial
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============== ============ ========== ==========
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crc32() 0x00000000 None 0xEDB88320
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crc32_ieee() 0xFFFFFFFF 0xFFFFFFFF 0xEDB88320
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============== ============ ========== ==========
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Example: computing CRC32 of the string ``"123456789"``:
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- ``crc32("123456789", 9)`` returns ``0x2DFD2D88``
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- ``crc32_ieee("123456789", 9)`` returns ``0xCBF43926`` (matches Linux/zlib)
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Incremental Calculation
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-----------------------
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Both variants support incremental (chunked) CRC calculation via the ``_part``
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functions. For ``crc32_ieee``, the first call should pass ``0`` as the initial
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value:
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.. code-block:: c
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uint32_t crc = 0;
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crc = crc32_ieeepart(chunk1, len1, crc);
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crc = crc32_ieeepart(chunk2, len2, crc);
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/* crc now contains the final CRC of chunk1+chunk2 */
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Other CRC32 Variants
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--------------------
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NuttX also provides CRC32 variants using different polynomials:
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- ``crc32h04c11db7()`` / ``crc32h04c11db7_part()`` — Castagnoli (CRC-32C)
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- ``crc32hf4acfb13()`` / ``crc32hf4acfb13_part()`` — CRC-32Q
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CRC16 and CRC8
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==============
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CRC16 and CRC8 follow the same naming pattern with multiple variant functions.
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See ``include/nuttx/crc16.h`` and ``include/nuttx/crc8.h`` for available
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variants.
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