ref: 2459e34d77e2e21ef829c0dfaafd99433899494f
dir: /man/2/convcs/
.TH CONVCS 2
.SH NAME
Convcs, Btos, Stob \- character set conversion suite
.SH SYNOPSIS
.EX
include "convcs.m";
convcs := load Convcs Convcs->PATH;
Btos: module {
init: fn(arg: string): string;
btos: fn(s: Convcs->State, b: array of byte, nchars: int)
: (Convcs->State, string, int);
};
Stob: module {
init: fn (arg: string): string;
stob: fn(s: Convcs->State, str: string)
: (Convcs->State, array of byte);
};
Convcs: module {
State: type string;
Startstate: con "";
init: fn(csfile: string): string;
getbtos: fn(cs: string): (Btos, string);
getstob: fn(cs: string): (Stob, string);
enumcs: fn(): list of (string, string, int);
aliases: fn(cs: string): (string, list of string);
};
.EE
.SH DESCRIPTION
The
.I Convcs
suite is a collection of
modules for converting various standard
coded character sets and character encoding schemes
to and from the Limbo strings.
.PP
The
.B Convcs
module provides an entry point to the suite, mapping character set names and aliases
to their associated
converter implementation.
.SS "The Convcs module"
.TP
.BI init( csfile )
.B Init
should be called once to initialise the internal state of
.BR Convcs .
The
.I csfile
argument specifies the path of the converter mapping file.
If this argument is nil, the default mapping file
.B /lib/convcs/charsets
is used.
.TP
.BI getbtos( cs )
.B Getbtos
returns an initialised
.B Btos
module for converting from the requested encoding.
.IP
The return value is a tuple, holding the module reference and an error string.
If any errors were encountered in locating, loading or initialising the requested
converter, the module reference will be nil and the string will contain an explanation.
.IP
The character set name,
.IR cs ,
is normalised by mapping all upper-case latin1 characters to lower-case before
comparison with character set names and aliases from the
.B charsets
file.
.TP
.BI getstob( cs )
.B Getstob
returns an initialised
.B Stob
module for converting from strings to the requested encoding.
Apart from the different module type, the return value and semantics are
the same as for
.BR getbtos() .
.TP
.B enumcs()
Returns a list describing the set of available converters.
The list is comprised of
.RI ( name ,
.IR desc ,
.IR mode )
tuples.
.IP
.I Name
is the standard name of the character set.
.IP
.I Desc
is a more friendly description taken directly from the
.B desc
attribute given in the
.I charsets
file.
If the attribute is not given then
.I desc
is set to
.IR name .
.IP
.I Mode
is a bitmap that details the converters available for the given character set.
Valid
.I mode
bits are
.B Convcs->BTOS
and
.BR Convcs->STOB .
For convenience,
.B Convcs->BOTH
is also defined.
.TP
.BI aliases( cs )
Returns the aliases for the character set name
.IR cs .
The return value is the tuple
.BI ( desc ", " aliases ).
.IP
.I Desc
is the descriptive text for the character set, as returned by
.BR enumcs() .
.IP
.I Aliases
lists all the known aliases for
.IR cs .
The first name in the list is the default character set name \- the name
that all the other aliases refer to.
If the
.I aliases
list is
.B nil
then there was an error in looking up the character set name and
.I desc
will detail the error.
.SS "Using a Btos converter"
The
.B Btos
module returned by
.B getbtos()
is already initialised and is ready to start the conversion.
Conversions can be made on a individual basis,
or in a `streamed' mode.
.PP
.IB converter "->btos(" s ,
.IB b ,
.IB nchars )
.RS
Converts raw byte codes of the character set encoding to a Limbo string.
.PP
The argument
.I s
is a converter state as returned from the previous call to
.B btos
on the same input stream.
The first call to
.B btos
on a particular input stream should give
.B Convcs->Startstate
(or
.BR nil)
as the value for
.IR s .
The argument
.I b
is the bytes to be converted.
The argument
.I nchars
is the maximal length of the string to be returned.
If this argument is
.B -1
then as much of
.I b
will be consumed as possible.
A value of
.B 0
indicates to the converter that there is no more data and
that any pending state should be flushed.
.PP
The return value of
.B btos
is the tuple
.RI ( state ,
.IR str ,
.IR nbytes )
where
.I state
is the new state of the converter,
.I str
is the converted string, and
.I nbytes
is the number of bytes from
.I b
consumed by the conversion.
.PP
The same converter module can be used for multiple conversion streams
by maintaining a separate
.I state
variable for each stream.
.RE
.SS "Using an Stob converter"
The
.B Stob
module returned by
.B getstob()
is already initialised and is ready to start the conversion.
.PP
.IB converter "->stob(" s ,
.IB str )
.RS
Converts the limbo string
.I str
to the raw byte codes of the character set encoding.
The argument
.I s
represents the converter state and is treated in the same way as for
.IB converter ->btos()
described above.
To terminate a conversion
.B stob
should be called with an empty string in order to flush
the converter state.
.PP
The return value of
.B stob
is the tuple
.RI ( state ,
.IR bytes )
where
.I state
is the new state of the converter and
.I bytes
is the result of the conversion.
.RE
.SS "Conversion errors"
When using
.IB converter "->btos()"
to convert data to Limbo strings,
any byte sequences that are not valid for the specific character encoding scheme
will be converted to the Unicode error character 16rFFFD.
.PP
When using
.IB converter "->stob()"
to convert Limbo strings,
any Unicode characters that can not be mapped
into the character set will normally be substituted by the US-ASCII code for `?'.
Note that this may be inappropriate for certain conversions, such converters will use
a suitable error character for their particular character set and encoding scheme.
.SS "Charset file format"
The file
.B /lib/convcs/charsets
provides the mapping between character set names and their implementation modules.
The file format conforms to that supported by
.IR cfg (2).
The following description relies on terms defined in the
.IR cfg (2)
manual page.
.PP
Each record name defines a character set name.
If the primary value of the record is non-empty then the name is an alias,
the value being the real name.
An alias record must point to an actual converter record, not to another alias, as
.I Convcs
only follows one level of aliasing.
.PP
Each converter record consists of a set of tuples with the following primary attributes:
.TP
.B desc
A more descriptive name for the character set encoding.
This attribute is used for the description returned by
.BR enumcs() .
.TP
.B btos
The path to the
.B Btos
module implementation of this converter.
.TP
.B stob
The path to the
.B Stob
module implementation of this converter.
.PP
Both the
.B btos
and
.B stob
tuples can have an optional
.B arg
attribute which is passed to the
.B init()
function of the converter when initialised by
.IR Convcs .
If a converter record has neither an
.B stob
nor a
.B btos
tuple, then it is ignored.
.PP
The following example is an extract from the standard Inferno
.I charsets
file:
.PP
.EX
cp866=ibm866
866=ibm866
ibm866=
desc='Russian MS-DOS CP 866'
stob=/dis/lib/convcs/cp_stob.dis arg=/lib/convcs/ibm866.cp
btos=/dis/lib/convcs/cp_btos.dis arg=/lib/convcs/ibm866.cp
.EE
.PP
This entry defines
.I Stob
and
.I Btos
converters for the character set called
.BR ibm866 .
The converters are actually the generic codepage converters
.B cp_stob
and
.B cp_btos
paramaterized with a codepage file.
The entry also defines the aliases
.B cp866
and
.B 866
for the name
.BR ibm866 .
.SH FILES
.TP
.B /lib/convcs/charsets
The default mapping between character set names and their implementation modules.
.TP
.BI /lib/convcs/ csname .cp
Codepage files for use by the generic codepage converters
.B cp_stob
and
.BR cp_btos .
Each file consists of 256 unicode runes mapping codepage byte values to unicode by their index.
The runes are stored in the utf-8 encoding.
.SH SOURCE
.TF /appl/lib/convcs/convcs.b
.TP
.B /appl/lib/convcs/convcs.b
Implementation of the
.B Convcs
module.
.TP
.B /appl/lib/convcs/cp_btos.b
Generic
.I Btos
codepage converter.
.TP
.B /appl/lib/convcs/cp_stob.b
Generic
.I Stob
codepage converter.
.br
.PP
.SH SEE ALSO
.IR tcs (1),
.IR cfg (2)