[PATCH v3] remove attribute access from regexec

[Paul Eggert]

On 8/19/21 4:50 PM, Martin Sebor wrote:

> Is <regex.h> for some reason not considered a system header in your
> test environment?

Yes, because Coreutils etc. currently use Gnulib regex.h instead of 
/usr/include/regex.h, as Glibc regex has a bug and the fix (in Gnulib) 
hasn't made its way back to Glibc yet.

This sort of thing is all too common, as regex development (such as it 
is) often occurs in Gnulib first, which means it's common for 
'configure' to discover that the current Glibc has a bug and to 
substitute Gnulib regex instead. And when we compile Gnulib headers we 
don't consider them to be system headers, because we want the extra 
static checking that gcc does for non system headers.

I think the simplest workaround for the warning is to disable -Wvla for 
that particular declaration. Please see attached Gnulib patch for how I 
suggest doing this.

> I didn't know mixing and matching two implementations like this
> was even possible.  Thanks for explaining it (though it seems
> like a pretty cumbersome arrangement).

Yes, it is a bit awkward. I am tempted to clean it up but that really 
should be a separate patch and so I will focus only on this thread's issue.

> +#ifndef __ARG_NELTS
> +# ifdef __ARG_NELTS

A clear typo. The suggestion was to define and use _ARG_NELTS_ instead.

Some other thoughts.

If we're going to change regexec, we should change __compat_regex to be 
consistent.

We should also change regexec.c's internal functions to be consistent 
with regexec. This includes re_search_internal, update_regs, etc.

These internal functions don't have regexec's quirk that pmatch is 
ignored when not substituting, so their parameters can use the syntax 
"regmatch_t pmatch[__ARG_NELTS (static nmatch)]" to let the compiler 
know that the array must be of the given size.

Proposed Gnulib patch attached. It should be easy to aplly to glibc 
though glibc also needs its own regex.h patched. I have not installed 
this into Gnulib yet.

At some point we should sync Gnulib and Glibc regex of course.
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