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Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to scl
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH v3 08/21] s390x: move sclp_service_call() to sclp.h |
Date: |
Mon, 02 Oct 2017 09:01:45 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Thomas Huth <address@hidden> writes:
> On 08.09.2017 14:29, Markus Armbruster wrote:
>> Thomas Huth <address@hidden> 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 <address@hidden>
>>>> ---
>>>> 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.
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