[cfe-dev] Clang ignores nonnull attributes on declarations following definitions when compiling as C++

[Richard Smith]

On Mon, Jul 13, 2015 at 8:15 AM, Tom Honermann <thonermann at coverity.com>
wrote:

> Clang (3.4-trunk) ignores nonnull attributes on declarations following
> definitions when compiling as C++, but not as C.  I presume the difference
> has to do with tentative definitions in C.
>
> A test case is below.  Recent versions of gcc and EDG both allow nonnull
> attributes on declarations following a definition, at least for this test
> case.  Is it desirable for Clang to act differently in this respect?
>

We don't support adding attributes after a definition. (In the case where
the attributes affect the generated code for the definition, that way lies
madness, and we don't distinguish between those attributes and others for
consistency -- note that nonnull *does* affect code generation in some
cases.) The difference between C and C++ here is most likely exactly what
you suspect (the first declaration is only a tentative definition in C).


> Tom.
>
> $ cat t.c
> void (*pf)(int *p1);
> extern void (*pf)(int *p1)
>     __attribute__((nonnull));
> void f() {
>     pf(0);
> }
>
>
> # Compiling as C, no warning is emitted, the nonnull attribute
> # is present in the resulting AST:
> $ clang -c -Xclang -ast-dump t.c
> t.c:5:9: warning: null passed to a callee that requires a non-null
> argument [-Wnonnull]
>     pf(0);
>        ~^
> ...
> `-VarDecl 0x822dda0 prev 0x822dc50 <line:2:1, col:26> col:15 pf 'void
> (*)(int *)' extern
>   `-NonNullAttr 0x822de00 <line:3:20>
> ...
>
>
> # Compiling as C++, a warning is emitted and the nonnull attribute
> # is not present in the resulting AST:
> $ clang -c -Xclang -ast-dump -x c++ t.c
> t.c:3:20: warning: attribute declaration must precede definition
> [-Wignored-attributes]
>     __attribute__((nonnull));
>                    ^
> t.c:1:8: note: previous definition is here
> void (*pf)(int *p1);
>        ^
> ...
> `-VarDecl 0x82e41e0 prev 0x82e4090 <line:2:1, col:26> col:15 pf 'void
> (*)(int *)' extern
> ...
> 1 warning generated.
>
>
> # gcc (4.6.3) emits a warning when invoking *pf with a null argument
> regardless of whether the code is compiled as C or C++:
> $ gcc -Wall -c t.c
> t.c: In function 'f':
> t.c:5:5: warning: null argument where non-null required (argument 1)
> [-Wnonnull]
> $ gcc -Wall -c -x c++ t.c
> t.c: In function 'void f()':
> t.c:5:9: warning: null argument where non-null required (argument 1)
> [-Wnonnull]
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