FieldFlags.jl Documentation

FieldFlags.jl is a small package without dependencies, giving users the ability to create structs containing packed integers of various bitsizes.

The two main exports of this package are the two macros @bitflags and @bitfield, for creating bit packed boolean flag structs and bit packed integer field structs respectively.

This package is heavily inspired by C-style bitfields, though there are some limitations.


  • Low/Negligible overhead
    • I can get a bextract on my machine from extremely high level julia code
  • High performance
  • Good optimization by the compiler (constant folding, elimination of error paths)
    • The package should "feel" as if there were no special implementation
  • Good debuggability with JET.jl


  • Thread safety
    • Accessing the objects produced by this package is not thread safe and atomic access is not planned to be supported. Users are advised to use proper locking to ensure safety.
  • Size of the objects
    • Due to a compiler limitation, the size of all objects created by this package is a multiple of 8 bits. This restriction may be removed in the future.
  • Type parameters cannot be supported - the size of a field needs to be known at definition time, so that the various bitshifts and masking operations done internally can be compiled away.
  • The widest a field can currently be is 8*sizeof(UInt) bits, as UInt is currently the default return type for fields (other than those of width 1, which return a Bool).

Planned Features

  • Custom field type annotations
    • This would look like a regular field type annotation for exact sizes (i.e. a::UInt16), or like e.g. a:3::UInt16 for objects that want to store the lower 3 bits of an UInt16 and want to get that type back out when accessing the field.
    • Due to the nature of how these objects are stored internally, the types will need to be at least isbitstype, possibly even isprimitivetype, as it's unclear whether the padding potentially contained in an isbitstype is legal to observe (I suspect it isn't).
    • <: Signed types will need to be at least 2 bits in size, to store the sign bit.
    • See #9 for the issue tracking this.
  • Narrower field types
    • Currently, all field accesses (unless accessing a single bit field) return an UInt. This is not guaranteed, and may be narrowed in the future, such that a field annotated with width 2 returns an UInt8 by default, width 9 an UInt16 etc.
    • See #7 for the issue tracking this.