Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h

[Markus Armbruster <armbru@redhat.com>]

Thomas Huth <thuth@redhat.com> writes:

> On 08.09.2017 14:29, Markus Armbruster wrote:
>> Thomas Huth <thuth@redhat.com> writes:
>> 
>>> On 07.09.2017 22:13, David Hildenbrand wrote:
>>>> Implemented in sclp.c, so let's move it to the right include file.
>>>> Fix up one include. Do a forward declaration of CPUS390XState to fix the
>>>> two sclp consoles complaining.
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>  include/hw/s390x/sclp.h    | 2 ++
>>>>  target/s390x/cpu.h         | 1 -
>>>>  target/s390x/misc_helper.c | 1 +
>>>>  3 files changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/hw/s390x/sclp.h b/include/hw/s390x/sclp.h
>>>> index a72d096081..4b86a8a293 100644
>>>> --- a/include/hw/s390x/sclp.h
>>>> +++ b/include/hw/s390x/sclp.h
>>>> @@ -242,5 +242,7 @@ sclpMemoryHotplugDev *init_sclp_memory_hotplug_dev(void);
>>>>  sclpMemoryHotplugDev *get_sclp_memory_hotplug_dev(void);
>>>>  void sclp_service_interrupt(uint32_t sccb);
>>>>  void raise_irq_cpu_hotplug(void);
>>>> +typedef struct CPUS390XState CPUS390XState;
>>>> +int sclp_service_call(CPUS390XState *env, uint64_t sccb, uint32_t code);
>>>
>>> That's dangerous and likely does not work with certain versions of GCC.
>>> You can't do a "forward declaration" with typedef in C, I'm afraid. See
>>> for example:
>>>
>>>  https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg01454.html
>>>  https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg03337.html
>>>  https://stackoverflow.com/questions/8367646/redefinition-of-typedef
>>>
>>> All this typedef'ing in QEMU is pretty bad ... we run into this problem
>>> again and again. include/qemu/typedefs.h is just a work-around for this.
>>> I know people like typedefs for some reasons (I used to do that, too,
>>> before I realized the trouble with them), but IMHO we should rather
>>> adopt the typedef-related rules from the kernel coding conventions instead:
>>>
>>>  https://www.kernel.org/doc/html/v4.13/process/coding-style.html#typedefs
>> 
>> I prefer the kernel style for typedefs myself.  But it's strictly a
>> matter of style.  Excessive typedeffing makes code harder to understand,
>> it isn't wrong.  The part that's wrong is defining things more than
>> once, and that applies to everything, not just typedefs.
>> 
>> Sometimes you get away with defining something more than once.  It's
>> still wrong.
>> 
>> You're not supposed to define the same variable more than once, either
>> (although many C compilers let you get away with it, see -fno-common).
>> You define it in *one* place.  If you need to declare it, declare it in
>> *one* place, and make sure that place is in scope at the definition, so
>> the compiler can check the two match.
>> 
>> Likewise, you're not supposed to define the same struct tag more than
>> once, and you should declare it in just one place.
>
> AFAIK it's perfect valid C to do a forward declaration of a struct
> multiple times by just writing e.g. "struct CPUS390XState;" somewhere in
> your code. This is also what is done in various Linux kernel headers all
> over the place.

"Define it in one place" is dictated by the language, i.e. violating the
rule is wrong.

"Declare it in one place" is merely style.  I insist on it for
declarations the compiler can meaningfully check against definitions,
such as function declarations.  It can't for struct tags, and I consider
"one place" a matter of taste there.

>> Likewise, you're not supposed to define (with typedef) the same type
>> more than once.  There is no such thing as a typedef declaration.
>> 
>> qemu/typedefs.h is not a work-around for a typedef-happy style.  Its
>> purpose is breaking inclusion cycles.  Secondary purpose is reducing the
>> need for non-cyclic includes.  If we didn't typedef, we'd still put our
>> struct declarations there.
>
> No, since it's not required for struct forward declarations, only to
> avoid multiple typedef definitions.

I definitely would, because I prefer sticking to "declare in one place"
style.