Re: new module suggestion: fprintftime-check

[Assaf Gordon]

Hello Bruno,

On 2018-12-28 9:34 a.m., Bruno Haible wrote:
> > This function enables syntax-check of the format string.
>
> First question: Should this syntax-check be integrated into the
> nstrftime() and fprintftime() functions? These functions are gnulib
> inventions, therefore they could be extended to return
>    - an error indicator (maybe EINVAL?),
>    - a detailed error message, if you like,
>    - a pointer to the wrong directive in the format string, if you like.
> Or is it better to have a function that does only the syntax check,
> and document that nstrftime() and fprintftime() should only be called
> after the syntax check has been done?
> (I have no preference. Just asking.)

Both nstrtime and fprintftime silently ignore bad formats:
https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/nstrftime.c#n1478

The comment even says:
      /* Unknown format; output the format, including the '%',
         since this is most likely the right thing to do if a
         multibyte string has been misparsed.  */

This has been the case since 1996 when strftime.c was imported from libc
(gnulib commit afabd949).

I suspect that changing this behavior would be a disruptive
backwards-incompatible change (but other opinions are welcomed).


> > I'd like to suggest the following new module: fprintftime-check.
>
> For a module that does only the syntax check, I would suggest a
> different name:
>    - The prefix 'fprint' indicates output to a 'FILE *'.
>    - The prefix 'str' indicates output to a 'char *' buffer.
> Why not call it 'ftime-check' or 'check-ftime'?

Good idea. I'll change it.


> tzalloc() may fail and return NULL. (And why would parsing the format
> string be dependent on a particular time zone at all?)

nstrtime's __strftime_internal() is a complicated beast, I don't
claim to fully understand it.
But the function contains this comment:
   /* POSIX.1 requires that local time zone information be used as
     though strftime called tzset.  */
https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/nstrftime.c#n540

And there is even an output "bool * tzset_called" parameter, to
let the caller know if it has been called or not - so I assume it is
critical to correct behavior of both nstrtime and fprintftime.
I will try to see if it can be disabled for the syntax-check code.

-assaf