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From: "jg at jguk dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108646] nonnull attribute does not detect variables that
 are NULL being passed
Date: Fri, 03 Feb 2023 12:08:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108646

--- Comment #3 from Jonny Grant <jg at jguk dot org> ---
(In reply to Jonathan Wakely from comment #2)
> It already does detect it:
>=20
> n.c: In function =E2=80=98test=E2=80=99:
> n.c:6:2: warning: argument 1 null where non-null expected [-Wnonnull]
>     6 |  mem2(dest);
>       |  ^~~~~~~~~~
> n.c:2:8: note: in a call to function =E2=80=98mem2=E2=80=99 declared =E2=
=80=98nonnull=E2=80=99
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
>=20
> You need to enable optimization, otherwise there is no data flow analysis.
>=20
> Without optimization, it detects it with -fanalyzer
>=20
> n.c: In function =E2=80=98void test()=E2=80=99:
> n.c:6:6: warning: use of NULL =E2=80=98dest=E2=80=99 where non-null expec=
ted [CWE-476]
> [-Wanalyzer-null-argument]
>     6 |  mem2(dest);
>       |  ~~~~^~~~~~
>   =E2=80=98void test()=E2=80=99: events 1-2
>     |
>     |    5 |  char *dest =3D NULL;
>     |      |        ^~~~
>     |      |        |
>     |      |        (1) =E2=80=98dest=E2=80=99 is NULL
>     |    6 |  mem2(dest);
>     |      |  ~~~~~~~~~~
>     |      |      |
>     |      |      (2) argument 1 (=E2=80=98dest=E2=80=99) NULL where non-=
null expected
>     |
> n.c:2:8: note: argument 1 of =E2=80=98void* mem2(void*)=E2=80=99 must be =
non-null
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
>=20
>=20
> And it's also detected at runtime using -fsanitize=3Dundefined:
>=20
> n.c:6:6: runtime error: null pointer passed as argument 1, which is decla=
red
> to never be null


That's great it works with optimization, I should remember to always do
that.(In reply to Jonathan Wakely from comment #2)
> It already does detect it:
>=20
> n.c: In function =E2=80=98test=E2=80=99:
> n.c:6:2: warning: argument 1 null where non-null expected [-Wnonnull]
>     6 |  mem2(dest);
>       |  ^~~~~~~~~~
> n.c:2:8: note: in a call to function =E2=80=98mem2=E2=80=99 declared =E2=
=80=98nonnull=E2=80=99
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
>=20
> You need to enable optimization, otherwise there is no data flow analysis.
>=20
> Without optimization, it detects it with -fanalyzer
>=20
> n.c: In function =E2=80=98void test()=E2=80=99:
> n.c:6:6: warning: use of NULL =E2=80=98dest=E2=80=99 where non-null expec=
ted [CWE-476]
> [-Wanalyzer-null-argument]
>     6 |  mem2(dest);
>       |  ~~~~^~~~~~
>   =E2=80=98void test()=E2=80=99: events 1-2
>     |
>     |    5 |  char *dest =3D NULL;
>     |      |        ^~~~
>     |      |        |
>     |      |        (1) =E2=80=98dest=E2=80=99 is NULL
>     |    6 |  mem2(dest);
>     |      |  ~~~~~~~~~~
>     |      |      |
>     |      |      (2) argument 1 (=E2=80=98dest=E2=80=99) NULL where non-=
null expected
>     |
> n.c:2:8: note: argument 1 of =E2=80=98void* mem2(void*)=E2=80=99 must be =
non-null
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
>=20
>=20
> And it's also detected at runtime using -fsanitize=3Dundefined:
>=20
> n.c:6:6: runtime error: null pointer passed as argument 1, which is decla=
red
> to never be null


That's great. I should have remembered to add -O2

Is it worth -Wnonnull emitting a warning message that it needs optimization=
 to
get the needed data flow analysis?=