Andrew Main (Zefram)


PerlIO::bitswap - I/O layer to swap bits and bytes


    open($fh, "<:bitswap(7)", $filename);
    open($fh, ">:bitswap(7)", $filename);


This PerlIO layer swaps the order of bits, nybbles, or bytes within bytes or words. It is a convenience when working with a file that uses a different endianness from the program, or when some other part of the system applies unwanted swaps.

The layer takes one argument, which must be a non-negative integer, specified in any of the ways that are by default permitted in Perl source. Each bit of the argument controls one of a set of swaps that are available, and the swaps for all set bits occur simultaneously. Generically, bit N (which has numeric value 2**N) causes the two 2**N-bit halves of each aligned 2**(N+1)-bit unit to be swapped. So, for example, an argument of 16 (only bit 4 set) swaps the two two-octet halves of each four-octet unit, while preserving the order of the octets within each two-octet unit and the order of the bits within each octet. The most useful swap arguments are:


Reverse the order of the eight bits within each octet, leaving the sequence of octets unmodified.


Swap octets within each two-octet word.


Reverse the order of the four octets within each four-octet word.


Reverse the order of the eight octets within each eight-octet word.

If octets or larger units are being swapped (argument 8 or greater), any incomplete swap block will result in an I/O error. It is permitted for individual reads and writes to involve incomplete swap blocks, but any sequence of reads and writes must cover an integral number of swap blocks. Seeks, similarly, must always travel an integral number of swap blocks. The logic assumes that the end of a regular file is always at a block boundary, and will yield incorrect results if that is not the case.


PerlIO, PerlIO::encoding, "open" in perlfunc, "pack" in perlfunc


Andrew Main (Zefram) <>


Copyright (C) 2010, 2011, 2017 Andrew Main (Zefram) <>


This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.