Using the Bitfield Types Generated by Bindgen

Bitfield Strategy Overview

As Rust does not support bitfields, Bindgen generates a struct for each with the following characteristics

• Immutable getter functions for each bitfield named <bitfield>
• Setter functions for each contiguous block of bitfields named set_<bitfield>
• For each contiguous block of bitfields, Bindgen emits an opaque physical field that contains one or more logical bitfields
• A static constructor new_bitfield_{1, 2, ...} with a parameter for each bitfield contained within the opaque physical field.

To keep bindgen from generating the bitfield unit struct, it can be blocklisted like any other type, i.e. --blocklist-type "__BindgenBitfieldUnit". This may be useful if you want to define a custom implementation, or your generated bindings import a pre-existing definition for the bitfield unit type.

Bitfield examples

For this discussion, we will use the following C type definitions and functions.

typedef struct {
    unsigned int a: 1;
    unsigned int b: 1;
    unsigned int c: 2;
    
} StructWithBitfields;

// Create a default bitfield
StructWithBitfields create_bitfield();

// Print a bitfield
void print_bitfield(StructWithBitfields bfield);

Bindgen creates a set of field getters and setters for interacting with the bitset. For example,

    let mut bfield = unsafe { create_bitfield() };
    
    bfield.set_a(1);
    println!("a set to {}", bfield.a());
    bfield.set_b(1);
    println!("b set to {}", bfield.b());
    bfield.set_c(3);
    println!("c set to {}", bfield.c());
    
    unsafe { print_bitfield(bfield) };

will print out

a set to 1
b set to 1
c set to 3
StructWithBitfields: a:1, b:1, c:3

Overflowing a bitfield will result in the same behavior as in C/C++: the bitfield will be set to 0.

    let mut bfield = unsafe { create_bitfield() };
    bfield.set_a(1);
    bfield.set_b(1);
    bfield.set_c(12);
    println!("c set to {} due to overflow", bfield.c());
    
    unsafe { print_bitfield(bfield) };

will print out

c set to 0 due to overflow
StructWithBitfields: a:1, b:1, c:0

To create a new bitfield in Rust, use the bitfield allocation unit constructor.

Note: This requires the Builder's derive_default to be set to true, otherwise the necessary Default functions won't be generated.

    let bfield = StructWithBitfields{
        _bitfield_1: StructWithBitfields::new_bitfield_1(0,0,0),
        ..Default::default()
    };
    
    unsafe { print_bitfield(bfield) };

This will print out

StructWithBitfields: a:0, b:0, c:0