On 10/21/19 10:38 AM, Andrii Nakryiko wrote:
> On Mon, Oct 21, 2019 at 10:18 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>>
>> Andrii Nakryiko <andriin@xxxxxx> writes:
>>
>>> LIBBPF_OPTS is implemented as a mix of field declaration and memset
>>> + assignment. This makes it neither variable declaration nor purely
>>> statements, which is a problem, because you can't mix it with either
>>> other variable declarations nor other function statements, because C90
>>> compiler mode emits warning on mixing all that together.
>>>
>>> This patch changes LIBBPF_OPTS into a strictly declaration of variable
>>> and solves this problem, as can be seen in case of bpftool, which
>>> previously would emit compiler warning, if done this way (LIBBPF_OPTS as
>>> part of function variables declaration block).
>>>
>>> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
>>> ---
>
> [...]
>
>>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>>> index 0fdf086beba7..bf105e9e866f 100644
>>> --- a/tools/lib/bpf/libbpf.h
>>> +++ b/tools/lib/bpf/libbpf.h
>>> @@ -77,12 +77,13 @@ struct bpf_object_open_attr {
>>> * bytes, but that's the best way I've found and it seems to work in practice.
>>> */
>>> #define LIBBPF_OPTS(TYPE, NAME, ...) \
>>> - struct TYPE NAME; \
>>> - memset(&NAME, 0, sizeof(struct TYPE)); \
>>> - NAME = (struct TYPE) { \
>>> - .sz = sizeof(struct TYPE), \
>>> - __VA_ARGS__ \
>>> - }
>>> + struct TYPE NAME = ({ \
>>> + memset(&NAME, 0, sizeof(struct TYPE)); \
>>> + (struct TYPE) { \
>>> + .sz = sizeof(struct TYPE), \
This ({ statements; ...; value; }) is used by bcc rewriter as well.
>>
>> Wait, you can stick arbitrary code inside a variable initialisation
>> block like this? How does that work? Is everything before the (struct
>> type) just ignored (and is that a cast)?
>
> Well, you definitely can still arbitrary code into a ({ }) expression
> block, that's not that surprising.
> The surprising bit that I discovered just recently was that stuff like
> this compiles and works correctly, try it:
>
> void *x = &x;
> printf("%lx == %lx\n", x, &x);
'void *x' just takes the address of the 'x' in the current scope.
It may looks like a use before define. but it actually works.
LGTM.
Acked-by: Yonghong Song <yhs@xxxxxx>
>
> So I'm using the fact that variable address is available inside
> variable initialization block.
>
> Beyond that, it's just a fancy, but standard (struct bla){ ...
> initializer list ...} syntax (it's not a struct initializer syntax,
> mind you, it's a struct assignment from struct literal). Fancy for
> sure, but it works and solves problems I mentioned in commit
> description.
>
>>
>> -Toke
>>
