Re: Commit 6016af "[media] v4l2: use __u32 rather than enums in ioctl() structs" breaks C++ users of V4L2

From: Rémi Denis-Courmont
Date: Tue Jul 17 2012 - 00:50:03 EST

Le lundi 16 juillet 2012 23:40:01 Jason L Tibbitts III, vous avez écrit :
> I ran into problems compiling the program ZoneMinder on Fedora rawhide
> (currently using something around 3.5rc6) which do not appear with 3.4
> kernels. With help this was traced to commit
> 6016af82eafcb6e086a8f2a2197b46029a843d68, "[media] v4l2: use __u32
> rather than enums in ioctl() structs" which changed videodev2.h in a way
> which appears to be incompatible with C++.
> This results in code such as the following:
> enum v4l2_buf_type type = v4l2_data.fmt.type;

That code is dangerous, probably even more so in C++. If the running kernel
version is more recent than the kernel header version that userland was
compiled with, the field value could be outside the range of the enumeration.

> failing to compile with:
> zm_local_camera.cpp:1523:49: error: invalid conversion from '__u32
> {aka unsigned int}' to 'v4l2_buf_type' [-fpermissive]
> but only when compiled with the headers from a 3.5 kernel.
> I'm very far from a C++ expert. I talked with some people who do grok
> it and the issue comes down to restrictions on assignments of ints to
> enums

> and additionally that enums in C++ don't have defined size.

That cannot be true. Your C++ compiler must agree with the kernel's C compiler
on the size of enum. If they did not, V4L2 would not have ever worked from C++
code in previous kernel versions.

Rémi Denis-Courmont
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