linux-kernel - Re: [PATCH v3] Input: Add EVIOC mechanism for MT slots

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Message-ID: <4F2D7730.3090704@gmail.com>
Date:	Sat, 04 Feb 2012 19:21:36 +0100
From:	Chase Douglas <chasedouglas@...il.com>
To:	Henrik Rydberg <rydberg@...omail.se>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Input: Add EVIOC mechanism for MT slots

On 02/03/2012 08:27 AM, Henrik Rydberg wrote:
> Hi Chase,
> 
>>> +#define INPUT_MT_REQUEST(num_slots)					\
>>> +	{								\
>>> +		__u32 code;						\
>>> +		__s32 values[num_slots];				\
>>
>> I think this assumes a userspace C compiler that can handle variable
>> length arrays. This would require only compiling in C source code at the
>> C99 standard or later. It looks like C++ doesn't even allow variable
>> length arrays, though gcc handles it. According to:
>>
>> http://www.cplusplus.com/forum/beginner/1601/
>>
>> it looks like Borland c++ compilers may not be able to compile this :(.
> 
> This is resolved on the preprocessor level, so C99 or not does not
> enter the problem. Compile-time constant, as you can see in the code
> example in the patch summary.

You're right, I didn't catch that. It will be compatible with all C
compilers if you use a static number of slots.

However, it will break if you use a non-C99 C compiler and the code
wants to do dynamic number of slots calculations. I imagine most callers
would do:

EVIOCGABS call on ABS_MT_SLOT;
int num_slots = ABS_MT_SLOT.max - ABS_MT_SLOT.min
struct INPUT_MT_REQUEST(num_slots) req;

This will break on non-C99 C compilers and other language compilers. It
also will lead to head-scratcher bugs when someone compiles it just fine
in their C99 project, copies the code to another project with a
different compiler, and is confronted with the issue.

I think this issue should be enough to rethink the interface.

-- Chase
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