Re: -Wvla vs dcnpgettext_expr's VLA decl

[Jim Meyering]

Bruno Haible wrote:

> Hi Jim,
>
>> I tried enabling -Wvla for coreutils and found that it provoked hundreds
>> of warnings, all due to this:
>>
>>     ../lib/gettext.h:262:3: error: variable length array 'msg_ctxt_id'\
>>     is used [-Werror=vla]
>>
>> That comes from this inline function from gettext.h:
>>
>>     static const char *
>>     dcnpgettext_expr (const char *domain,
>>                       const char *msgctxt, const char *msgid,
>>                       const char *msgid_plural, unsigned long int n,
>>                       int category)
>>     {
>>       size_t msgctxt_len = strlen (msgctxt) + 1;
>>       size_t msgid_len = strlen (msgid) + 1;
>>       const char *translation;
>>     #if _LIBGETTEXT_HAVE_VARIABLE_SIZE_ARRAYS
>>       char msg_ctxt_id[msgctxt_len + msgid_len];
>>     #else
>>     ...
>
> Yes, as you can see this code uses variable-length arrays only
> when the compiler supports it.
>
>> Do we have a guarantee that that array dimension is reasonable?
>
> Yes. While msgctxt and msgid normally rarely exceed 1 KB (because

That sounds like convention.  Is there a guarantee?

> of the principle to split at paragraph boundaries, so that translators
> can compare an old and a new msgid, for msgids with plural, the
> string is most often only a single sentence, that is, rarely larger
> than 200 bytes.

What I was wondering is whether there were some guard, say
in all callers, that would prevent VLA abuse.

>> Have you compared the trade-offs of VLA-vs-malloc here?
>
> Yes I did.
>
> The problem is that the compiler does not know that I did.
>
> You may want to propose a #pragma diagnostic ignore patch.

Such a #pragma affects a compilation unit, so putting it
in the .h file would affect every .c file that includes it.
Putting such a #pragma in every file from which gettext.h
is included is not really an option.

I can live with the status quo, but it would make -Wvla more useful
not to put such constructs in commonly-included header files.