Data::Bitfield - manage data packed into bytes of multiple bit fields


   use Data::Bitfield qw( bitfield boolfield enumfield );

   # The stat(2) st_mode field on Linux
   bitfield MODE =>
      format      => enumfield(12,
         undef,     "fifo", "char", undef, "dir",    undef, "block", undef,
         "regular", undef,  "link", undef, "socket", undef, undef,   undef ),
      set_uid     => boolfield(11),
      set_gid     => boolfield(10),
      sticky      => boolfield(9),
      user_read   => boolfield(8),
      user_write  => boolfield(7),
      user_exec   => boolfield(6),
      group_read  => boolfield(5),
      group_write => boolfield(4),
      group_exec  => boolfield(3),
      other_read  => boolfield(2),
      other_write => boolfield(1),
      other_exec  => boolfield(0);

   my %modebits = unpack_MODE( stat($path)->mode );

   # The flag register of a Z80
   bitfield FLAGS =>
      sign      => boolfield(7),
      zero      => boolfield(6),
      halfcarry => boolfield(4),
      parity    => boolfield(2),
      subtract  => boolfield(1),
      carry     => boolfield(0);


This module provides a single primary function, bitfield, which creates helper functions in the package that calls it, to assist in managing data that is encoded in sets of bits, called bitfields. This may be useful when interacting with a low-level networking protocol, binary file format, hardware devices, or similar purposes.


   bitfield $name, %fields

Creates two new functions in the calling package whose names are derived from the string $name passed here. These functions will be symmetric opposites, which convert between a key/value list of field values, and their packed binary byte-string or integer representation.

   $packed_value = pack_$name( %field_values )

   %field_values = unpack_$name( $packed_value )

These two functions will work to a set of field names that match those field definitions given to the bitfield function that declared them.

Each field has a name and a definition. Its definition comes from one of the following field-declaration functions.

Additional options may be passed by giving a HASH reference as the first argument, before the structure name.

   bitfield { %options }, $name, %fields

Recognised options are:

format => "bytes-LE" | "bytes-BE" | "integer"

Defines the format that the pack_NAME function will return and the unpack_NAME function will expect to receive as input. The two bytes-* formats describe a packed binary string in either little- or big-endian direction, and integer describes an integer numerical value.

Note that currently the integer format is limited to values 32bits wide or smaller.

Optional; will default to integer if not supplied. This default may change in a later version - make sure to always specify it for now.

unrecognised_ok => BOOL

If true, the pack_ function will not complain about unrecognised field names; they will simply be ignored.



   boolfield( $bitnum )

Declares a single bit-wide field at the given bit index, whose value is a simple boolean truth.


   intfield( $bitnum, $width )

Declares a field of $width bits wide, starting at the given bit index, whose value is an unsigned integer. It will be shifted appropriately.


   signed_intfield( $bitnum, $width )

Since version 0.04.

Declares a field of $width bits wide, starting at the given bit index, whose value is a signed integer. It will be shifted appropriately.


   enumfield( $bitnum, @values )

Declares a field some number of bits wide, sufficient to store an integer big enough to act as an index into the list of values, starting at the given bit index. Its value will be automatically converted to or from one of the values given, which should act sensibly as strings for comparison purposes. Holes can be placed in the range of supported values by using undef.


   constfield( $bitnum, $width, $value )

Declares a field some number of bits wide that stores a constant value. This value will be packed automatically.

Unlike other field definitions, this field is not named. It returns a 2-element list directly for use in the bitfield list.


  • More flexible error-handling - missing/extra values to pack_, extra bits to unpack_.

  • Allow truely-custom field handling, including code to support discontiguous fields.


Paul Evans <>