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RFC-0030: FIDL is little endian

RFC-0030: FIDL is little endian
  • FIDL

Declare that FIDL is little endian.

Date submitted (year-month-day)2019-01-30
Date reviewed (year-month-day)2019-01-30


Declare that FIDL is little endian.


Moves FIDL closer to being a persistable format by having a specified, and portable, endianness.

The initial design specifically chose host memory order (and representations) to avoid any need to swizzle memory during transfer. This is key to safely represent FIDL messages as C structs. However, we recognize that the likelihood of running Fuchsias on a big-endian machine anytime soon is small, and hence it is a practical decision to set FIDL to being little-endian.


Currently FIDL is documented to use host-endianness, however all extant hosts are little-endian. In order to move towards a subset of FIDL that is serializable, it's proposed that we declare that FIDL is little endian for now (this amounts to documentation cleanup).

Should we ever need to support a big endian platform many other pieces of code and documentation will need to be updated, and it's proposed that any changes to FIDL be dealt with at that time.

Implementation strategy

Prepare a CL to change FIDL documentation.

Capture the expectation that bindings are only used on little-endian machines as asserts in code.


No change to ergonomics.

Documentation and examples

Spec changes only.

Backwards compatibility

No change currently. Will limit FIDL on a potential future big endian device to need to do some swizzling (minimally for persisted data structures).


No change to current performance. A FIDL revision may be needed should we ever support big-endian platforms.


No change to security.


No change to testing.

Drawbacks, alternatives, and unknowns

There are two big alternatives:

  1. Not providing serializability — but this limits FIDL's applicability in some use cases.
  2. Providing a separate persistable format — but this results in a secondary serialization path needing to be supported everywhere.


While the technical decision to fix FIDL to be little-endian was not particularly controversial, as with many things, a long thread ensued. In it, we learned:

  • MIPS used to do both BE and LE depending on the init vector you clocked into the processor as it came out of reset (a long time ago when MIPS mattered, and when you could buy discrete MIPS CPUs). Some products even switched endianness upon reboot (don't ask why).

    While MIPS is not really much of a thing anymore, we would expect that the gates are embedded in a SoC, and that the endianness is probably fixed (and probably fixed to be little).

  • All ARM cores implement both big and little endian.

    arm64 can select it per EL in the SCTLR. You can switch endian mode at exception level transitions.

    arm(32) selects via the SETEND instruction. It can switch endianness at any point during runtime. Your compiler is unlikely to support this, but it can be useful for some hand-coded assembly.

  • IEEE 802.11 is little endian: the management and control planes for 802.11 traffic use little endian in their fields. All the encapsulated protocols are still big endian, which the 802.11 stack barely touches.

    History goes up to year 1982, when Xerox invented Ethernet. WLAN mostly inherits that decision. The reason to choose the Little Endian? It was an arbitrary choice:

    "The Ethernet itself is also totally insensitive to the interpretation of bits within an octet as constituting the digits of an 8-digit binary numeric value. Since some uniform convention is helpful, however, in avoiding needless incompatibility among different station types, the interpretation is arbitrarily defined to be that ETHERNET SPECIFICATION: Data Link Layer the left-most bit (first transmitted) is the low-order (2^0) digit and the right-most bit (last transmitted) is the high-order (2^7) digit"

Figure: Data Link Layer Frame Format

  • USB is little endian.

  • Trivia: MAC addresses aren't impacted by byte ordering but you can definitely shave a few cycles in your IP routing routine if addresses are big-endian... and perhaps even initiate a lower-latency "cut-through" routing of the packet as it trickles its way in, byte by byte, on an ancient ~1 Mbps link (~128 bytes per millisecond).

  • Fun fact: FAT file systems use little-endian for most, but not all entries in the header. If the checksum over the entire FAT boot sector, read as big-endian, is 0x1234, then the FAT filesystem can be used as a bootable device for an m68k processor on your Atari ST, which implies that the current year is actually still 1985.

  • Fun fact: There is also middle-endian, which standard GCC defines, and notes:

    "If __BYTE_ORDER__ is equal to __ORDER_PDP_ENDIAN__, then bytes in 16-bit words are laid out in a little-endian fashion, whereas the 16-bit subwords of a 32-bit quantity are laid out in big-endian fashion."