25.2. Simple project
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## 25.2. Simple project

Let's start with a very simple project, the "Hello world" program for example

    /* hello.c */
    #include <stdio.h>
    int main(int argc, char* argv[])
    {
      printf("Hello world\n");
      return 0;
    }

Prepare a `SConstruct` to compile the program as usual.

    # SConstruct
    env = Environment()
    hello = Program(["hello.c"])

Now we'll convert the project to a multilingual one. If you don't already have GNU gettext utilities installed, install them from your preferred package repository, or download from http://ftp.gnu.org/gnu/gettext/. For the purpose of this example, you should have following three locales installed on your system: `en_US`, `de_DE` and `pl_PL`. On Debian, for example, you may enable certain locales through dpkg-reconfigure locales.

First prepare the `hello.c` program for internationalization. Change the previous code so it reads as follows:

    /* hello.c */
    #include <stdio.h>
    #include <libintl.h>
    #include <locale.h>
    int main(int argc, char* argv[])
    {
      bindtextdomain("hello", "locale");
      setlocale(LC_ALL, "");
      textdomain("hello");
      printf(gettext("Hello world\n"));
      return 0;
    }

Detailed recipes for such conversion can be found at http://www.gnu.org/software/gettext/manual/gettext.html#Sources. The `gettext("...")` has two purposes. First, it marks messages for the xgettext(1) program, which we will use to extract from the sources the messages for localization. Second, it calls the `gettext` library internals to translate the message at runtime.

Now we shall instruct SCons how to generate and maintain translation files. For that, use the `Translate` builder and `MOFiles` builder. The first one takes source files, extracts internationalized messages from them, creates so-called `POT` file (translation template), and then creates `PO` translation files, one for each requested language. Later, during the development lifecycle, the builder keeps all these files up-to date. The `MOFiles` builder compiles the `PO` files to binary form. Then install the `MO` files under directory called `locale`.

The completed `SConstruct` is as follows:

    # SConstruct
    env = Environment( tools = ['default', 'gettext'] )
    hello = env.Program(["hello.c"])
    env['XGETTEXTFLAGS'] = [
      '--package-name=%s' % 'hello',
      '--package-version=%s' % '1.0',
    ]
    po = env.Translate(["pl","en", "de"], ["hello.c"], POAUTOINIT = 1)
    mo = env.MOFiles(po)
    InstallAs(["locale/en/LC_MESSAGES/hello.mo"], ["en.mo"])
    InstallAs(["locale/pl/LC_MESSAGES/hello.mo"], ["pl.mo"])
    InstallAs(["locale/de/LC_MESSAGES/hello.mo"], ["de.mo"])

Generate the translation files with scons po-update. You should see the output from SCons similar to this:

    user@host:$ scons po-update
    scons: Reading SConscript files ...
    scons: done reading SConscript files.
    scons: Building targets ...
    Entering '/home/ptomulik/projects/tmp'
    xgettext --package-name=hello --package-version=1.0 -o - hello.c
    Leaving '/home/ptomulik/projects/tmp'
    Writting 'messages.pot' (new file)
    msginit --no-translator -l pl -i messages.pot -o pl.po
    Created pl.po.
    msginit --no-translator -l en -i messages.pot -o en.po
    Created en.po.
    msginit --no-translator -l de -i messages.pot -o de.po
    Created de.po.
    scons: done building targets.

If everything is right, you should see following new files.

    user@host:$ ls *.po*
    de.po  en.po  messages.pot  pl.po

Open `en.po` in poedit and provide the English translation to message `"Hello world\n"`. Do the same for `de.po` (deutsch) and `pl.po` (polish). Let the translations be, for example:

- `en: "Welcome to beautiful world!\n"`

- `de: "Hallo Welt!\n"`

- `pl: "Witaj swiecie!\n"`

Now compile the project by executing scons. The output should be similar to this:

    user@host:$ scons
    scons: Reading SConscript files ...
    scons: done reading SConscript files.
    scons: Building targets ...
    msgfmt -c -o de.mo de.po
    msgfmt -c -o en.mo en.po
    gcc -o hello.o -c hello.c
    gcc -o hello hello.o
    Install file: "de.mo" as "locale/de/LC_MESSAGES/hello.mo"
    Install file: "en.mo" as "locale/en/LC_MESSAGES/hello.mo"
    msgfmt -c -o pl.mo pl.po
    Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
    scons: done building targets.

SCons automatically compiled the `PO` files to binary format `MO`, and the `InstallAs` lines installed these files under `locale` folder.

Your program should be now ready. You may try it as follows (Linux):

    user@host:$ LANG=en_US.UTF-8 ./hello
    Welcome to beautiful world

    user@host:$ LANG=de_DE.UTF-8 ./hello
    Hallo Welt

    user@host:$ LANG=pl_PL.UTF-8 ./hello
    Witaj swiecie

To demonstrate the further life of translation files, let's change Polish translation (poedit pl.po) to `"Witaj drogi swiecie\n"`. Run scons to see how scons reacts to this

    user@host:$scons
    scons: Reading SConscript files ...
    scons: done reading SConscript files.
    scons: Building targets ...
    msgfmt -c -o pl.mo pl.po
    Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
    scons: done building targets.

Now, open `hello.c` and add another one `printf` line with new message.

    /* hello.c */
    #include <stdio.h>
    #include <libintl.h>
    #include <locale.h>
    int main(int argc, char* argv[])
    {
      bindtextdomain("hello", "locale");
      setlocale(LC_ALL, "");
      textdomain("hello");
      printf(gettext("Hello world\n"));
      printf(gettext("and good bye\n"));
      return 0;
    }

Compile project with scons. This time, the msgmerge(1) program is used by SCons to update `PO` file. The output from compilation is like:

    user@host:$scons
    scons: Reading SConscript files ...
    scons: done reading SConscript files.
    scons: Building targets ...
    Entering '/home/ptomulik/projects/tmp'
    xgettext --package-name=hello --package-version=1.0 -o - hello.c
    Leaving '/home/ptomulik/projects/tmp'
    Writting 'messages.pot' (messages in file were outdated)
    msgmerge --update de.po messages.pot
    ... done.
    msgfmt -c -o de.mo de.po
    msgmerge --update en.po messages.pot
    ... done.
    msgfmt -c -o en.mo en.po
    gcc -o hello.o -c hello.c
    gcc -o hello hello.o
    Install file: "de.mo" as "locale/de/LC_MESSAGES/hello.mo"
    Install file: "en.mo" as "locale/en/LC_MESSAGES/hello.mo"
    msgmerge --update pl.po messages.pot
    ... done.
    msgfmt -c -o pl.mo pl.po
    Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
    scons: done building targets.

The next example demonstrates what happens if we change the source code in such way that the internationalized messages do not change. The answer is that none of translation files (`POT`, `PO`) are touched (i.e. no content changes, no creation/modification time changed and so on). Let's append another line to the program (after the last printf), so its code becomes:

    /* hello.c */
    #include <stdio.h>
    #include <libintl.h>
    #include <locale.h>
    int main(int argc, char* argv[])
    {
      bindtextdomain("hello", "locale");
      setlocale(LC_ALL, "");
      textdomain("hello");
      printf(gettext("Hello world\n"));
      printf(gettext("and good bye\n"));
      printf("----------------\n");
      return a;
    }

Compile the project. You'll see on your screen

    user@host:$scons
    scons: Reading SConscript files ...
    scons: done reading SConscript files.
    scons: Building targets ...
    Entering '/home/ptomulik/projects/tmp'
    xgettext --package-name=hello --package-version=1.0 -o - hello.c
    Leaving '/home/ptomulik/projects/tmp'
    Not writting 'messages.pot' (messages in file found to be up-to-date)
    gcc -o hello.o -c hello.c
    gcc -o hello hello.o
    scons: done building targets.

As you see, the internationalized messages didn't change, so the `POT` and the rest of translation files have not even been touched.

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