- Add some hints to reduce strings to localize when using NLS

Reviewed by:	rene

en_US.ISO8859-1/books/developers-handbook/l10n/chapter.sgml

@@ -221,6 +221,60 @@ printf(getstr(1));
 #ifndef WITHOUT_NLS
 	catclose(catalog);
 #endif</programlisting>
+
+	<sect3>
+	  <title>Reducing Strings to Localize</title>
+
+	  <para>There is a good way of reducing the strings that
+	    need to be localized by using <application>libc</application>
+	    error messages.  This is also useful to just avoid duplication
+	    and provide consistent error messages for the common errors
+	    that can be encountered by a great many of programs.</para>
+
+	  <para>First, here is an example that does not use
+	    <application>libc</application> error messages:</para>
+
+	  <programlisting>
+#include &lt;err.h&gt;
+...
+if (!S_ISDIR(st.st_mode))
+	err(1, "argument is not a directory");
+	  </programlisting>
+
+	  <para>This can be transformed to print an error message by
+	    reading <varname>errno</varname> and printing an error message
+	    accordingly:</para>
+
+	  <programlisting>
+#include &lt;err.h&gt;
+#include &lt;errno.h&gt;
+...
+if (!S_ISDIR(st.st_mode)) {
+	errno = ENOTDIR;
+	err(1, NULL);
+}
+	  </programlisting>
+
+	  <para>In this example, the custom string is eliminated, thus
+	    translators will have less work when localizing the program
+	    and users will see the usual <quote>Not a directory</quote>
+	    error message when they encounter this error.  This message
+	    will probably seem more familiar to them.  Please note that
+	    it was necessary to include <filename
+	      class="headerfile">errno.h</filename> in order to directly
+	    access <varname>errno</varname>.</para>
+
+	  <para>It is worth to note that there are cases when
+	    <varname>errno</varname> is set automatically by a preceding
+	    call, so it is not necessary to set it explicitly:</para>
+
+	  <programlisting>
+#include &lt;err.h&gt;
+...
+if ((p = malloc(size)) == NULL)
+	err(1, NULL);
+	  </programlisting>
+	</sect3>
       </sect2>
 
       <sect2 id="nls-mk">