X-Git-Url: https://git.distorted.org.uk/~mdw/mLib/blobdiff_plain/173ff44a439dc7cf51a3f7a433c4d0d444b59293..236f657b6dab66f31f4902cecfc03b4673f5bb98:/codec/codec.3 diff --git a/codec/codec.3 b/codec/codec.3 new file mode 100644 index 0000000..96174f3 --- /dev/null +++ b/codec/codec.3 @@ -0,0 +1,256 @@ +.\" -*-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 " +.B "#include " +.B "#include " +.B "#include " + +.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_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 +.I 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,