harvey/sys/doc/utf.ms

.TL
Hello World
.br
or
.br
.ft R
Καλημέρα κόσμε
.ft
.br
or
.br
\f(Jpこんにちは 世界\fP
.AU
Rob Pike
Ken Thompson
.sp
rob,ken@plan9.bell-labs.com
.AB
.FS
Originally appeared, in a slightly different form, in
.I
Proc. of the Winter 1993 USENIX Conf.,
.R
pp. 43-50,
San Diego
.FE
Plan 9 from Bell Labs has recently been converted from ASCII
to an ASCII-compatible variant of the Unicode Standard, a 16-bit character set.
In this paper we explain the reasons for the change,
describe the character set and representation we chose,
and present the programming models and software changes
that support the new text format.
Although we stopped short of full internationalization\(emfor
example, system error messages are in Unixese, not Japanese\(emwe
believe Plan 9 is the first system to treat the representation
of all major languages on a uniform, equal footing throughout all its
software.
.AE
.SH
Introduction
.PP
The world is multilingual but most computer systems
are based on English and ASCII.
The first release of Plan 9 [Pike90], a new distributed operating
system from Bell Laboratories, seemed a good occasion
to correct this chauvinism.
It is easier to make such deep changes when building new systems than
by refitting old ones.
.PP
The ANSI C standard [ANSIC] contains some guidance on the matter of
`wide' and `multi-byte' characters but falls far short of
solving the myriad associated problems.
We could find no literature on how to convert a
.I system
to larger character sets, although some individual
.I programs
had been converted.
This paper reports what we discovered as we
explored the problem of representing multilingual
text at all levels of an operating system,
from the file system and kernel through
the applications and up to the window system
and display.
.PP
Plan 9 has not been `internationalized':
its manuals are in English,
its error messages are in English,
and it can display text that goes from left to right only.
But before we can address these other problems,
we need to handle, uniformly and comfortably,
the textual representation of all the major written languages.
That subproblem is richer than we had anticipated.
.SH
Standards
.PP
Our first step was to select a standard.
At the time (January 1992),
there were only two viable options:
ISO 10646 [ISO10646] and Unicode [Unicode].
The documents describing both proposals were still in the draft stage.
.PP
The draft of ISO 10646 was not
very attractive to us.
It defined a sparse set of 32-bit characters,
which would be
hard to implement
and have punitive storage requirements.
Also, the draft attempted to
mollify national interests by allocating
16-bit subspaces to national committees
to partition individually.
The suggested mode of use was to
``flip'' between separate national
standards to implement the international standard.
This did not strike us as a sound basis for a character set.
As well, transmitting 32-bit values in a byte stream,
such as in pipes, would be expensive and hard to implement.
Since the standard does not define a byte order for such
transmission, the byte stream would also have to carry
state to enable the values to be recovered.
.PP
The Unicode Standard is a proposal by a consortium of mostly American
computer companies formed
to protest the technical
failings of ISO 10646.
It defines a uniform 16-bit code based on the
principle of unification:
two characters are the same if they look the
same even though they are from different
languages.
This principle, called Han unification,
allows the large Japanese, Chinese, and Korean
character sets to be packed comfortably into a 16-bit representation.
.PP
We chose the Unicode Standard for its technical merits and because its
code space was better defined.
Moreover,
the Unicode Consortium was derailing the
ISO 10646 standard.
(Now, in 1995,
ISO 10646 is a standard
with one 16-bit group defined,
which is almost exactly the Unicode Standard.
As most people expected, the two standards bodies
reached a détente and
ISO 10646 and Unicode represent the same character set.)
.PP
The Unicode Standard defines an adequate character set
but an unreasonable representation.
It states that all characters
are 16 bits wide and are communicated and stored in
16-bit units.
It also reserves a pair of characters
(hexadecimal FFFE and FEFF) to detect byte order
in transmitted text, requiring state in the byte stream.
(The Unicode Consortium was thinking of files, not pipes.)
To adopt this encoding,
we would have had to convert all text going
into and out of Plan 9 between ASCII and Unicode, which cannot be done.
Within a single program, in command of all its input and output,
it is possible to define characters as 16-bit quantities;
in the context of a networked system with
hundreds of applications on diverse machines
by different manufacturers,
it is impossible.
.PP
We needed a way to adapt the Unicode Standard to the tools-and-pipes
model of text processing embodied by the Unix system.
To do that, we
needed an ASCII-compatible textual
representation of Unicode characters for transmission
and storage.
In the draft ISO standard there was an informative
(non-required)
Annex
called UTF
that provided a byte stream encoding
of the 32-bit ISO code.
The encoding uses multibyte sequences composed
from the 190 printable characters of Latin-1
to represent character values larger
than 159.
.PP
The UTF encoding has several good properties.
By far the most important is that
a byte in the ASCII range 0-127 represents
itself in UTF.
Thus UTF is backward compatible with ASCII.
.PP
UTF has other advantages.
It is a byte encoding and is
therefore byte-order independent.
ASCII control characters appear in the byte stream
only as themselves, never as an element of a sequence
encoding another character,
so newline bytes separate lines of UTF text.
Finally, ANSI C's
.CW strcmp
function applied to UTF strings preserves the ordering of Unicode characters.
.PP
To encode and decode UTF is expensive (involving multiplication,
division, and modulo operations) but workable.
UTF's major disadvantage is that the encoding
is not self-synchronizing.
It is in general impossible to find the character
boundaries in a UTF string without reading from
the beginning of the string, although in practice
control characters such as newlines,
tabs, and blanks provide synchronization points.
.PP
In August 1992,
X-Open circulated a proposal for another UTF-like
byte encoding of Unicode characters.
Their major concern was that an embedded character
in a file name
(in particular a slash)
could be part of an escape sequence in UTF and
therefore confuse a traditional file system.
Their proposal would allow all 7-bit ASCII characters
to represent themselves
.I "and only themselves"
in text.
Multibyte sequences would contain only characters
with the high bit set.
We proposed a modification to the new UTF that
would address our synchronization problem.
Our proposal, which was  originally known informally as UTF-2 and FSS-UTF,
is now referred to as UTF-8 and has been approved by ISO to become
Annex P to ISO 10646.
.PP
The model for text in Plan 9 is chosen from these
three standards*:
.FS
* ``That's the nice thing about standards\(emthere's so many to choose from.'' \- Andy Tannenbaum (no, the other one)
.FE
the Unicode character set encoded as a byte stream by
UTF-8, from
(soon to be) Annex P of ISO 10646.
Although this mixture may seem like a precarious position for us to adopt,
it is not as bad as it sounds.
ISO 10646 and the Unicode Standard have converged,
other systems such as Linux have adopted the same character set and encoding,
and the general feeling seems to be that Unicode and UTF-8 will be accepted as the way
to exchange text between systems.
The prognosis for wide acceptance is good.
.PP
There are a couple of aspects of the Unicode Standard we have not faced.
One is the issue of right-to-left text such as Hebrew or Arabic.
Since that is an issue of display, not representation, we believe
we can defer that problem for the moment without affecting our
ability to solve it later.
Another issue is diacriticals and `combining characters',
which cause overstriking of multiple Unicode characters.
Although necessary for some scripts, such as Thai, Arabic, and Hebrew,
such characters confuse the issues for Latin languages because they
generate multiple representations for accented characters.
ISO 10646 describes three levels of implementation;
in Plan 9 we decided not to address the issue.
Again, this can be labeled as a display issue and its finer points are still being debated,
so we felt comfortable deferring.  Mañana.
.PP
Although we converted Plan 9 in the altruistic interests of
serving foreign languages, we have found the large character
set attractive for other reasons.  The Unicode Standard includes many
characters\(emmathematical symbols, scientific notation,
more general punctuation, and more\(emthat we now use
daily in our work.  We no longer test our imaginations
to find ways to include non-ASCII symbols in our text;
why type
.CW :-)
when you can use the character ☺?
Most compelling is the ability to absorb documents
and data that contain non-ASCII characters; our browser for the
Oxford English Dictionary
lets us see the dictionary as it really is, with pronunciation
in the IPA font, foreign phrases properly rendered, and so on,
.I "in plain text.
.PP
In the rest of this paper, except when
stated otherwise, the term `UTF' refers to the UTF-8 encoding
of Unicode characters as adopted by Plan 9.
.SH
C Compiler
.PP
The first program to be converted to UTF
was the C Compiler.
There are two levels of conversion.
On the syntactic level,
input to the C compiler
is UTF; on the semantic level,
the C language needs to define
how compiled programs manipulate
the UTF set.
.PP
The syntactic part is simple.
The ANSI C language standard defines the
source character set to be ASCII.
Since UTF is backward compatible with ASCII,
the compiler needs little change.
The only places where a larger character set
is allowed are in character constants, strings, and comments.
Since 7-bit ASCII characters can represent only
themselves in UTF,
the compiler does not have to be careful while looking
for the termination of a string or comment.
.PP
The Plan 9 compiler extends ANSI C to treat any Unicode
character with a value outside of the ASCII range as
an alphabetic.
To a Greek programmer or an English mathematician,
α is a sensible and now valid variable name.
.PP
On the semantic level, ANSI C allows,
but does not tie down,
the notion of a
.I "wide character
and admits string and character constants
of this type.
We chose the wide character type to be
.CW unsigned
.CW short .
In the libraries, the word
.CW Rune
is defined by a
.CW typedef
to be equivalent to
.CW unsigned
.CW short
and is
used to signify a Unicode character.
.PP
There are surprises; for example:
.P1
L'x'	\f1is 120\fP
\&'x'	\f1is 120\fP
L'ÿ'	\f1is 255\fP
\&'ÿ'	\f1is -1, stdio \fPEOF\f1 (if \fPchar\f1 is signed)\fP
L'\f1α\fP'	\f1is 945\fP
\&'\f1α\fP'	\f1is illegal\fP
.P2
In the string constants,
.P1
"\f(Jpこんにちは 世界\fP"
L"\f(Jpこんにちは 世界\fP",
.P2
the former is an array of
.CW chars
with 22 elements
and a null byte,
while the latter is an array of
.CW unsigned
.CW shorts
.CW Runes ) (
with 8 elements and a null
.CW Rune .
.PP
The Plan 9 library provides an output conversion function,
.CW print
(analogous to
.CW printf ),
with formats
.CW %c ,
.CW %C ,
.CW %s ,
and
.CW %S .
Since
.CW print
produces text, its output is always UTF.
The character conversion
.CW %c
(lower case) masks its argument
to 8 bits before converting to UTF.
Thus
.CW L'ÿ'
and
.CW 'ÿ'
printed under
.CW %c
will be identical,
but
.CW L'\f1α\fP'
will print as the Unicode
character with decimal value 177.
The character conversion
.CW %C
(upper case) masks its argument
to 16 bits before converting to UTF.
Thus
.CW L'ÿ'
and
.CW L'\f1α\fP'
will print correctly under
.CW %C ,
but
.CW 'ÿ'
will not.
The conversion
.CW %s
(lower case)
expects a pointer to
.CW char
and copies UTF sequences up to a null byte.
The conversion
.CW %S
(upper case) expects a pointer to
.CW Rune
and
performs sequential
.CW %C
conversions until a null
.CW Rune
is encountered.
.PP
Another problem in format conversion
is the definition of
.CW %10s :
does the number refer to bytes or characters?
We decided that such formats were most
often used to align output columns and
so made the number count characters.
Some programs, however, use the count
to place blank-padded strings
in fixed-sized arrays.
These programs must be found and corrected.
.PP
Here is a complete example:
.P1
#include <u.h>

char c[] = "\f(Jpこんにちは 世界\fP";
Rune s[] = L"\f(Jpこんにちは 世界\fP";

main(void)
{
	print("%d, %d\en", sizeof(c), sizeof(s));
	print("%s\en", c);
	print("%S\en", s);
}
.P2
.PP
This program prints
.CW 23,
.CW 18
and then two identical lines of
UTF text.
In practice,
.CW %S
and
.CW L"..."
are rare in programs; one reason is
that most formatted I/O is done in unconverted UTF.
.SH
Ramifications
.PP
All programs in Plan 9 now read and write text as UTF, not ASCII.
This change breaks two deep-rooted symmetries implicit in most C programs:
.IP 1.
A character is no longer a
.CW char .
.IP 2.
The internal representation (Rune) of a character now differs from its
external representation (UTF).
.PP
In the sections that follow,
we show how these issues were faced in the layers of
system software from the operating system up to the applications.
The effects are wide-reaching and often surprising.
.SH
Operating system
.PP
Since UTF is the only format for text in Plan 9,
the interface to the operating system had to be converted to UTF.
Text strings cross the interface in several places:
command arguments,
file names,
user names (people can log in using their native name),
error messages,
and miscellaneous minor places such as commands to the I/O system.
Little change was required: null-terminated UTF strings
are equivalent to null-terminated ASCII strings for most purposes
of the operating system.
The library routines described in the next section made that
change straightforward.
.PP
The window system, once called
.CW 8.5 ,
is now rightfully called
.CW 8½ .
.SH
Libraries
.PP
A header file included by all programs (see [Pike92]) declares
the
.CW Rune
type to hold 16-bit character values:
.P1
typedef unsigned short Rune;
.P2
Also defined are several constants relevant to UTF:
.P1
enum
{
    UTFmax    = 3,    /* maximum bytes per rune */
    Runesync  = 0x80, /* can't appear in UTF sequence (<) */
    Runeself  = 0x80, /* rune==UTF sequence (<) */
    Runeerror = 0x80, /* decoding error in UTF */
};
.P2
(With the original UTF,
.CW Runesync
was hexadecimal 21 and
.CW Runeself
was A0.)
.CW UTFmax
bytes are sufficient
to hold the UTF encoding of any Unicode character.
Characters of value less than
.CW Runesync
only appear in a UTF string as
themselves, never as part of a sequence encoding another character.
Characters of value less than
.CW Runeself
encode into single bytes
of the same value.
Finally, when the library detects errors in UTF input\(embyte sequences
that are not valid UTF sequences\(emit converts the first byte of the
error sequence to the character
.CW Runeerror .
There is little a rune-oriented program can do when given bad data
except exit, which is unreasonable, or carry on.
Originally the conversion routines, described below,
returned errors when given invalid UTF,
but we found ourselves repeatedly checking for errors and ignoring them.
We therefore decided to convert a bad sequence to a valid rune
and continue processing.
(The ANSI C routines, on the other hand, return errors.)
.PP
This technique does have the unfortunate property that converting
invalid UTF byte strings in and out of runes does not preserve the input,
but this circumstance only occurs when non-textual input is
given to a textual program.
The Unicode Standard defines an error character, value FFFD, to stand for
characters from other sets that it does not represent.
The
.CW Runeerror
character is a different concept, related to the encoding rather than the character set, so we
chose a different character for it.
.PP
The Plan 9 C library contains a number of routines for
manipulating runes.
The first set converts between runes and UTF strings:
.P1
extern	int	runetochar(char*, Rune*);
extern	int	chartorune(Rune*, char*);
extern	int	runelen(long);
extern	int	fullrune(char*, int);
.P2
.CW Runetochar
translates a single
.CW Rune
to a UTF sequence and returns the number of bytes produced.
.CW Chartorune
goes the other way, reporting how many bytes were consumed.
.CW Runelen
returns the number of bytes in the UTF encoding of a rune.
.CW Fullrune
examines a UTF string up to a specified number of bytes
and reports whether the string begins with a complete UTF encoding.
All these routines use the
.CW Runeerror
character to work around encoding problems.
.PP
There is also a set of routines for examining null-terminated UTF strings,
based on the model of the ANSI standard
.CW str
routines, but with
.CW utf
substituted for
.CW str
and
.CW rune
for
.CW chr :
.P1
extern	int	utflen(char*);
extern	char*	utfrune(char*, long);
extern	char*	utfrrune(char*, long);
extern	char*	utfutf(char*, char*);
.P2
.CW Utflen
returns the number of runes in a UTF string;
.CW utfrune
returns a pointer to the first occurrence of a rune in a UTF string;
and
.CW utfrrune
a pointer to the last.
.CW Utfutf
searches for the first occurrence of a UTF string in another UTF string.
Given the synchronizing property of UTF-8,
.CW utfutf
is the same as
.CW strstr
if the arguments point to valid UTF strings.
.PP
It is a mistake to use
.CW strchr
or
.CW strrchr
unless searching for a 7-bit ASCII character, that is, a character
less than
.CW Runeself .
.PP
We have no routines for manipulating null-terminated arrays of
.CW Runes .
Although they should probably exist for completeness, we have
found no need for them, for the same reason that
.CW %S
and
.CW L"..."
are rarely used.
.PP
Most Plan 9 programs use a new buffered I/O library, BIO, in place of
Standard I/O.
BIO contains routines to read and write UTF streams, converting to and from
runes.
.CW Bgetrune
returns, as a
.CW Rune
within a
.CW long ,
the next character in the UTF input stream;
.CW Bputrune
takes a rune and writes its UTF representation.
.CW Bungetrune
puts a rune back into the input stream for rereading.
.PP
Plan 9 programs use a simple set of macros to process command line arguments.
Converting these macros to UTF automatically updated the
argument processing of most programs.
In general,
argument flag names can no longer be held in bytes and
arrays of 256 bytes cannot be used to hold a set of flags.
.PP
We have done nothing analogous to ANSI C's locales, partly because
we do not feel qualified to define locales and partly because we remain
unconvinced of that model for dealing with the problems.
That is really more an issue of internationalization than conversion
to a larger character set; on the other hand,
because we have chosen a single character set that encompasses
most languages, some of the need for
locales is eliminated.
(We have a utility,
.CW tcs ,
that translates between UTF and other character sets.)
.PP
There are several reasons why our library does not follow the ANSI design
for wide and multi-byte characters.
The ANSI model was designed by a committee, untried, almost
as an afterthought, whereas
we wanted to design as we built.
(We made several major changes to the interface
as we became familiar with the problems involved.)
We disagree with ANSI C's handling of invalid multi-byte sequences.
Also, the ANSI C library is incomplete:
although it contains some crucial routines for handling
wide and multi-byte characters, there are some serious omissions.
For example, our software can exploit
the fact that UTF preserves ASCII characters in the byte stream.
We could remove that assumption by replacing all
calls to
.CW strchr
with
.CW utfrune
and so on.
(Because of the weaker properties of the original UTF,
we have actually done so.)
ANSI C cannot:
the standard says nothing about the representation, so portable code should
.I never
call
.CW strchr ,
yet there is no ANSI equivalent to
.CW utfrune .
ANSI C simultaneously invalidates
.CW strchr
and offers no replacement.
.PP
Finally, ANSI did nothing to integrate wide characters
into the I/O system: it gives no method for printing
wide characters.
We therefore needed to invent some things and decided to invent
everything.
In the end, some of our entry points do correspond closely to
ANSI routines\(emfor example
.CW chartorune
and
.CW runetochar
are similar to
.CW mbtowc
and
.CW wctomb \(embut
Plan 9's library defines more functionality, enough
to write real applications comfortably.
.SH
Converting the tools
.PP
The source for our tools and applications had already been converted to
work with Latin-1, so it was `8-bit safe', but the conversion to the Unicode
Standard and UTF is more involved.
Some programs needed no change at all:
.CW cat ,
for instance,
interprets its argument strings, delivered in UTF,
as file names that it passes uninterpreted to the
.CW open
system call,
and then just copies bytes from its input to its output;
it never makes decisions based on the values of the bytes.
(Plan 9
.CW cat
has no options such as
.CW -v
to complicate matters.)
Most programs, however, needed modest change.
.PP
It is difficult to
find automatically the places that need attention,
but
.CW grep
helps.
Software that uses the libraries conscientiously can be searched
for calls to library routines that examine bytes as characters:
.CW strchr ,
.CW strrchr ,
.CW strstr ,
etc.
Replacing these by calls to
.CW utfrune ,
.CW utfrrune ,
and
.CW utfutf
is enough to fix many programs.
Few tools actually need to operate on runes internally;
more typically they need only to look for the final slash in a file
name and similar trivial tasks.
Of the 170 C source programs in the top levels of
.CW /sys/src/cmd ,
only 23 now contain the word
.CW Rune .
.PP
The programs that
.I do
store runes internally
are mostly those whose
.I raison
.I d'être
is character manipulation:
.CW sam
(the text editor),
.CW sed ,
.CW sort ,
.CW tr ,
.CW troff ,
.CW 8½
(the window system and terminal emulator),
and so on.
To decide whether to compute using runes
or UTF-encoded byte strings requires balancing the cost of converting
the data when read and written
against the cost of converting relevant text on demand.
For programs such as editors that run a long time with a relatively
constant dataset, runes are the better choice.
There are space considerations too, but they are more complicated:
plain ASCII text grows when converted to runes; UTF-encoded Japanese
shrinks.
.PP
Again, it is hard to automate the conversion of a program from
.CW chars
to
.CW Runes .
It is not enough just to change the type of variables; the assumption
that bytes and characters are equivalent can be insidious.
For instance, to clear a character array by
.P1
memset(buf, 0, BUFSIZE)
.P2
becomes wrong if
.CW buf
is changed from an array of
.CW chars
to an array of
.CW Runes .
Any program that indexes tables based on character values needs
rethinking.
Consider
.CW tr ,
which originally used multiple 256-byte arrays for the mapping.
The naïve conversion would yield multiple 65536-rune arrays.
Instead Plan 9
.CW tr
saves space by building in effect
a run-encoded version of the map.
.PP
.CW Sort
has related problems.
The cooperation of UTF and
.CW strcmp
means that a simple sort\(emone with no options\(emcan be done
on the original UTF strings using
.CW strcmp .
With sorting options enabled, however,
.CW sort
may need to convert its input to runes: for example,
option
.CW -t\f1α\fP
requires searching for alphas in the input text to
crack the input into fields.
The field specifier
.CW +3.2
refers to 2 runes beyond the third field.
Some of the other options are hopelessly provincial:
consider the case-folding and dictionary order options
(Japanese doesn't even have an official dictionary order) or
.CW -M
which compares by case-insensitive English month name.
Handling these options involves the
larger issues of internationalization and is beyond the scope
of this paper and our expertise.
Plan 9
.CW sort
works sensibly with options that make sense relative to the input.
The simple and most important options are, however, usually meaningful.
In particular,
.CW sort
sorts UTF into the same order that
.CW look
expects.
.PP
Regular expression-matching algorithms need rethinking to
be applied to UTF text.
Deterministic automata are usually applied to bytes;
converting them to operate on variable-sized byte sequences is awkward.
On the other hand, converting the input stream to runes adds measurable
expense
and the state tables expand
from size 256 to 65536; it can be expensive just to generate them.
For simple string searching,
the Boyer-Moore algorithm works with UTF provided the input is
guaranteed to be only valid UTF strings; however, it does not work
with the old UTF encoding.
At a more mundane level, even character classes are harder:
the usual bit-vector representation within a non-deterministic automaton
is unwieldy with 65536 characters in the alphabet.
.PP
We compromised.
An existing library for compiling and executing regular expressions
was adapted to work on runes, with two entry points for searching
in arrays of runes and arrays of chars (the pattern is always UTF text).
Character classes are represented internally as runs of runes;
the reserved value
.CW FFFF
marks the end of the class.
Then
.I all
utilities that use regular expressions\(emeditors,
.CW grep ,
.CW awk ,
etc.\(emexcept the shell, whose notation
was grandfathered, were converted to use the library.
For some programs, there was a concomitant loss of performance,
but there was also a strong advantage.
To our knowledge, Plan 9 is the only Unix-like system
that has a single definition and implementation of
regular expressions; patterns are written and interpreted
identically by all the programs in the system.
.PP
A handful of programs have the notion of character built into them
so strongly as to confuse the issue of what they should do with UTF input.
Such programs were treated as individual special cases.
For example,
.CW wc
is, by default, unchanged in behavior and output; a new option,
.CW -r ,
counts the number of correctly encoded runes\(emvalid UTF sequences\(emin
its input;
.CW -b
the number of invalid sequences.
.PP
It took us several months to convert all the software in the system
to the Unicode Standard and the old UTF.
When we decided to convert from that to the new UTF,
only three things needed to be done.
First, we rewrote the library routines to encode and decode the
new UTF.  This took an evening.
Next, we converted all the files containing UTF
to the new encoding.
We wrote a trivial program to look for non-ASCII bytes in
text files and used a Plan 9 program called
.CW tcs
(translate character set) to change encodings.
Finally, we recompiled all the system software;
the library interface was unchanged, so recompilation was sufficient
to effect the transformation.
The second two steps were done concurrently and took an afternoon.
We concluded that the actual encoding is relatively unimportant to the
software; the adoption of large characters and a byte-stream encoding
.I per
.I se
are much deeper issues.
.SH
Graphics and fonts
.PP
Plan 9 provides only minimal support for plain text terminals.
It is instead designed to be used with all character input and
output mediated by a window system such as
.CW 8½ .
The window system and related software are responsible for the
display of UTF text as Unicode character images.
For plain text, the window system must provide a user-settable
.I font
that provides a (possibly empty) picture for each Unicode character.
Fancier applications that use bold and Italic characters
need multiple fonts storing multiple pictures for each
Unicode value.
All the issues are apparent, though,
in just the problem of
displaying a single image for each character, that is, the
Unicode equivalent of a plain text terminal.
With 128 or even 256 characters, a font can be just
an array of bitmaps.  With 65536 characters,
a more sophisticated design is necessary.  To store the ideographs
for just Japanese as 16×16×1 bit images,
the smallest they can reasonably be, takes over a quarter of a
megabyte.  Make the images a little larger, store more bits per
pixel, and hold a copy in every running application, and the
memory cost becomes unreasonable.
.PP
The structure of the bitmap graphics services is described at length elsewhere
[Pike91].
In summary, the memory holding the bitmaps is stored in the same machine that has
the display, mouse, and keyboard: the terminal in Plan 9 terminology,
the workstation in others'.
Access to that memory and associated services is provided
by device files served by system
software on the terminal.  One of those files,
.CW /dev/bitblt ,
interprets messages written upon it as requests for actions
corresponding to entry points in the graphics library:
allocate a bitmap, execute a raster operation, draw a text string, etc.
The window system
acts as a multiplexer that mediates access to the services
and resources of the terminal by simulating in each client window
a set of files mirroring those provided by the system.
That is, each window has a distinct
.CW /dev/mouse ,
.CW /dev/bitblt ,
and so on through which applications drive graphical
input and output.
.PP
One of the resources managed by
.CW 8½
and the terminal is the set of active
.I subfonts.
Each subfont holds the
bitmaps and associated data structures for a sequential set of Unicode
characters.
Subfonts are stored in files and loaded into the terminal by
.CW 8½
or an application.
For example, one subfont
might hold the images of the first 256 characters of the Unicode space,
corresponding to the Latin-1 character set;
another might hold the standard phonetic character set, Unicode characters
with value 0250 to 02E9.
These files are collected in directories corresponding to typefaces:
.CW /lib/font/bit/pelm
contains the Pellucida Monospace character set, with subfonts holding
the Latin-1, Greek, Cyrillic and other components of the typeface.
A suffix on subfont files encodes (in a subfont-specific
way) the size of the images:
.CW /lib/font/bit/pelm/latin1.9
contains the Latin-1 Pellucida Monospace characters with lower
case letters 9 pixels high;
.CW /lib/font/bit/jis/jis5400.16
contains 16-pixel high
ideographs starting at Unicode value 5400.
.PP
The subfonts do not identify which portion of the Unicode space
they cover.  Instead, a
font file, in plain text,
describes how to assemble subfonts into a complete
character set.
The font file is presented as an argument to the window system
to determine how plain text is displayed in text windows and
applications.
Here is the beginning of the font file
.CW /lib/font/bit/pelm/jis.9.font ,
which describes the layout of a font covering that portion of
the Unicode Standard for which we have characters of typical
display size, using Japanese characters
to cover the Han space:
.P1
18	14
0x0000	0x00FF	latin1.9
0x0100	0x017E	latineur.9
0x0250	0x02E9	ipa.9
0x0386	0x03F5	greek.9
0x0400	0x0475	cyrillic.9
0x2000	0x2044	../misc/genpunc.9
0x2070	0x208E	supsub.9
0x20A0	0x20AA	currency.9
0x2100	0x2138	../misc/letterlike.9
0x2190	0x21EA	../misc/arrows
0x2200	0x227F	../misc/math1
0x2280	0x22F1	../misc/math2
0x2300	0x232C	../misc/tech
0x2500	0x257F	../misc/chart
0x2600	0x266F	../misc/ding
.P2
.P1
0x3000	0x303f	../jis/jis3000.16
0x30a1	0x30fe	../jis/katakana.16
0x3041	0x309e	../jis/hiragana.16
0x4e00	0x4fff	../jis/jis4e00.16
0x5000	0x51ff	../jis/jis5000.16
\&...
.P2
The first two numbers set the interline spacing of the font (18
pixels) and the distance from the baseline to the top of the
line (14 pixels).
When characters are displayed, they are placed so as best
to fit within those constraints; characters
too large to fit will be truncated.
The rest of the file associates subfont files
with portions of Unicode space.
The first four such files are in the Pellucida Monospace typeface
and directory; others reside in other directories.  The file names
are relative to the font file's own location.
.PP
There are several advantages to this two-level structure.
First, it simultaneously breaks the huge Unicode space into manageable
components and provides a unifying architecture for
assembling fonts from disjoint pieces.
Second, the structure promotes sharing.
For example, we have only one set of Japanese
characters but dozens of typefaces for the Latin-1 characters,
and this structure permits us to store only one copy of the
Japanese set but use it with any Roman typeface.
Also, customization is easy.
English-speaking users who don't need Japanese characters
but may want to read an on-line Oxford English Dictionary can
assemble a custom font with the
Latin-1 (or even just ASCII) characters and the International
Phonetic Alphabet (IPA).
Moreover, to do so requires just editing a plain text file,
not using a special font editing tool.
Finally, the structure guides the design of
caching protocols to improve performance and memory usage.
.PP
To load a complete Unicode character set into each application
would consume too
much memory and, particularly on slow terminal lines, would take
unreasonably long.
Instead, Plan 9 assembles a multi-level cache structure for
each font.
An application opens a font file, reads and parses it,
and allocates a data structure.
A message written to
.CW /dev/bitblt
allocates an associated structure held in the terminal, in particular,
a bitmap to act as a cache
for recently used character images.
Other messages copy these images to bitmaps such as the screen
by loading characters from subfonts into the cache on demand and
from there to the destination bitmap.
The protocol to draw characters is in terms of cache indices,
not Unicode character number or UTF sequences.
These details are hidden from the application, which instead
sees only a subroutine to draw a string in a bitmap from a
given font, functions to discover character size information,
and routines to allocate and to free fonts.
.PP
As needed, whole
subfonts are opened by the graphics library, read, and then downloaded
to the terminal.
They are held open by the library in an LRU-replacement list.
Even when the program closes a subfont, it is retained
in the terminal for later use.
When the application opens the subfont, it asks the terminal
if it already has a copy to avoid reading it from the file
server if possible.
This level of cache has the property that the bitmaps for, say,
all the Japanese characters are stored only once, in the terminal;
the applications read only size and width information from the terminal
and share the images.
.PP
The sizes of the character and subfont caches held by the
application are adaptive.
A simple algorithm monitors the cache miss rate to enlarge and
shrink the caches as required.
The size of the character cache is limited to 2048 images maximum,
which in practice seems enough even for Japanese text.
For plain ASCII-like text it naturally stays around 128 images.
.PP
This mechanism sounds complicated but is implemented by only about
500 lines in the library and considerably less in each of the
terminal's graphics driver and
.CW 8½ .
It has the advantage that only characters that are
being used are loaded into memory.
It is also efficient: if the characters being drawn
are in the cache the extra overhead is negligible.
It works particularly well for alphabetic character sets,
but also adapts on demand for ideographic sets.
When a user first looks at Japanese text, it takes a few
seconds to read all the font data, but thereafter the
text is drawn almost as fast as regular text (the images
are larger, so draw a little slower).
Also, because the bitmaps are remembered by the terminal,
if a second application then looks at Japanese text
it starts faster than the first.
.PP
We considered
building a `font server'
to cache character images and associated data
for the applications, the window system, and the terminal.
We rejected this design because, although isolating
many of the problems of font management into a separate program,
it didn't simplify the applications.
Moreover, in a distributed system such as Plan 9 it is easy
to have too many special purpose servers.
Making the management of the fonts the concern of only
the essential components simplifies the system and makes
bootstrapping less intricate.
.SH
Input
.PP
A completely different problem is how to type Unicode characters
as input to the system.
We selected an unused key on our ASCII keyboards
to serve as a prefix for multi-keystroke
sequences that generate Unicode characters.
For example, the character
.CW ü
is generated by the prefix key
(typically
.CW ALT
or
.CW Compose )
followed by a double quote and a lower-case
.CW u .
When that character is read by the application, from the file
.CW /dev/cons ,
it is of course presented as its UTF encoding.
Such sequences generate characters from an arbitrary set that
includes all of Latin-1 plus a selection of mathematical
and technical characters.
An arbitrary Unicode character may be generated by typing the prefix,
an upper case X, and four hexadecimal digits that identify
the Unicode value.
.PP
These simple mechanisms are adequate for most of our day-to-day needs:
it's easy to remember to type `ALT 1 2' for ½\^ or `ALT accent letter'
for accented Latin letters.
For the occasional unusual character, the cut and paste features of
.CW 8½
serve well.  A program called (perhaps misleadingly)
.CW unicode
takes as argument a hexadecimal value, and prints the UTF representation of that character,
which may then be picked up with the mouse and used as input.
.PP
These methods
are clearly unsatisfactory when working in a non-English language.
In the native country of such a language
the appropriate keyboard is likely to be at hand.
But it's also reasonable\(emespecially now that the system handles Unicode characters\(emto
work in a language foreign to the keyboard.
.PP
For alphabetic languages such as Greek or Russian, it is
straightforward to construct a program that does phonetic substitution,
so that, for example, typing a Latin `a' yields the Greek `α'.
Within Plan 9, such a program can be inserted transparently
between the real keyboard and a program such as the window system,
providing a manageable input device for such languages.
.PP
For ideographic languages such as Chinese or Japanese the problem is harder.
Native users of such languages have adopted methods for dealing with
Latin keyboards that involve a hybrid technique based on phonetics
to generate a list of possible symbols followed by menu selection to
choose the desired one.
Such methods can be
effective, but their design must be rooted in information about
the language unknown to non-native speakers.
.CW Cxterm , (
a Chinese terminal emulator built by and for
Chinese programmers,
employs such a technique
[Pong and Zhang].)
Although the technical problem of implementing such a device
is easy in Plan 9\(emit is just an elaboration of the technique for
alphabetic languages\(emour lack of familiarity with such languages
has restrained our enthusiasm for building one.
.PP
The input problem is technically the least interesting but perhaps
emotionally the most important of the problems of converting a system
to an international character set.
Beyond that remain the deeper problems of internationalization
such as multi-lingual error messages and command names,
problems we are not qualified to solve.
With the ability to treat text of most languages on an equal
footing, though, we can begin down that path.
Perhaps people in non-English speaking countries will
consider adopting Plan 9, solving the input problem locally\(emperhaps
just by plugging in their local terminals\(emand begin to use
a system with at least the capacity to be international.
.SH
Acknowledgements
.PP
Dennis Ritchie provided consultation and encouragement.
Bob Flandrena converted most of the standard tools to UTF.
Brian Kernighan suffered cheerfully with several
inadequate implementations and converted
.CW troff
to UTF.
Rich Drechsler converted his Postscript driver to UTF.
John Hobby built the Postscript ☺.
We thank them all.
.SH
References
.LP
[ANSIC] \f2American National Standard for Information Systems \-
Programming Language C\f1, American National Standards Institute, Inc.,
New York, 1990.
.LP
[ISO10646]
ISO/IEC DIS 10646-1:1993
\f2Information technology \-
Universal Multiple-Octet Coded Character Set (UCS) \(em
Part 1: Architecture and Basic Multilingual Plane\fP.
.LP
[Pike90] R. Pike, D. Presotto, K. Thompson, H. Trickey,
``Plan 9 from Bell Labs'',
UKUUG Proc. of the Summer 1990 Conf.,
London, England,
1990.
.LP
[Pike91] R. Pike, ``8½, The Plan 9 Window System'', USENIX Summer
Conf. Proc., Nashville, 1991, reprinted in this volume.
.LP
[Pike92] R. Pike, ``How to Use the Plan 9 C Compiler'', this volume.
.LP
[Pong and Zhang] Man-Chi Pong and Yongguang Zhang, ``cxterm:
A Chinese Terminal Emulator for the X Window System'',
.I
Software\(emPractice and Experience,
.R
Vol 22(1), 809-926, October 1992.
.LP
[Unicode]
\f2The Unicode Standard,
Worldwide Character Encoding,
Version 1.0, Volume 1\f1,
The Unicode Consortium,
Addison Wesley,
New York,
1991.