CONVCS(2) CONVCS(2)
NAME
Convcs, Btos, Stob - character set conversion suite
SYNOPSIS
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);
};
DESCRIPTION
The Convcs suite is a collection of modules for converting
various standard coded character sets and character encoding
schemes to and from the Limbo strings.
The Convcs module provides an entry point to the suite, map-
ping character set names and aliases to their associated
converter implementation.
The Convcs module
init(csfile)
Init should be called once to initialise the internal
state of Convcs. The csfile argument specifies the
path of the converter mapping file. If this argument
is nil, the default mapping file /lib/convcs/charsets
is used.
getbtos(cs)
Getbtos returns an initialised Btos module for
Page 1 Plan 9 (printed 11/27/24)
CONVCS(2) CONVCS(2)
converting from the requested encoding.
The return value is a tuple, holding the module refer-
ence and an error string. If any errors were encoun-
tered in locating, loading or initialising the
requested converter, the module reference will be nil
and the string will contain an explanation.
The character set name, cs, is normalised by mapping
all upper-case latin1 characters to lower-case before
comparison with character set names and aliases from
the charsets file.
getstob(cs)
Getstob returns an initialised Stob module for convert-
ing from strings to the requested encoding. Apart from
the different module type, the return value and seman-
tics are the same as for getbtos().
enumcs()
Returns a list describing the set of available convert-
ers. The list is comprised of (name, desc, mode)
tuples.
Name is the standard name of the character set.
Desc is a more friendly description taken directly from
the desc attribute given in the charsets file. If the
attribute is not given then desc is set to name.
Mode is a bitmap that details the converters available
for the given character set. Valid mode bits are
Convcs->BTOS and Convcs->STOB. For convenience,
Convcs->BOTH is also defined.
aliases(cs)
Returns the aliases for the character set name cs. The
return value is the tuple (desc, aliases).
Desc is the descriptive text for the character set, as
returned by enumcs().
Aliases lists all the known aliases for cs. The first
name in the list is the default character set name -
the name that all the other aliases refer to. If the
aliases list is nil then there was an error in looking
up the character set name and desc will detail the
error.
Using a Btos converter
The Btos module returned by getbtos() is already initialised
and is ready to start the conversion. Conversions can be
Page 2 Plan 9 (printed 11/27/24)
CONVCS(2) CONVCS(2)
made on a individual basis, or in a `streamed' mode.
converter->btos(s, b, nchars)
Converts raw byte codes of the character set encoding
to a Limbo string.
The argument s is a converter state as returned from
the previous call to btos on the same input stream.
The first call to btos on a particular input stream
should give Convcs->Startstate (or nil) as the value
for s. The argument b is the bytes to be converted.
The argument nchars is the maximal length of the string
to be returned. If this argument is -1 then as much of
b will be consumed as possible. A value of 0 indicates
to the converter that there is no more data and that
any pending state should be flushed.
The return value of btos is the tuple (state, str,
nbytes) where state is the new state of the converter,
str is the converted string, and nbytes is the number
of bytes from b consumed by the conversion.
The same converter module can be used for multiple con-
version streams by maintaining a separate state vari-
able for each stream.
Using an Stob converter
The Stob module returned by getstob() is already initialised
and is ready to start the conversion.
converter->stob(s, str)
Converts the limbo string str to the raw byte codes of
the character set encoding. The argument s represents
the converter state and is treated in the same way as
for converter->btos() described above. To terminate a
conversion stob should be called with an empty string
in order to flush the converter state.
The return value of stob is the tuple (state, bytes)
where state is the new state of the converter and bytes
is the result of the conversion.
Conversion errors
When using converter->btos() to convert data to Limbo
strings, any byte sequences that are not valid for the spe-
cific character encoding scheme will be converted to the
Unicode error character 16rFFFD.
When using 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
Page 3 Plan 9 (printed 11/27/24)
CONVCS(2) CONVCS(2)
conversions, such converters will use a suitable error char-
acter for their particular character set and encoding
scheme.
Charset file format
The file /lib/convcs/charsets provides the mapping between
character set names and their implementation modules. The
file format conforms to that supported by cfg(2). The fol-
lowing description relies on terms defined in the cfg(2)
manual page.
Each record name defines a character set name. If the pri-
mary 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 Convcs only follows one level of aliasing.
Each converter record consists of a set of tuples with the
following primary attributes:
desc A more descriptive name for the character set encoding.
This attribute is used for the description returned by
enumcs().
btos The path to the Btos module implementation of this con-
verter.
stob The path to the Stob module implementation of this con-
verter.
Both the btos and stob tuples can have an optional arg
attribute which is passed to the init() function of the con-
verter when initialised by Convcs. If a converter record has
neither an stob nor a btos tuple, then it is ignored.
The following example is an extract from the standard
Inferno charsets file:
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
This entry defines Stob and Btos converters for the charac-
ter set called ibm866. The converters are actually the gen-
eric codepage converters cp_stob and cp_btos paramaterized
with a codepage file. The entry also defines the aliases
cp866 and 866 for the name ibm866.
FILES
Page 4 Plan 9 (printed 11/27/24)
CONVCS(2) CONVCS(2)
/lib/convcs/charsets
The default mapping between character set names and
their implementation modules.
/lib/convcs/csname.cp
Codepage files for use by the generic codepage convert-
ers cp_stob and 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.
SOURCE
/appl/lib/convcs/convcs.b Implementation of the Convcs mod-
ule.
/appl/lib/convcs/cp_btos.b Generic Btos codepage converter.
/appl/lib/convcs/cp_stob.b Generic Stob codepage converter.
SEE ALSO
tcs(1), cfg(2)
Page 5 Plan 9 (printed 11/27/24)