In C programs strings are often used within calls of functions from the
printf family. The special thing about these format strings is
that they can contain format specifiers introduced with %. Assume
we have the code
printf (gettext ("String `%s' has %d characters\n"), s, strlen (s));
A possible German translation for the above string might be:
"%d Zeichen lang ist die Zeichenkette `%s'"
A C programmer, even if he cannot speak German, will recognize that
there is something wrong here. The order of the two format specifiers
is changed but of course the arguments in the
printf don’t have.
This will most probably lead to problems because now the length of the
string is regarded as the address.
To prevent errors at runtime caused by translations, the
tool can check statically whether the arguments in the original and the
translation string match in type and number. If this is not the case
and the ‘-c’ option has been passed to
will give an error and refuse to produce a MO file. Thus consistent
use of ‘msgfmt -c’ will catch the error, so that it cannot cause
problems at runtime.
If the word order in the above German translation would be correct one would have to write
"%2$d Zeichen lang ist die Zeichenkette `%1$s'"
The routines in
msgfmt know about this special notation.
Because not all strings in a program will be format strings, it is not
msgfmt to test all the strings in the .po file.
This might cause problems because the string might contain what looks
like a format specifier, but the string is not used in
xgettext adds a special tag to those messages it
thinks might be a format string. There is no absolute rule for this,
only a heuristic. In the .po file the entry is marked using the
c-format flag in the
#, comment line (see The Format of PO Files).
The careful reader now might say that this again can cause problems.
The heuristic might guess it wrong. This is true and therefore
xgettext knows about a special kind of comment which lets
the programmer take over the decision. If in the same line as or
the immediately preceding line to the
xgettext program finds a comment containing the words
xgettext:c-format, it will mark the string in any case with
c-format flag. This kind of comment should be used when
xgettext does not recognize the string as a format string but
it really is one and it should be tested. Please note that when the
comment is in the same line as the
gettext keyword, it must be
before the string to be translated. Also note that a comment such as
xgettext:c-format applies only to the first string in the same
or the next line, not to multiple strings.
This situation happens quite often. The
printf function is often
called with strings which do not contain a format specifier. Of course
one would normally use
fputs but it does happen. In this case
xgettext does not recognize this as a format string but what
happens if the translation introduces a valid format specifier? The
printf function will try to access one of the parameters but none
exists because the original code does not pass any parameters.
xgettext of course could make a wrong decision the other way
round, i.e. a string marked as a format string actually is not a format
string. In this case the
msgfmt might give too many warnings and
would prevent translating the .po file. The method to prevent
this wrong decision is similar to the one used above, only the comment
to use must contain the string
If a string is marked with
c-format and this is not correct the
user can find out who is responsible for the decision. See
xgettext Program to see how the
--debug option can be
used for solving this problem.