Bitfield

This is a Go code generation tool to emulate C bitfields and unions.

Quickstart

Install with:

go install github.com/arl/bitfield@latest

Say you want to generate, in Go, the following C struct:

struct scroll
{
    unsigned X : 5;
    unsigned Y : 5;
};

You'd declare the following Go struct in a file called scroll_gen.go, only used to define the bit field, that's why it's guarded with an go:build ignore tag.

//go:build ignore

package mypkg

//go:generate bitfield -out scroll.go

type Scroll struct {
	X uint8 `bitfield:"5"`
	Y uint8 `bitfield:"5"`
}

And then run:

go generate scroll_gen.go

This creates the following scroll.go:

type Scroll uint16

func (s Scroll) X() uint8 {
	return uint8(s & 0x1f)
}

func (s *Scroll) SetX(val uint8) {
	*s &^= 0x1f
	*s |= Scroll(val & 0x1f)
}

func (s Scroll) Y() uint8 {
	return uint8((s >> 5) & 0x1f)
}

func (s *Scroll) SetY(val uint8) {
	*s &^= 0x1f << 5
	*s |= Scroll(val&0x1f) << 5
}

Note that both getters are defined on the value and setters on the pointer.

Supported field types

These are the allowed types for the fields of the input struct:

• bool
• uint8
• uint16
• uint32
• uint64

bool can only be defined on a 1-bit wide field.

For the generated type, bitfield automatically uses the smallest unsigned integer to accomodate for all the bits of the field.

Bit fields with anonymous unions

The following C union:

union Addr
{
    struct
    {
        unsigned cX : 5;  // Coarse X.
        unsigned cY : 5;  // Coarse Y.
        unsigned nt : 2;  // Nametable.
        unsigned fY : 3;  // Fine Y.
    };
    struct
    {
        unsigned l : 8;
        unsigned h : 7;
    };

    unsigned val : 14;
};

can be defined with:

type Addr struct {
	cX uint8 `bitfield:"5,union=scroll"`
	cY uint8 `bitfield:"5,union=scroll"`
	nt uint8 `bitfield:"2,union=scroll"`
	fY uint8 `bitfield:"3,union=scroll"`

	l uint8 `bitfield:"8,union=lohi"`
	h uint8 `bitfield:"7,union=lohi"`

	// When not specified, implicitely uses the 'default' union.
	val uint8 `bitfield:"14"`
}

Usage

bitfield -h
Usage of bitfield:
  -in string
        INPUT file name (necessary unless within a go:generate comment)
  -out string
        output file name (defaults to standard output)
  -pkg string
        package name (defaults to INPUT file package)
  -type string
        name of the type to convert (defaults to all structs) (default "all")

License

This project is licensed under the MIT - see the LICENSE file for details.