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

[Thomas Huth <thuth@redhat.com>]

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.

> 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.

 Thomas