Peter Maydell <peter.maydell@linaro.org> writes:
> On Mon, 29 Nov 2021 at 20:05, Peter Maydell <peter.maydell@linaro.org> wrote:
>>
>> qemu-common.h has a comment at the top:
>>
>> * This file is supposed to be included only by .c files. No header file should
>> * depend on qemu-common.h, as this would easily lead to circular header
>> * dependencies.
>
> As a side note, that comment was added back in 2012 when qemu-common.h
> was bigger, included other headers, and did some of the work we currently
> use osdep.h for. As it stands today qemu-common.h includes no other
> files so it isn't a source of possible circular dependencies -- it's
> just a grab-bag of miscellaneous prototypes that in an ideal world
> would be in more focused individual headers[*]. So there's an argument
> for deleting this comment...
First, thank you for this cleanup series.
The comment is from commit 04509ad939a:
qemu-common.h: Comment about usage rules
Every time we make a tiny change on a header file, we often find
circular header dependency problems. To avoid this nightmare, we need to
stop including qemu-common.h from other headers, and we should gradually
move the declarations from the catch-all qemu-common.h header to their
specific headers.
This simply adds a comment documenting the rules about qemu-common.h,
hoping that people will see it before including qemu-common.h from other
header files, and before adding more declarations to qemu-common.h.
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
You're right: we've since moved all #include out of qemu-common.h, so
the risk of circular header dependency is gone, and the comment's claim
"this would easily lead to circular header dependencies" is no longer
correct.
In my opinion, including qemu-common.h in a header is a bad idea
regardless. Headers should include the headers they need to make them
self-contained, and no more, because more risks slower compiles, in
particular frequent recompiles of pretty much everything. Previously
discussed at
https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg06291.html
Message-ID: <877eac82il.fsf@dusky.pond.sub.org>
Nothing in qemu-common.h should be required to make another header
self-contained.
This is likely the case for other headers as well, which don't carry a
comment forbidding inclusion into headers. You could argue that
qemu-common.h should not carry one, either. I can accept that. I'm not
attached to the comment. I am interested in keeping unwanted #include
in headers under control.
> [*] A cleanup that would be nice, and I'm about to send out a patchset
> that splits out the rtc related functions; but the grab-bag at the
> bottom of osdep.h is probably higher priority because that header
> gets pulled in by an order of magnitude more C files.
By all "normal" ones, in fact.