https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644
Bug ID: 93644
Summary: -Wreturn-local-addr July regression: new
false-positive warning
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: jim at meyering dot net
Target Milestone: ---
Created attachment 47808
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47808&action=edit
derived from gnulib's lib/careadlinkat.c
Using -Wreturn-local-addr can now evoke this false positive:
$ gcc -c -O2 -Wreturn-local-addr k.c
k.c: In function 'careadlinkat':
cc1: warning: function may return address of local variable
[-Wreturn-local-addr]
k.c:24:8: note: declared here
24 | char stack_buf[1024];
| ^~~~~~~~~
I was able to easily narrow down that this behavior changed on gcc/git master
built from:
2019-07-19 (no warning) to
2019-07-23 (warning emitted).
--
I was surprised not to be able to convince the compiler that the code is ok OR
even to suppress the warning.
Per discussion in the thread at
https://lists.gnu.org/r/coreutils/2020-02/msg00006.html, I first tried this:
>> Would an `assure (buf != stack_buf)` before the `return buf`
>> indicate that constraint to gcc with minimal runtime overhead?
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, even with that pragma, the warning was still emitted.
I ended up having to disable this normally-high-signal warning for all of
coreutils/lib.
The attached file is derived from gnulib's lib/careadlinkat.c.
