#bitfield #bit #bitfields

macro bitf

This crate provides a procedural macro to easily create a bitfield out of a struct

9 releases (5 stable)

1.3.0 Jan 19, 2022
1.2.0 Jan 15, 2022
0.11.1 Jan 15, 2022
0.10.1 Jan 6, 2022
0.9.0 Jan 1, 2022

#914 in Data structures

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Rust procedural macro to quickly generate bitfield from a structure.


  • Any size from 8 to 128 bits
  • Auto implementation of getters and setters, and Default.
  • Supports the use of other attribute on the structure
  • Declaration of fields either from the Least Significant Bit or the Most Significant Bit
  • Supports custom return types (primitives and custom types)
  • Supports custom visibility for each field
  • Skip implementation of fields marked as reserved
  • Implementation of a Pretty Print associated function: pprint()

By default:

  • starts declaration of fields from the Least Significant Bit;
  • declares all fields as public;
  • does not implement the pretty print function;

Usage and syntax

The macro can be used as following:

#[bitf(size, opt_arg1, opt_arg2, opt_arg3)]

Where size can be:

There are 3 optional parameters:
Order:  can be 'lsb' or 'msb'
Visibility: 'no_pub'
Pretty Print: 'pp'


The size parameter will constrain the total size of the bitfield.


The order parameter is optional and will alter the order in which the fields are declared. By default this parameter is set to lsb. When setting the order parameter to msb, the first declared field of the struct will be set on the most significant bit, and the other way around when using the lsb mode.


The visibility parameter is optional and will alter the visibility of the declared field. It can be set only to no_pub. By default, all fields are declared as public, using the flag no_pub will deactivate this behaviour and rely on the visibility declared by the user.

Hence, the size and position of the field is based on the field declaration :

use bitf::bitf;

#[bitf(u8, lsb, pp)]
struct Example
    any_case_name_2: (),        // () is used to specify to use the raw type defined in the attribute (here is u8)
    _reserved_4:     (),        // This field will not be implemented as the name is _reserved
    name_B_2:        u16,	// Return type override. The `get` method implemented will return a u16
    				// Custom types can be used, be will need to implement the From trait
				// Please see the test file in "test/attribute_macro.rs" for an example

// The internal, full value of the field can be accessed as :

let e = Example::default();
println!("{}", e.raw);

// and the representation of the bitfield can be accessed via

When combined to other attributes, make sure to implement it BEFORE any #[derive(..)] attribute, or the expansion order might (will) fail.

use bitf::bitf;

struct MyStruct
    fieldA_4:	(),
    fieldB_4:	(),

Pretty Print

The Pretty Print parameter is set throught the pp switch. This switch will implement an associated set of functions on the structure, accessible through pprint(). This function will produce the following output (for a 128 bits bitfield):

64     60  59   57  56                 40      35         27     23   21    18  17           7   6     3   2    0
│ 1111 │ 1 │ 01 │ 0 │ rrrrrrrrrrrrrrrr │ 01101 │ 11110101 │ 0110 │ 00 │ 010 │ 0 │ 1000110101 │ 0 │ 110 │ 0 │ 10 │

The field noted as "rrrrrrrr..." symbolizes a reserved field. Such fields are defined when declared with the name _reserved_usize

Please note that there is not any mechanism of paging or any clever system to adapt the output to the shell size. Hence, it will probably fail if you try to print a bitfield of 128 1-byte wide fields, unless you have an exceptionnaly wide screen

Reserved fields: skipping the implementation of a field

You can use the following syntax when declaring a field to skip its implementation. _reserved_intSize

In the previous example, the field _reserved_4 will not have its 4 bits implemented. No accessor will be generated for this field.


Considering the following bitfield:

7             0
0 0 0 0 0 0 0 0
| | | | | | | |_ field_a    - Size 1
| | | | | | |___ fieldB     - Size 1
| | | | | |_____ fieldC     - Size 1
| |  \|/________ reserved   - Size 3
\ /_____________ field_D    - Size 2

It can be achieved with the following declaration and macro usage

use bitf::bitf;

struct MyStruct
    field_a_1:  (),
    fieldB_1:   (),
    FieldC_1:   (),
    _reserved_3: (),
    Field_D_2:  (),

This will generate the following structure and associated methods

struct MyStruct
  pub raw:  u8,

impl MyStruct
    pub fn field_a(self: &Self) -> u8 { /* bitwise logic */ 0 }
    pub fn set_field_a(self: &Self, val: u8) { /* bitwise logic */ }
    pub fn fieldB(self: &Self) -> u8 { /* bitwise logic */ 0 }
    pub fn set_fieldB(self: &Self, val: u8) { /* bitwise logic */ }
     * And so on...

impl Default for MyStruct 
    fn default() -> Self
        MyStruct { raw: 0x0 }

//So you can easily set and read values of each defined bitfield:

let mut bf = MyStruct::default();

println!("{:#010b}", bf.field_a());


  • A short-sighted decision made it that currently the macro is assuming that the format of the declared field is of the form CamelCaseName_Size. Would be better to implement the form Any_Case_Size
  • Generate proper rust documentation
  • Implement a pretty print for easy bitfield reading
  • Skip the implementation of the fields defined as reserved (or not?). Done: you can mark a field as reserved using the naming convention _reserved_intSize
  • Implement a check to fail if the bitfield is too small to hold every declared field
  • Add lsb/msb as optional param, make lsb default
  • Add visibility modifier param. Either all declared field are implemented as pub (default) or specified by user
  • Add custom return type for each declared field
  • Support the addition of attribute to the structure
  • ???


~25K SLoC