Messages in this thread | | | Date | Sat, 04 Feb 2012 19:21:36 +0100 | From | Chase Douglas <> | Subject | Re: [PATCH v3] Input: Add EVIOC mechanism for MT slots |
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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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