codec/codec.3: Minor formatting fix.

[mLib] / codec / codec.3

.\" -*-nroff-*-
.TH codec 3 "9 January 2009" "Straylight/Edgeware" "mLib utilities library"
.SH NAME
codec \- binary encoding and decoding
.\" @codec_class
.\" @codec_strerror
.\" @null_codec_class
.\" @base64_class
.\" @file64_class
.\" @base64url_class
.\" @base32_class
.\" @base32hex_class
.\" @hex_class
.SH SYNOPSIS
.nf
.B "#include <mLib/codec.h>"
.B "#include <mLib/base64.h>"
.B "#include <mLib/base32.h>"
.B "#include <mLib/hex.h>"

.B "codec_class null_codec_class;"
.B "codec_class base64_class, file64_class, base64url_class;"
.B "codec_class base32_class, base32hex_class;"
.B "codec_class hex_class;"

.BI "const char *codec_strerror(int " err ");"
.fi
.SH DESCRIPTION
The
.B codec
system provides an object-based interface to functions which encode
binary data as plain text and decode the result to recover the original
binary data.  The interface makes it easy to support multiple encodings
and select an appropriate one at runtime.
.SS "The codec_class structure"
The
.B codec_class
structure represents a particular encoding format.  The structure has
the following members.
.TP
.B "const char *name"
The name of the class, as a null-terminated string.  The name should not
contain whitespace characters.
.TP
.BI "codec *(*encoder)(unsigned " flags ", const char *" indent ", unsigned " maxline ")"
Pointer to a function which constructs a new encoder object, of type
.BR codec .
The
.I flags
configure the behaviour of the object; the
.I indent
string is written to separate lines of output; the integer
.I maxline
is the maximum length of line to be produced, or zero to forbid line
breaking.
.TP
.BI "codec *(*decoder)(unsigned " flags ")"
Pointer to a function which constructs a new decoder object, also of
type
.BR codec .
The
.I flags
configure the behaviour of the object.
.PP
The
.I flags
to the
.B encoder
and
.B decoder
functions have the following meanings.
.TP
.B CDCF_LOWERC
For codecs which produce output using a single alphabetic case (e.g.,
.BR base32 ,
.BR hex ),
emit and accept only lower case; the default to emit and accept only
upper case, for compatibility with RFC4648.  If the codec usually
produces mixed-case output, then this flag is ignored.
.TP
.B CDCF_IGNCASE
For codecs which produce output using a single alphabetic case, ignore
the case of the input when decoding.  If the codec usually produces
mixed-case output, then this flag is ignored.
.TP
.B CDCF_NOEQPAD
For codecs which usually pad their output (e.g.,
.BR base64 ,
.BR base32 ),
do not emit or accept padding characters.  If the codec does not usually
produce padding, or the padding is not redundant, then this flag is
ignored.
.TP
.B CDCF_IGNEQPAD
For codecs which usually pad their output, do not treat incorrect (e.g.,
missing or excessive) padding as an error when decoding.  If the codec
does not usually produce padding, or the padding is required for
unambiguous decoding, then this flag is ignored.
.TP
.B CDCF_IGNEQMID
For codecs which usually pad their output, ignore padding characters
wherever they may appear when decoding.  Usually padding characters
indicate the end of the input, and further input characters are
considered erroneous.  If the codec does not usually produce padding, or
it is impossible to resume decoding correctly having seen padding
characters, then this flag is ignored.
.TP
.B CDCF_IGNZPAD
For codecs which need to pad their input, ignore unusual padding bits
when decoding.  (This is not at all the same thing as the padding
characters controlled by the flags above: they deal with padding the
length of the encoding
.I output
up to a suitable multiple of characters; this option deals with padding
of the
.I input
prior to encoding.)  If the codec does not add padding bits, or specific
values are required for unambiguous decoding, then this flag is ignored.
.TP
.B CDCF_IGNNEWL
Ignore newline (and carriage-return) characters when decoding: the
default for RFC4648 codecs is to reject newline characters.  If these
characters are significant in the encoding, then this flag is ignored.
.TP
.B CDCF_IGNSPC
Ignore whitespace characters (other than newlines) when decoding: the
default for RFC4648 codecs is to reject whitespace characters.  If these
characters are significant in the encoding, then this flag is ignored.
.TP
.B CDCF_IGNINVCH
Ignore any other invalid characters appearing in the input when
decoding.
.TP
.B CDCF_IGNJUNK
Ignore all `junk' in the input.  This should suppress almost all
decoding errors.
.PP
If you do not set any of the
.BR CDCF_IGN ...
flags, a decoder should only accept the exact encoding that the
corresponding encoder would produce (with
.I maxline
= 0 to inhibit line-breaking).
.SS "The codec and codec_ops structures"
The
.B codec
structure represents the state of an encoder or decoder, as returned by
the
.B encoder
and
.B decoder
functions described above, contains a single member.
.TP
.B "const codec_ops *ops"
Pointer to a
.B codec_ops
structure which contains operations and metadata for use with the
encoder or decoder.
.PP
The
.B codec_ops
structure contains the following members.
.TP
.B "const codec_class *c"
Pointer back to the
.B codec_class
which was used to construct the
.B codec
object.
.TP
.BI "int (*code)(codec *" c ", const void *" p ", size_t " sz ", dstr *" d ")"
Encode or decode, using the codec
.IR c ,
the data in the buffer at address
.I p
and continuing for
.I sz
bytes, appending the output to the dynamic string
.I d
(see
.BR dstr (3)).
If the operation was successful, the function returns zero; otherwise it
returns a nonzero error code, as described below.
.TP
.BI "void (*destroy)(codec *" c ")"
Destroy the codec object
.IR c ,
freeing any resources it may hold.
.PP
A codec may buffer its input (e.g., if needs to see more in order to
decide what output to produce next); it may also need to take special
action at the end of the input (e.g., flushing buffers, and applying
padding).  To signal the codec that there is no more input, call the
.B code
function with a null
.I p
pointer.  It will then write any final output to
.IR d .
.PP
The following error conditions may be reported.
.TP
.B CDCERR_INVCH
An invalid character was encountered while decoding.  This includes
encoutering padding characters if padding is disabled using the
.B CDCF_NOEQPAD
flag.
.TP
.B CDCERR_INVEQPAD
Invalid padding characters (e.g., wrong characters, or too few, too
many, or none at all) were found during decoding.  This may also
indicate that the input is truncated, even if the codec does not usually
perform output padding.
.TP
.B CDCERR_INVZPAD
Invalid padding bits were found during decoding.
.PP
The
.B codec_strerror
function converts these error codes to brief, (moderately)
human-readable strings.
.SS "Provided codecs"
The library provides a number of standard codecs.
.TP
.B base64
Implements Base64 encoding, as defined by RFC4648.  Output is
mixed-case, so the
.B CDCF_LOWERC
and
.B CDCF_IGNCASE
flags are ignored.
.TP
.B safe64
Implements a variant of the Base64 encoding which uses
.RB ` % '
in place of
.RB ` / ',
so that its output is suitable for use as a Unix filename.
.TP
.B base64url
Implements the filename- and URL-safe variant of Base64 encoding, as
defined by RFC4648.
.TP
.B base32
Implements Base32 encoding, as defined by RFC4648.  Output is in upper
case by default.
.TP
.B base32hex
Implements the extended-hex variant of Base32, as defined by RFC4648.
This encoding has the property that the encoding preserves the ordering
of messages if padding is suppressed.
.TP
.B hex
Implements hex encoding, defined by RFC4648 under the name Base16.  For
compatibility with that specification, output is in upper case by
default.
.SH "SEE ALSO"
.BR bincode (1),
.BR dstr (3),
.BR mLib (3).
.SH AUTHOR
Mark Wooding, <mdw@distorted.org.uk>