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LOCALE TUTORIAL
Written by Patrick D'Cruze (pdcruze@orac.iinet.com.au)
with contributions from Mitchum DSouza
(m.dsouza@mrc-applied-psychology.cambridge.ac.uk)
Topics:
1 An introduction to locale and catalogs
1.1 What is locale?
1.2 What are message catalogs?
1.3 What is the format of a message catalog?
2 What routines are involved?
2.1 Setlocale()
2.2 Catopen()
2.3 Catgets()
2.4 Catclose()
2.5 Xtract
2.6 Gencat
3 Writing locale software
3.1 Writing and modifying software to support message catalogs
3.2 Writing software that is to be used on locale and non-locale systems
4 Where are the message catalogs stored?
5 Frequently Asked Questions
Section 1. Introduction to locales and catalogs
1.1 What is a locale?
There are many attributes that are needed to define a country's cultural
conventions. These attributes include the country's native language,
the formatting of the date and time, the representation of numbers, the
symbols for currency, etc. These local "rules" are termed the
country's locale. The locale represents the knowledge needed to support
the country's native attributes.
There are 5 major areas which may vary between countries and hence locales.
Characters and Codesets
The codeset most commonly used through out the USA and most English
speaking parts of the world is the ASCII codeset. However, there are
many characters needed by various locales that are not found within this
codeset. The 8-bit ISO 8859-1 code set has most of the special
characters needed to handle the major European languages. However, in
many cases, the ISO 8859-1 font is not adequate. Hence each locale will
need to specify which codeset they need to use and will need to have the
appropriate character handling routines to cope with the codeset.
Currency
The symbols used vary from country to country as does the position used
by the symbol. Software needs to be able to transparently display
currency figures in the native mode for each locale.
Dates
The format of date varies between locales. eg, Christmas day in 1994,
is written as 12/25/94 in the USA and as 25/12/94 in Australia. Some
locales require time to be specified in 24-hour mode rather than as AM
or PM.
Numbers
Numbers can be represented differently in different locales. eg, the
following numbers are all written correctly for their respective locales:
12,345.67 English
12.345,67 French
1,2345.67 Asia
Messages
The most obvious area is the language support within a locale. An easy
mechanism has to be provided for developers and users to easily change
the language that the software uses to communicate to the user.
This Locale tutorial will only concentrate on the area of native message
support for software. At a later stage, it will be updated to
illustrate the ease with which developers can add support for other
locale attributes. In addition it must be emphasized that the locale
routines and functions are used most frequently by text-based software
ie, software which operates within an xterm or a virtual console.
Different routines exist for software that interacts with X Windows, and
these too will be covered in a later revision of this document.
1.2 What is a Message Catalog?
Software communicates to users by writing text messages to the screen.
These messages can be scattered throughout many lines of source code.
To support various languages, it is necessary to translate these text
messages into different languages. It is infeasible to hardcode these
messages into the source code for two reasons:
1). To translate the messages into another language, translators would
have to go hunting through the source code for these messages. This is
obviously inefficient and many times, translators may not even have
access to the source code.
2). Supporting a new language will mean that the text messages within
the code, needs to be translated, and then the code needs to be
recompiled. This needs to be done for every language.
The solution is to have all textual message stored in an external
message catalog. Whenever the software needs to display a message, the
software tells the operating system to look up the appropriate message
in the catalog and display it on the screen.
The benefits this brings is that:
a) the catalog can be translated without needing access to the source code
b) the source code only needs to be compiled once. To support a new
language, its only a matter of translating the message catalog and
shipping the translated catalog to the user
c) All of the message are collated into one place.
1.3 What is the format of a message catalog?
Once the text messages have been extracted from the source code, they
are stored within an ordinary text file which is commonly referred to as
a message file. The text file often has the following structure:
1 Cannot open file foo.bar
2 Cannot write to file foo.bar
3 Cannot access directory
... ...
While this is a useful representation for programmers and translators,
it is an inefficient form for the operating system to access. The
operating system would be able to access the text messages a lot faster
if they were stored in some sort of binary database form. And this is
indeed what is done.
A message catalog is a binary representation of the messages used within
the software. The message text files are compiled using the gencat
software into a binary message catalog. The compiled message catalog is
in a machine-specific format and is not portable between different
machines and architectures, however this is of little concern. It is
trivial to recompile the message text files on other platforms - the
gencat software operates identically on other platforms.
Programmers and translators store the text messages used by their
software within message files and these files are then compiled into a
message catalog. However, a single piece of software may contain
hundreds of printf() statements, each one consisting of a unique
message. Each of these messages needs to be stored in a message file.
It is entirely unreasonable to expect to have all of these stored within
a single message text file. Editing, changing, deleting, and adding new
messages would grow to be a major inconvenience.
The solution is to break up messages into sets. Each set contains
messages for a different part of the software. Combining all of the
sets together gives the sum total of all messages used within the
software. These sets can then be compiled into a single message
catalog. The software can then access a particular message within a
particular set within the message catalog.
This makes the programmers job, (and the translators job) a lot easier.
The programmer can assign separate sets for major subroutines. Then
when a subroutine is modified or changed, only its corresponding message
set needs to be changed. All others sets can be left alone.
eg,
For software gnubar we have two major areas requiring communication to
the user - displaying errors, and reporting results.
So we create 2 message files (or sets):
errors.m
results.m
(We adopt the practice of using .m to signify a message file).
All of the error messages are stored within the errors.m file, and
similarly, result messages are stored in the results.m file. We then
modify the software so that whenever an error message needs to be
printed, the software accesses the errors set, and prints the
corresponding error message. Similarly for the results set.
Both of these files are then compiled to form the message catalog for
gnubar. The resulting catalog is usually named:
gnubar.cat
This catalog consists of 2 sets - errors and results, each of which
contains numerous messages.
To access a particular message, the software needs to specify which set
the message is located in, and the message number to be displayed from
that set.
Section 2. What routines are involved?
The 4 core routines for accessing and dealing with message catalogs
within your source code are setlocale(), catopen(), catgets(), and
catclose().
NB. Remember that Message Catalogs are but one element of a locale.
Other elements will be covered in later revisions of this document.
Note for Linux users: To access and use the locale functions you will
need to use libc.so.4.4.4c or greater (I'd recommend using at least
libc.so.4.5.26 or higher as this includes a lot of improvements in the
locale routines). You will also need the include files locale.h and
nl_types.h - if you have a libc that supports locale functions, then you
will also most likely have these include files too.
2.1 SETLOCALE()
The first thing a program needs to do is to establish the locale to use.
It does this using the setlocale() function. This is defined as:
#include <locale.h>
char *setlocale(int category, const char *locale);
The category argument tells the setlocale() function which attributes to
set. The choices are:
LC_COLLATE Changes the behavior of the strcoll() and strxfrm() functions.
LC_CTYPE Changes the behavior of the character-handling functions:
isalpha(), islower(), isupper(), isprint(), ...
LC_MESSAGES Changes the language in which messages are displayed.
LC_MONETARY Changes the information returned by localeconv().
LC_NUMERIC Changes the radix character for numeric conversions.
LC_TIME Changes the behavior of the strftime() function.
LC_ALL Changes all of the above.
In our examples, we will only be dealing with the Message catalogs,
hence we only need to set the LC_MESSAGES category within the
setlocale() function. The LC_ALL category could also be used. However
it is good programming practise to only use those categories that you
need within your software. The reason for this will be explained
shortly.
The locale argument is the name of a locale. Two special locale names are:
C this makes all attributes function as defined in the
C standard.
POSIX this is the same as the above.
Usually, the locale argument will be:
""
(empty quotes). This will select the user's native locale. This is
done by the operating system as follows:
1. If the user has an environment variable LC_ALL defined, and it is not
null, then the value of this environment variable is used as the locale
argument.
2. If the user has an environment variable that has the same name as the
category, and which is not null, then this is used as the locale
argument.
3. If the LANG environment variable is defined and is not null, then
this value is used as the locale argument.
If the resulting value is the same as a valid, supported locale, then
the locale is changed. If the value however does not name a supported
locale and is not null, setlocale() will return a NULL pointer and the
locale will not be changed from the default "C" locale.
At program startup, the operating system performs the following
setlocale() function:
setlocale(LC_ALL, "C");
This if your software doesn't make any setlocale() calls, or cannot
change the locale (due to no valid environment variables being set),
then the software will use the default C locale.
If setlocale() is unable to change the locale, then NULL is returned.
Good programming practice dictates that you should only use the locale
categories suitable for your software. An example will illustrate why.
eg,
main()
{
setlocale(LC_ALL, "");
....
}
The software will now set all the locale categories to the value of
either the LC_ALL environment variable if set, or else the value of the
LANG environment variable. Otherwise, it will use the default "C"
locale.
Now suppose, the user wishes to have all messages displayed on their
screen in English, but wishes to use the other attributes from the
French locale. The user does this by pointing the LC_MESSAGES variable
to the English locale, but setting the LANG variable to the French
locale.
Now the above example (using LC_ALL) will ignore the LC_MESSAGES
environment variable and will instead use the LANG variable. Hence
messages will be displayed in French. The user can either have all
attributes set for English or all the attributes set for French.
Admittedly this would be a very rare situation but if your software only
needs to access the Messages attribute, then only this category needs to
be set. If your software needs to access 4 categories, then you should
use 4 setlocale() functions.
It is the user's responsibility to correctly set their environment
variables. It is also easy for a user to alter their environment,
simply by changing their environment variables. It is wise to include
information on the correct setting of these variables with your software
as many users may be unaware of the correct procedures or settings.
These issues will be covered in a later section.
2.2 CATOPEN()
The setlocale() function only establishes the correct locale for the
program to use. To access a catalog, the catalog must first be opened.
The catopen() function is used for this. It is defined as follows:
#include <nl_types.h>
nl_catd catopen(char *name, int flag);
Catopen() opens a message catalog and returns a catalog descriptor.
name specifies the name of the message catalog to be opened. If name
specifies an absolute path, (i.e. contains a `/') then name specifies a
pathname for the message catalog. Otherwise, the environment variable
NLSPATH is used with name substituted for %N. If NLSPATH does not exist
in the environment, or if a message catalog cannot be opened in any of
the paths specified by NLSPATH, then the following paths are searched in
order
/usr/lib/locale/LC_MESSAGES
/usr/lib/locale/name/LC_MESSAGES
In all cases LC_MESSAGES stands for the current setting of the
LC_MESSAGES category of locale from a previous call to setlocale() and
defaults to the "C" locale. In the last search path name refers to the
catalog name.
The flag argument to catopen is used to indicate the type of loading
desired. This should be either MCLoadBySet or MCLoadAll. The former
value indicates that only the required set from the catalog is loaded
into memory when needed, whereas the latter causes the initial call to
catopen() to load the entire catalog into memory.
catopen() returns a message catalog descriptor of type nl_catd on
success. On failure, it returns -1.
Sample usage:
static nl_catd catfd = 0;
catfd = catopen("foo.cat", MCLoadBySet);
if (catfd == -1)
printf("Failed to open the message catalog");
2.3 CATGETS()
Once a message catalog has been opened, we need a routine to access the
catalog and retrieve messages from it. This is the purpose of the
catgets() routine. It is defined as:
#include <nl_types.h>
char *catgets(nl_catd catfd, int set_number, int message_number, char
*message);
catgets() reads the message message_number, in set set_number, from the
message catalog identified by catfd. catfd is a catalog descriptor
returned from an earlier call to catopen(3). The fourth argument
message points to a default message string which will be returned by
catgets() if the identified message catalog is not currently open, or
damaged. The message-text is contained in an internal buffer area and
should be copied by the application if it is to be saved or modified.
The return string is always terminated with a null byte.
On success, catgets() returns a pointer to an internal buffer area
containing the null-terminated message string. catgets() returns a
pointer to message if it fails because the message catalog specified by
catfd is not currently open. Otherwise, catgets() returns a pointer to
an empty string if the message catalog is available but does not
contain the specified message.
Sample usage:
printf(catgets(catfd, 3, 7, "Error accessing block %d"), block_num);
The above routine attempts to access the 7th message in the 3rd set of
the message catalog. If this message cannot be accessed for any reason,
then the message "Error accessing block %d" is printed instead.
2.4 CATCLOSE()
Once the software has finished using a particular message catalog, the
catalog should be closed so that the operating system can free up the
memory used to store the catalog. The catalog is closed by the use of
the catclose() function. It is defined as:
#include <nl_types.h>
void catclose(nl_catd catfd);
catclose() closes the message catalog identified by catfd. It
invalidates any subsequent references to the message catalog defined by
catfd.
catclose() returns 0 on success, or -1 on failure.
Sample usage:
....
catclose(catfd);
exit(0);
}
These are the 4 C routines needed to access catalogs within your
software. The next section will cover tools that are available to help
you extract existing messages from your software, and will detail the
gencat software for compiling message text files into message catalogs.
Before we discuss xtract and gencat, we'll outline the format of the
text message files. Gencat requires the message file to be in a
specific format so that it can compile the messages into a message
catalog.
A sample message file is given below:
$set 2 #chmod
$ #1 Original Message:(invalid mode)
# invalid mode
$ #2 Original Message:(virtual memory exhausted)
# virtual memory exhausted
...
The first line is used to establish the set number for this message
file. The "set" keyword must exist in all message files. The second
field is the set number for this message file and must be unique for the
message catalog. The third field (minus the # sign) is the name which
can also be used to identify this set (the set number can also be used).
(More on this later).
The second line is the unique id for this message. The only important
things here are the $ sign and the second field (the #num). The $ sign
is always needed to distinguish between a text message, and a message id
(or set command). The second field (minus the # sign) is the message
id. Everything after this second field is ignored. It is often helpful
to include the original message to aid translators and others who have
to modify or edit the message file.
The third line (minus the # sign) is the actual text message. In this
case, it is the text message for the first message in this second set.
Similarly, the fifth line is the text for the second message in this
second set.
When translating message files into other languages, it is only
necessary to translate the "text" lines, ie lines starting with a #
sign. Anything with a $ sign at the beginning should not be touched.
The above format for the message file matches the arguments for the
catgets() routine perfectly. The catgets() routine requires the
set_number and the message_number to be integers, which of course they
are in the message file structure outlined above. Thus to print the
first message from the second set:
$set 2 #chmod
+------------^
| +--------v
| | $ #1 Original Message:(invalid mode)
| | # invalid mode
| |
| | $ #2 Original Message:(virtual memory exhausted)
| | # virtual memory exhausted
| | ...
| |
| | we use the following arguments:
| |
| | printf(catgets(catfd, 2, 1, "invalid mode"));
| +------------------------------^
+-----------------------------^
While the locale functions and routines will function perfectly, it
doesn't make for an intuitive way of writing software. ie, whenever a
software developer needs to print a text message, they first need to
look up the message, find its set number and message number, and then
copy these into the software. This can become unwieldy when software
needs to access several sets or catalogs or messages. Looking up these
hard to remember numbers is a pain.
Instead of using an integer to refer to a set number or message number,
it would be much easier to use names or ascii text to refer to them. We
can do this if we use #defines to map the ascii names to integers.
To do this requires a few additional steps (over using the standard
integer access methods). The first thing to do is to change the message
identifiers from numbers to ascii names. So instead of having:
...
$ #1
# text for message 1
$ #2
# text for message 2
...
We will have:
...
$ #Label1
# text for message 1
$ #Label2
# text for message 2
Note we do not need to make any alterations for the set numbering as a
name is already present for this. The first line of every message file
contains 3 fields:
$set 2 #chmod
The second field determines that this is the second set within this
message catalog. The third field (minus the # sign) is the name which
can also be used to access this set.
The new message file looks like this:
$set 2 #chmod
$ #Invalid_Mode Original Message:(invalid mode)
# invalid mode
$ #VM_exhausted Original Message:(virtual memory exhausted)
# virtual memory exhausted
...
To access the second message from this second set we can now use the
following code:
printf(catgets(catfd, chmodSet, chmodVM_exhausted, "virtual memory
exhausted"));
The set_number argument in the catopen() routine is always the set name
(chmod) appended with the word "Set" => "chmodSet". The message_number
argument is always the set name (chmod) appended with the message id
string (VM_exhausted) => chmodVM_exhausted.
In order to use these ascii names however, the software needs to
associate these names with an integer because the catopen() routine only
accepts integers for the set_number and message_number arguments. We
make this association by asking the gencat software (explained further
below in detail) to generate an include file which is used by the
software to map these names to integers.
For the above message file, the generated include file looks like this:
#define chmodSet 0x2
#define chmodInvalid_Mode 0x1
#define chmodVM_exhausted 0x2
...
This header file was generated from the chmod.m message file. We adopt
the practice of naming these header files as xxx-nls.h so in our case
this header file is called:
chmod-nls.h
We now have one thing left to do and that is to include this header file
in the software. So we now include the line:
#include "chmod-nls.h"
at the beginning of our software. With that, we can now take advantage
of a much more flexible and intuitive means of referring to message sets
and messages.
2.5 XTRACT
xtract is some software written using yacc to extract messages from
source code. It needs to be compiled into a binary and can be found on
sunsite.unc.edu:/pub/Linux/utils/nls/catalogs/locale-package.tar.gz
xtract searches through the source code for any string messages
contained within quotes, and prints out any it finds to stdout.
It is used as follows:
xtract <source_code.c> message_file.m
eg, to extract the messages from file foobar.c and place them in the
message file foobar.m:
xtract <foobar.c> foobar.m
The resulting message file contains all the messages that xtract could
find within the source. The messages have all been placed in the
correct format.
A little bit of editing however is required of the resulting message
file. The first two lines need to be deleted and in their place, an
appropriate "set" line needs to be inserted.
ie,
the original message file will look like this:
$ #0 Original Message:(configuration probelms)
# configuration problems
$ #1 Original Message:(cannot open file)
# cannot open file
$ #2 Original Message:(error accessing file)
# error accessing file
....
This is not in the correct message file format because it is lacking a
line to establish the set number for this message file. Thus the
following line needs to be inserted at the very beginning of the message
file:
$set X #descriptor
where X = the set number for this message file
and descriptor is a suitable text descriptor for this set
Thus thus the resulting message file would look something like this:
$set 17 #database
$ #0 Original Message:(configuration probelms)
# configuration problems
$ #1 Original Message:(cannot open file)
# cannot open file
$ #2 Original Message:(error accessing file)
# error accessing file
....
2.6 GENCAT
Gencat is the software used to compile message files into message
catalogs. The command line switches it understands are detailed below:
gencat [-new] [-lang C|C++|ANSIC] catfile msgfile [-h <header-file>]
A description of the flags:
-new Erase the msg catalog and start a new one.
The default behavior is to update the catalog with the
specified msgfile(s). This will instead cause the old
one to be deleted and a whole new one started.
-lang <l> This governs the form of the include file.
Currently supported is C, C++ and ANSIC. The latter two are
identical in output. This argument is position dependent,
you can switch the language back and forth in between
include files if you care to.
-h <hfile> Output identifiers to the specified header files.
This creates a header file with all of the appropriate
#define's in it. Without this it would be up to you to
ensure that you keep your code in sync with the catalog file.
The header file is created from all of the previous msgfiles
on the command line, so the order of the command line is
important. This means that if you just put it at the end of
the command line, all the defines will go in one file
gencat foo.m bar.m zap.m -h all.h
If you prefer to keep your dependencies down you can specify
one after each message file, and each .h file will receive
only the identifiers from the previous message file
gencat foo.m -h foo.h bar.m -h bar.h zap.m -h zap.h
As an added bonus, if you run the following sequence:
gencat foo.m -h foo.h
the file foo.h will NOT be modified the second time. gencat
checks to see if the contents have changed before modifying
things. This means that you won't get spurious rebuilds of
your source every time you change a message. You can thus use
a Makefile rule such as:
MSGSRC=foo.m bar.m
GENFLAGS=-or -lang C
GENCAT=gencat
NLSLIB=nlslib/OM/C
$(NLSLIB): $(MSGSRC)
@for i in $?; do cmd="$(GENCAT) $(GENFLAGS) $@
$$i -h `b
asename $$i .m`.H"; echo $$cmd; $$cmd; done
foo.o: foo.h
The for-loop isn't too pretty, but it works. For each .m
file that has changed we run gencat on it. foo.o depends on
the result of that gencat (foo.h) but foo.h won't actually
be modified unless we changed the order (or added new members)
to foo.m.
The gencat software has two purposes and is usually used in 2 passes.
The first use is to generate the header files from the message files so
that the software can use descriptive names when referring to sets and
messages.
The following command will accomplish this:
gencat -new /dev/null foobar.m -h foobar-nls.h
The gencat software will take the foobar.m message file and produce a
header file called foobar-nls.h which can the be included in the
software. The -new and /dev/null flags indicate that gencat should also
generate a new message catalog but send the resultant catalog to the bit
bucket.
If you want to generate multiple header files for multiple message
files, you have to use the following command:
gencat -new /dev/null aaa.m -h aaa-nls.h bbb.m -h bbb-nls.m ....
This will generate a header file for each message file. For each
message set that your software accesses, you will need to include the
corresponding header file. If you would like to compile just one
solitary header file for all your message sets, the following command
can be used:
gencat -new /dev/null aaa.m bbb.m ccc.m -h foobar-nls.m
The other use for the gencat software is in generating message catalogs
from the message files. To generate a new message catalog, the
following command can be used:
gencat -new foobar.cat foobar.m
This will take the foobat.m message file and compile it into a message
catalog called foobar.cat. To compile multiple message sets into one
catalog, the following command can be used:
gencat -new foobar.cat foobar1.m foobar2.m foobar3.m ...
The usual way for compiling message catalogs is via a Makefile. In this
case, it is often easier to define a variable (say, MESSAGEFILES) to
contain the list of message files which need to be compiled into a
catalog. eg, in the above example we would have a line within the
Makefile reading:
MESSAGEFILES = foobar1.m foobar2.m foobar3.m ....
Then to compile these files into a catalog, we use the following line
within the Makefile:
gencat -new foobar.cat $(MESSAGEFILES)
SECTION 3. Writing locale software
3.1 Writing and modifying software to support message catalogs
So how do I modify or write new software that supports message catalogs?
Here are the steps involved.
STEP 1: (only applicable if modifying existing software)
The first thing to do is to extract text messages from the existing
software and place them into a message file. The xtract software is
used to do this. Its operation is covered elsewhere in this document,
but briefly you use it as follows:
source code == foobar.c
message file == foobar.m
xtract <foobar.c> foobar.m
We now have to insert the appropriate set number declaration at the
beginning of the message file. ie, insert a line:
$set X #bbb
where X = the set number for this message file
bbb = the variable name used to access this message set
STEP 2: (only applicable if creating a new message file)
If creating a new message file from scratch, it is important to
remember the correct order and structure of the message file. There are
3 key elements of a message file:
- the message set identifier
- the actual message identifier
- the text for each message identifier
The format of the message file has been covered in an earlier section
of this document. This format must be adhered to otherwise problems
will arise when compiling the message files into a message catalog.
Briefly, the format must be as follows:
$set 2 #chmod
$ #Invalid_Mode Original Message:(invalid mode)
# invalid mode
$ #VM_exhausted Original Message:(virtual memory exhausted)
# virtual memory exhausted
...
The first line is the message set identifier. All other lines starting
with a $ sign are message identifiers. The lines immediately following
these are the actual messages displayed.
STEP 3:
Whether modifying a message file extracted from step 1, or creating a
new message file from scratch, it is much easier to use names to refer
to messages and sets rather than numbers. To use names, we need to
assign a unique name to be the set identifier, and assign unique names
to the messages within that set.
The first line of every message file is the set identifier line. Its
format is as follows:
$set X #bbb
where X = the set number for this message file
bbb = the name used to access this message set
X must be a unique number for this set. So too does the name (bbb).
Subsequent accesses to this set can either use the number (X) as the set
identifier or the set name (bbb). It is up to you which you decide to
use. However if you do decide to use the set name, remember that in
your software, you must append the set name with the word "Set" to
access it. ie, the complete set name for accessing this set is "bbbSet".
STEP 4:
Now that we are using names as set identifiers and message identifiers,
we have to create a header file which maps these names to integers which
can be used by the message catalog routines within libc. The gencat
software is used to generate a header file from a message file. Its
operation is explained elsewhere in this document. But briefly, we use
the following command and arguments to generate the header file:
Message file == foobar.m
Header file == foobar-nls.h
gencat -new /dev/null foobar.m -h foobar-nls.h
Gencat will then take the message file listed and generate an
appropriate set of defines in the header file. This header file must
now be included in the software.
We recommend adopting the practise of naming your gencat generated
header files "xxx-nls.h". The "-nls" name will help you to distinguish
locale specific header files from other header files used by your
software.
STEP 5:
We are now ready to start modifying the source code. The first thing
we need to do is to include the appropriate header files. We will
usually need to include at least 3 files:
#include <locale.h"
#include <nl_types.h>
#include "foobar-nls.h"
The first header file <locale.h> defines various variables used by the
setlocale() and other C routines, such as the LC_* variables
(LC_MESSAGES, LC_TIME, LC_ALL, etc).
The second header file <nl_types.h> defines variables that are used by
the catopen() and catclose() routines and also defines the nl_catd
catalog file descriptor variable.
The third header file is the set of defines for the message file(s)
used by your software and allows you to use names in catgets() routines
when referring to message and sets.
STEP 6:
The next thing to do is to declare one or more global catalog
descriptor variables. We need a catalog descriptor when we access a
message catalog. Usually, software will only need to access their own
message catalog and hence we only need to define one message catalog
descriptor. This is defined before main():
/* Message catalog descriptor */
static nl_catd catfd = -1;
Now whenever we need to refer to or access the message catalog, we use
the catfd file descriptor variable.
STEP 7:
Within main() the first thing we need to do is to set the locale used
by the software. This is done by calling the setlocale() function. The
operation of the setlocale() routine is described elsewhere in this
document. However the usual arguments when dealing with message
catalogs is to use the following form:
setlocale(LC_MESSAGES,"");
This will set the LC_MESSAGE locale routines, to the appropriate
directory as specified by the user within their environment variables.
STEP 8:
We now have the software accessing the proper directories when it needs
to look for message catalogs and/or other locale information. We now
need to open the message catalog used by our software. This is achieved
by using the catopen() routine.
The easiest way to do this is to use the following line:
catfd = catopen("foobar",MCLoadBySet);
The catopen() routine has 2 arguments: the name of the message catalog
to open, and the type of loading desired. Message catalogs are usually
stored in the appropriate directory as:
foobar.cat
However, we do not need to include the ".cat" extension when using
catopen() to open the catalog. Indeed adding the ".cat" extension will
most likely cause the catopen() routine to fail to open the message
catalog and you will be left using the default message stored within
your software.
The type of loading desired is either to load the message catalog a set
at a time or to load the complete set into memory all at once.
Obviously loading the catalog set by set uses up less memory than
loading the complete catalog at once. However, access will be slightly
slower because each new access to a different set will require the new
set to be loaded into memory. The choice is left to the programmer.
A more robust way of opening and initializing the message catalog is
presented below. Software often spans multiple subroutines and files
and a message catalog may be opened and closed in many different places.
It can sometimes become tricky to keep track of whether a catalog is
open or closed. To alleviate this, it is helpful to define a catalog
initialization routine which checks to see if the catalog is currently
open. If not, it opens the catalog. This 5 line routine is presented
below:
catinit ()
{
if (catfd == (nl_catd)-1)
catfd = catopen("foobar",MCLoadBySet);
}
The routine first checks to see if the catalog is open. If it is, it
immediately returns. If not, it opens the message catalog and then
returns. It is thus fairly easy to insert this catinit() routine into
your source code and various subroutines. The first time you call this
routine should be immediately after the setlocale() line in main().
Thereafter, you can call this routine whenever you are unsure whether
the catalog is open or closed.
STEP 9:
Now we are finally ready to start accessing the message catalog and
retrieving messages from it. We do this via the catgets() routine.
The catgets() function has 4 arguments.
catgets(catfd, set_identifier, message_identifier, *message);
The catfd catalog descriptor is the descriptor returned from the
catinit() or catopen() routines. It is used by the catgets() function
to determine which message catalog to access (more than one message
catalog may be opened at one time within the software).
The set_identifier is the variable used to identify which set to access
within the message catalog. This can either be the set number or else
the set name (which needs to be appended with the word "Set").
The message_identifier is the name or number used to identifier a
particular message within the set. If the name is used, it must be
remembered that the name of the set must be prepended to the message
name.
The *message is the default string which is used if the catgets routine
cannot access the message catalog (perhaps it was not installed or
cannot be read). It can be a unique message.
eg,
catgets(catfd, errorsSet, errorsVM_exhausted, "Virtual memory has been
exhausted");
this will attempt to obtain the VM_exhausted message from the errors
set. If successful, the retrieved message is pointed to. If not, then
the text string "Virtual memory has been exhausted" is used in its place.
We recommend that you adopt the practice of always using the standard
English messages as the default string. If the catalog cannot be opened
for any reason, then the software will resort to using the standard
English messages which are stored internally within the compiled binary.
The catgets() routine merely returns a pointer to an internal buffer
area containing the null-terminated message string. We need to print
out this message string to the user. Hence we just encapsulate the
catgets() routine around a printf() statement. This will ensure the
message is printed out.
eg,
printf(catgets(catfd, errorsSet, errorsVM_Exhausted, "Virtual memory
has been exhausted"));
This will attempt to access the desired message and print it out. It
will either successfully retrieve the message and print it out, or else
print out the default message.
A few examples of the old approach (hard coded messages) versus the new
approach (message catalogs) will illustrate how to use the catgets()
function.
Example 1:
BEFORE:
printf("Incorrect read permission");
AFTER:
printf(catgets(catfd, errorsSet, errorsIncorrect_Perm, "incorrect read
permission");
Example 2:
BEFORE:
printf("Cannot change to directory %s", dir_name);
AFTER:
(extract from the message catalog)
...
$ #Cant_chdir
# Cannot change to directory %s
...
printf(catgets(catfd, errorsSet, errorsCant_chdir, "Cannot change to
directory %s"), dir_name);
Variables and other printf formatting codes are used transparently.
The codes can easily be included within the message files and catalogs
as can all escape codes.
STEP 10:
Just before the software is about to exit (or when we have finished
using a message catalog), we need to close the catalog. The simple line
to do this is:
catclose(catfd);
And that's basically it. Little or no error checking needs to be done.
If the catalog cannot be opened for any reason, then the software uses
the default stored message. It is a good idea though to check for
errors while debugging the software. There are many reasons why the
catalog cannot be opened by the operating system (incorrect directory
location, incorrect name, incorrect file permissions, incorrect set or
message identifiers, etc) and checking for these errors while debugging
can help correct these mistakes.
Below is a sample program that incorporates all of the features
necessary to employ message catalogs:
---
#include <stdio.h>
#include <nl_types.h>
#include <locale.h>
#include "foobar-nls.h"
static nl_catd catfd = -1;
void main()
{
char temp_name;
setlocale(LC_MESSAGES,"");
catinit ();
printf(catgets(catfd, foobarSet, foobarRandom_Name, "Random text with
string %s"), temp_name);
catclose(catfd);
exit(0);
}
catinit ()
{
if (catfd != (nl_catd)-1)
catfd = catopen("foobar",MCLoadBySet);
}
---
A Makefile for the above program is given below:
-------
all: foobar catalog
foobar: foobar.o
gcc -o foobar -O2 foobar.c
foobar.o: foobar-nls.h
foobar-nls.h: foobar-nls.m
gencat -new /dev/null foobar-nls.m -h foobar-nls.h
catalog:
gencat -new foobar.cat foobar.m
install: all
install -o root -m 0755 foobar /usr/local/bin
install -o root -m 0755 foobar.cat /etc/locale/C
clean:
/bin/rm -f foobar *.o foobar-nls.h foobar.cat core
-------
It is up to you where you group the message files. It may be easier to
group the message files in another directory and separate the source
code from the message files.
3.2 Writing software that is to be used on locale and non-locale systems
It is fairly easy to abstract out the locale specific functions from the
rest of the code. The usual method of doing this is via a define
statement.
eg, within the Makefile add the following:
DEFINES = -DNLS
foobar.o foobar.c
gcc $(DEFINES) foobar.c
Now within foobar.c we have the following:
#ifdef NLS
printf(catgets(catfd, chmodSet, chmodVM_exhausted, "Virtual Memory
exhausted"));
#else
printf("Virtual Memory exhausted");
#endif
These #ifdef/#endif statements will need to surround every locale
specific function. These will include the <locale.h> and <nl_types.h>
include files, the catfd static descriptor variable, the catinit()
routine, catopen(), catclose(), catgets(), and setlocale(). As can be
seen, this can get quite messy and can make the code very hard to read.
A solution to using all the #ifdef NLS/#endif statements involves using
a macro for the software.
The macro file would include all the #include and variable descriptors
for the locale specific version as well as defining routines to handle
printing messages in both a locale capable system and a non-capable
system. A sample macro package has been included below:
---
#ifdef NLS
#include <locale.h>
#include <nl_types.h>
extern nl_catd catfd;
void catinit ();
#endif
/* Define Macros used */
#ifdef NLS
#define NLS_CATCLOSE(catfd) catclose (catfd);
#define NLS_CATINIT catinit ();
#define NLS_CATGETS(catfd, arg1, arg2, fmt) \
catgets ((catfd), (arg1), (arg2), (fmt))
#else
#define NLS_CATCLOSE(catfd) /* empty */
#define NLS_CATINIT /* empty */
#define NLS_CATGETS(catfd, arg1, arg2, fmt) fmt
#endif
---
Now instead of having to do:
#ifdef NLS
printf(catgets(catfd, chmodSet, chmodVM_exhausted, "Virtual Memory
exhausted"));
#else
printf("Virtual Memory exhausted");
#endif
all the time, we could rewrite this as:
printf(NLS_CATGETS(catfd, chmodSet, chmodVM_exhausted, "Virtual Memory
exhausted"));
This will handle both cases very easily. Hence the changes now needed
to support a locale version and a non-locale version are:
- include a -DNLS define in the makefile if the system supports locale
functions
- #include the macro file into your source code
- surround your
#include "foobar-nls.h"
with #ifdef NLS/#endif statements.
Section 4. Where are the message catalogs stored?
The following is the situation as I have managed to ascertain from
various people. It should only be regarded as a very rough guide until
I have had time to check the X/Open Portability Guide 4 standards.
Message catalogs and other locale attributes are stored in a nest of
subdirectories. The nest has two possible base points:
/usr/lib/locale
/usr/local/lib/locale
The first is used by the software accompanying the base operating
system. The second is used by externally installed packages - packages
which are not considered part of the base OS.
Under these directories, we now have the following subdirectories:
LC_COLLATE
LC_CTYPE
LC_MESSAGES
LC_MONETARY
LC_NUMERIC
LC_TIME
Notes: These are not to be confused with the variables of the same name.
These are the actual subdirectory names and do not change (unlike their
variable counterparts). To avoid confusion, the variables will now be
referred to as $(LC_MESSAGES) etc.
Under these subdirectories are the various country subdirectories.
eg, under /usr/lib/locale/LC_MESSAGES we could have the following directories:
C
POSIX -> C
en_US.88591
de_DE.88591
fr_BE.88591
And under these directories, the language and code specific message
catalogs are stored. Hence, the message catalog for the "ls" binary on
an American English speaking system would be stored under:
/usr/lib/locale/LC_MESSAGES/en_US.88591/ls.cat
The general format is as follows:
/usr/lib/locale/LC_MESSAGES/xx_YY.ZZZ/mm.cat
^^^^^^^^^^^^^^^ ^^^^^^^^^^^ ^^^^^^^^^ ^^^^^^
root category lang catalog
The root does not change - its either /usr/lib/locale for system
software or /usr/local/lib/locale for externally installed software.
The category is only dependent upon the type of locale functions the
software is attempting to access. If the software was looking up
information on the monetary variables for the particular locale, then it
would be searching in:
/usr/lib/locale/LC_MONETARY/xx_YY.ZZZ/
for the information.
The lang component is possibly the most important and is the component
that determines which variables and directories the system searches in
to obtain the info it needs. The format of the lang component is as
follows:
language_country.characterset
The following examples will illustrate it:
en_US.88591 English language in the USA using the ISO 88591 character set
de_DE.88591 German language in Germany using the ISO 88591 character set
fr_BE.88591 French language in Belgium using the ISO 88591 character set
The lang component is set by the user through the $(LANG) environment
variable. The user will establish the correct language, country and
character set, and set his $(LANG) environment variable accordingly.
The OS will then use the $(LANG) environment variable when searching the
appropriate subdirectories to find the information or message catalogs
that it needs - as detailed by the setlocale() command.
We've outlined the two default places above that the system uses to
store message catalogs and other locale attributes. However, the system
must also be able to handle users who cannot install message catalogs in
either of these places (doing so usually requires superuser privileges)
and instead must install message catalogs within their own personal home
directories. The system can accommodate message catalogs store here (or
in any other non-standard place) by the use of the NLSPATH environment
variable.
The NLSPATH environment variable lists directories which the OS examines
to find the necessary message catalogs.
eg,
NLSPATH=/usr/lib/locale/LC_MESSAGES/%L/%N:/usr/local/lib/locale/LC_MESSAGES/%L
/%N:~/messages/%N
where %L represents the value of the LANG environment variable
and %N = the name of the catalog
These two values (%L and %N) are substituted by the OS at evaluation time.
The the user can store their own message catalogs within their home
directories and have the system automatically access them. They can
even override the default message catalogs stored on the system by
rearranging the order of the entries for the NLSPATH environment
variable.
Section 5. Frequently Asked Questions
Q. How do I know if the Unix platform I am using supports the locale routines?
A. A Unix platform that supports the full range of locale functions must
have two include files:
locale.h
and nl_types.h
These are usually found in /usr/include. If one or both of these files
are missing, then the OS may only support a subset of the locale
functions. Both are included with Linux.
The material covered in this document is variously copyrighted by
Alfalfa Software, Mitchum DSouza, and Patrick D'Cruze - 1989-1994.
Please send any suggestions, feedback, or notification of errors to the
author. I can be contacted at:
pdcruze@orac.iinet.com.au