* No warning about duplicate values in enum
@ 2023-03-11 12:54 Basile Starynkevitch
2023-03-11 16:48 ` Jonathan Wakely
0 siblings, 1 reply; 6+ messages in thread
From: Basile Starynkevitch @ 2023-03-11 12:54 UTC (permalink / raw)
To: Andrea Monaco; +Cc: gcc
Hello all,
Andrea observed that:
In gcc 8.3.0, compiling
enum
test
{
FIRST = 1,
SECOND = 1,
THIRD = 2
};
int
main (void)
{
return 0;
}
generates no warning even with -Wextra.
I believe that the C standard (which I don't have here, but see also https://port70.net/~nsz/c/c11/n1570.html or buy it from ISO) explicitly allow duplicate values in enum.
For readability of some weird code, or in the case of C code generated by other tools.
By the way, gcc version 12.2.0 (Debian 12.2.0-14) don't emit any warnings neither when used on your example as gcc -Wall -Wextra -fanalyzer /tmp/andrea.c -o /tmp/andrea
If you really want to get warnings, consider writing your GCC plugin (perhaps starting with https://github.com/bstarynk/bismon/ ....) or using static analysis tools like https://frama-c.com/
Regards.
NB my pet open source project is the RefPerSys open source inference engine (work in progress) on http://refpersys.org/
--
Basile Starynkevitch <basile@starynkevitch.net>
92340 Bourg-la-Reine, France
http://starynkevitch.net/Basile/ and http://refpersys.org/
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: No warning about duplicate values in enum
2023-03-11 12:54 No warning about duplicate values in enum Basile Starynkevitch
@ 2023-03-11 16:48 ` Jonathan Wakely
2023-03-13 13:29 ` Marek Polacek
0 siblings, 1 reply; 6+ messages in thread
From: Jonathan Wakely @ 2023-03-11 16:48 UTC (permalink / raw)
To: Basile Starynkevitch; +Cc: Andrea Monaco, gcc
[-- Attachment #1: Type: text/plain, Size: 686 bytes --]
On Sat, 11 Mar 2023, 12:53 Basile Starynkevitch, <basile@starynkevitch.net>
wrote:
> Hello all,
>
>
> Andrea observed that:
>
> In gcc 8.3.0, compiling
>
>
> enum
> test
> {
> FIRST = 1,
> SECOND = 1,
> THIRD = 2
> };
>
> int
> main (void)
> {
> return 0;
> }
>
>
> generates no warning even with -Wextra.
>
> I believe that the C standard (which I don't have here, but see also
> https://port70.net/~nsz/c/c11/n1570.html or buy it from ISO) explicitly
> allow duplicate values in enum.
>
Of course it does, it's perfectly valid. Nobody has said it should be
rejected. The request is for a warning, because for *some* uses of enums
duplicates are not wanted.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: No warning about duplicate values in enum
2023-03-11 16:48 ` Jonathan Wakely
@ 2023-03-13 13:29 ` Marek Polacek
2023-03-13 13:38 ` Basile Starynkevitch
0 siblings, 1 reply; 6+ messages in thread
From: Marek Polacek @ 2023-03-13 13:29 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: Basile Starynkevitch, Andrea Monaco, gcc
On Sat, Mar 11, 2023 at 04:48:14PM +0000, Jonathan Wakely via Gcc wrote:
> On Sat, 11 Mar 2023, 12:53 Basile Starynkevitch, <basile@starynkevitch.net>
> wrote:
>
> > Hello all,
> >
> >
> > Andrea observed that:
> >
> > In gcc 8.3.0, compiling
> >
> >
> > enum
> > test
> > {
> > FIRST = 1,
> > SECOND = 1,
> > THIRD = 2
> > };
> >
> > int
> > main (void)
> > {
> > return 0;
> > }
> >
> >
> > generates no warning even with -Wextra.
> >
> > I believe that the C standard (which I don't have here, but see also
> > https://port70.net/~nsz/c/c11/n1570.html or buy it from ISO) explicitly
> > allow duplicate values in enum.
> >
>
>
> Of course it does, it's perfectly valid. Nobody has said it should be
> rejected. The request is for a warning, because for *some* uses of enums
> duplicates are not wanted.
And as I said in the other thread about the very same issue, it's
<https://gcc.gnu.org/PR16186> which is assigned to me and I hope to
implement it for GC 14.
Marek
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: No warning about duplicate values in enum
2023-03-13 13:29 ` Marek Polacek
@ 2023-03-13 13:38 ` Basile Starynkevitch
0 siblings, 0 replies; 6+ messages in thread
From: Basile Starynkevitch @ 2023-03-13 13:38 UTC (permalink / raw)
To: Marek Polacek; +Cc: gcc
On 3/13/23 14:29, Marek Polacek wrote:
> On Sat, Mar 11, 2023 at 04:48:14PM +0000, Jonathan Wakely via Gcc wrote:
>> On Sat, 11 Mar 2023, 12:53 Basile Starynkevitch, <basile@starynkevitch.net>
>> wrote:
>>
>>> Hello all,
>>>
>>>
>>> Andrea observed that:
>>>
>>> In gcc 8.3.0, compiling
>>>
>>>
>>> enum
>>> test
>>> {
>>> FIRST = 1,
>>> SECOND = 1,
>>> THIRD = 2
>>> };
>>>
>>> int
>>> main (void)
>>> {
>>> return 0;
>>> }
>>>
>>>
>>> generates no warning even with -Wextra.
>>>
>>> I believe that the C standard (which I don't have here, but see also
>>> https://port70.net/~nsz/c/c11/n1570.html or buy it from ISO) explicitly
>>> allow duplicate values in enum.
>>>
>>
>> Of course it does, it's perfectly valid. Nobody has said it should be
>> rejected. The request is for a warning, because for *some* uses of enums
>> duplicates are not wanted.
> And as I said in the other thread about the very same issue, it's
> <https://gcc.gnu.org/PR16186> which is assigned to me and I hope to
> implement it for GCC 14.
>
> Marek
Then consider perhaps:
prototyping that warning in your own GCC plugin (and test it on a bunch
of existing open source software).
adding some explicit new warning option to enable that new warning or
adding another option to disable that warning in a compilation unit.
adding some explicit #pragma to disable that new warning (with the hope
that tools generating C++ code would later -in 2024- emit the pragma
appropriately)
Since there are some valid, probably weird, cases (notably generated C++
code, or low level driver code where two different enum names need to
share some common values, ....) where having duplicate values in enum
may make sense.
Regards
--
Basile Starynkevitch <basile@starynkevitch.net>
(only mine opinions / les opinions sont miennes uniquement)
92340 Bourg-la-Reine, France
web page: starynkevitch.net/Basile/ & refpersys.org
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: No warning about duplicate values in enum
2023-03-10 12:57 Andrea Monaco
@ 2023-03-10 13:05 ` Marek Polacek
0 siblings, 0 replies; 6+ messages in thread
From: Marek Polacek @ 2023-03-10 13:05 UTC (permalink / raw)
To: Andrea Monaco; +Cc: gcc
On Fri, Mar 10, 2023 at 01:57:06PM +0100, Andrea Monaco via Gcc wrote:
>
> In gcc 8.3.0, compiling
>
>
> enum
> test
> {
> FIRST = 1,
> SECOND = 1,
> THIRD = 2
> };
>
> int
> main (void)
> {
> return 0;
> }
>
>
> generates no warning even with -Wextra. That hit me today, because I
> had a large enum with many explicitly assigned constants and I
> accidentally used the same value twice, which is an obvious source of
> problems.
This is https://gcc.gnu.org/PR16186.
Marek
^ permalink raw reply [flat|nested] 6+ messages in thread
* No warning about duplicate values in enum
@ 2023-03-10 12:57 Andrea Monaco
2023-03-10 13:05 ` Marek Polacek
0 siblings, 1 reply; 6+ messages in thread
From: Andrea Monaco @ 2023-03-10 12:57 UTC (permalink / raw)
To: gcc
In gcc 8.3.0, compiling
enum
test
{
FIRST = 1,
SECOND = 1,
THIRD = 2
};
int
main (void)
{
return 0;
}
generates no warning even with -Wextra. That hit me today, because I
had a large enum with many explicitly assigned constants and I
accidentally used the same value twice, which is an obvious source of
problems.
Andrea Monaco
^ permalink raw reply [flat|nested] 6+ messages in thread
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2023-03-11 12:54 No warning about duplicate values in enum Basile Starynkevitch
2023-03-11 16:48 ` Jonathan Wakely
2023-03-13 13:29 ` Marek Polacek
2023-03-13 13:38 ` Basile Starynkevitch
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2023-03-10 12:57 Andrea Monaco
2023-03-10 13:05 ` Marek Polacek
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