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Re: gcc10's -Wreturn-local-addr gives FP warning about lib/careadlinkat

From: Jim Meyering
Subject: Re: gcc10's -Wreturn-local-addr gives FP warning about lib/careadlinkat
Date: Thu, 6 Feb 2020 09:05:13 -0800

On Thu, Feb 6, 2020 at 6:03 AM Pádraig Brady <address@hidden> wrote:
> On 06/02/2020 00:27, Jim Meyering wrote:
> > Building latest latest coreutils using latest-from-git gcc10 evokes
> > this false positive:
> >
> > lib/careadlinkat.c: In function 'careadlinkat':
> > cc1: error: function may return address of local variable
> > [-Werror=return-local-addr]
> > lib/careadlinkat.c:73:8: note: declared here
> >     73 |   char stack_buf[1024];
> >
> > I'm guessing improved flow analysis will eventually suppress this. I
> > hesitate to turn off the useful and normally-high-S/N
> > -Wreturn-local-addr globally. Maybe just disable it in that one file,
> > temporarily?
> The logic of the function looks fine.
> Would an `assure (buf != stack_buf)` before the `return buf`
> indicate that constraint to gcc with minimal runtime overhead?

I would have preferred that, but it has no effect.
I then tried to suppress that warning in the affected file by adding
these lines:

/* Without this pragma, gcc 10.0.1 20200205 reports that
   the "function may return address of local variable".  */
# pragma GCC diagnostic ignored "-Wreturn-local-addr"

But, surprisingly, that didn't help, either.
Also tried Kaz Kylheku's return-early suggestion, to no avail.

Hence, my last resort suggestion: disable that warning option for all
gnulib compiles. Note that simply omitting the -W... option was
insufficient (probably pulled in by some high-level one), so I
resorted to adding -Wno-return-local-addr:

Attachment: gcc10-FP-vs-careadlinkat.diff
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