On: Lesser known tricks, quirks and features of C

[dezgeg]

> you can't change the order of the formatted arguments.

You can with the $ syntax. Never seen it used though. Maybe it isn't very portable.

[skissane → dezgeg]

It is specified by POSIX, but not by ISO C (or C++). So most Unix(-like) systems support it. But the printf in Microsoft's C runtime doesn't. However, Microsoft does define an alternative printf function which does, printf_p, so `#define printf printf_p` will get past that.

I think the real reason you rarely see it, is it is only used with internationalisation–the idea being if you translate the format string, the translator may need to reorder the parameters for a natural translation, given differences in word order in different languages. However, a lot of software isn't internationalised, or if it is, the internationalisation is in end-user facing text, which nowadays usually ends up in a GUI or web UI, so printf has less to do with it. And the kind of lower-level tools/components for which people still often use C are less likely to be internationalised, since they are targeted at a technical audience who are expected to be able to read some level of English.

[tom_ → skissane]

printf_p is pretty neat, thanks for the pointer. But I would bet that you will still find at least one %d gets turned into a %s.

I like printf format strings, but as a way of handling localizable strings I don't think they are the best.

[skissane → tom_]

You are 100% correct. printf, even with the argument numbering feature, is insufficient for high quality internationalisation.

A good example of this is pluralisation. We've all done things like:

    printf("%d file(s) copied\n", count);

which is acceptable but kind of ugly. Some people want to make it nicer:

    printf("%d file%s copied\n", count, count != 1 ? "s" : "");

Which is fine for English, but doesn't work at all for other languages. The problem is not just that the plural ending is something other than `s` – if it was just that, it wouldn't be too hard. The problem is that the `count != 1` bit only works for English. For example, while 0 is plural in English, in French it is singular. Many other languages are much more complex. The GNU gettext manual has a chapter which goes into this in great detail – https://www.gnu.org/software/gettext/manual/html_node/Plural...

printf() has zero hope of coping with this complexity. gettext provides a special function to handle this, ngettext(), which is passed the number as a separate argument, so it can select which plural form to use. And then the translated message files contain a header defining how many plural forms that language has, and the rules to choose which one to use. And for some languages it is crazy complex. Arabic is the most extreme, for which the manual gives this plural rule:

    Plural-Forms: nplurals=6; \
        plural=n==0 ? 0 : n==1 ? 1 : n==2 ? 2 : n%100>=3 && n%100<=10 ? 3 \
        : n%100>=11 ? 4 : 5;