(gettext.info)c-format Flag

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4.6 Special Comments preceding Keywords
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   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 ‘msgfmt’
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 ‘msgfmt’, ‘msgfmt’ 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 useful for ‘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 ‘printf’.

   Therefore ‘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 (Note: 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 ‘gettext’ keyword the ‘xgettext’
program finds a comment containing the words ‘xgettext:c-format’, it
will mark the string in any case with the ‘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.

   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 ‘xgettext:no-c-format’.

   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 Note:
xgettext Invocation to see how the ‘--debug’ option can be used for
solving this problem.