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), \
>
> 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);
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
>
