Re: [Qemu-devel] [PATCH] Use standard header for offsetof

[Stefan Weil]

Anthony Liguori schrieb:
> Stefan Weil wrote:
> > Anthony Liguori schrieb:
> > > I don't understand why you're adding #include <stdef.h> to files 
> > > that do not define offsetof.  What's the rationale for that?
> > >
> > > Regards,
> > >
> > > Anthony Liguori
> > There were several possible ways to replace the defines for offsetof.
> > After removing the defines, I could
> >
> > 1) include stddef.h at the places where offsetof was defined formerly
>
> This seems like the most logically thing to do to me.  Otherwise, 
> you're unnecessarily increasing the number of #include's in C files.

exec-all.h is one of these places. Adding #include <stddef.h> there 
reduces the number of #include statements
in C sources, but now all sources which need exec-all.h also include 
stddef.h during compilation.
So this increases the number of included headers during a compilation.

If you prefer source files without many includes, you can put all system 
includes in some central project headers.
Many projects do this, and Qemu's qemu-common.h is an example for this 
approach. You could add the #include
for stddef.h there.

I prefer minimized dependencies and short compile times, so I include 
system headers only at the places which need them.

The choice between both alternatives also depends on your compiler: 
nowadays most C compilers are clever enough
not to parse a system header file more than once while compiling a C 
source which has several references to it.
This is in favour of my prefered choice. But other compilers use 
precompiled headers, and for MS C with precompiled
headers, few large header files are better. So there is no clear winner 
for all situations.

Qemu has no clear strategy for header file inclusion: qemu-common.h is 
used by many source files and includes stdio.h,
but some files (for example osdep.h) include both qemu-common.h and stdio.h.

Regards
Stefan