Re: [PATCH 5/8] v4l2-info: remove a strange sizeof usage

[Nicolas Dufresne <nicolas@ndufresne.ca>]

Le mercredi 21 avril 2021 à 11:33 +0200, Hans Verkuil a écrit :
> On 21/04/2021 11:19, Rosen Penev wrote:
> > 
> > > On Apr 21, 2021, at 01:23, Hans Verkuil <hverkuil@xs4all.nl> wrote:
> > > 
> > > > On 21/04/2021 09:20, Rosen Penev wrote:
> > > > The array has a nullptr and 0 member for some reason. Remove and convert
> > > > loop to a for range one.
> > > > 
> > > > Signed-off-by: Rosen Penev <rosenp@gmail.com>
> > > > ---
> > > > utils/common/v4l2-info.cpp | 33 +++++++++++++++------------------
> > > > 1 file changed, 15 insertions(+), 18 deletions(-)
> > > > 
> > > > diff --git a/utils/common/v4l2-info.cpp b/utils/common/v4l2-info.cpp
> > > > index cb3cb91f7..0359cf137 100644
> > > > --- a/utils/common/v4l2-info.cpp
> > > > +++ b/utils/common/v4l2-info.cpp
> > > > @@ -3,6 +3,8 @@
> > > >  * Copyright 2018 Cisco Systems, Inc. and/or its affiliates. All rights reserved.
> > > >  */
> > > > 
> > > > +#include <array>
> > > > +
> > > > #include <v4l2-info.h>
> > > > 
> > > > static std::string num2s(unsigned num, bool is_hex = true)
> > > > @@ -411,21 +413,20 @@ std::string mbus2s(unsigned flags, bool is_hsv)
> > > >    return flags2s(flags, mbus_ycbcr_def);
> > > > }
> > > > 
> > > > -static const flag_def selection_targets_def[] = {
> > > > -    { V4L2_SEL_TGT_CROP_ACTIVE, "crop" },
> > > > -    { V4L2_SEL_TGT_CROP_DEFAULT, "crop_default" },
> > > > -    { V4L2_SEL_TGT_CROP_BOUNDS, "crop_bounds" },
> > > > -    { V4L2_SEL_TGT_COMPOSE_ACTIVE, "compose" },
> > > > -    { V4L2_SEL_TGT_COMPOSE_DEFAULT, "compose_default" },
> > > > -    { V4L2_SEL_TGT_COMPOSE_BOUNDS, "compose_bounds" },
> > > > -    { V4L2_SEL_TGT_COMPOSE_PADDED, "compose_padded" },
> > > > -    { V4L2_SEL_TGT_NATIVE_SIZE, "native_size" },
> > > > -    { 0, nullptr }
> > > 
> > > The idea of having this sentinel is that this makes it easy to add new
> > > entries without having to update the array size.
> > Not following this. I assume it’s some C feature.
> 
> Standard programming techique:
> 
> https://en.wikipedia.org/wiki/Sentinel_value#:~:text=In%20computer%20programming%2C%20a%20sentinel,a%20loop%20or%20recursive%20algorithm.
> 
> > > 
> > > > +static constexpr std::array<flag_def, 8> selection_targets_def{
> > > 
> > > Something you need to do here, adding a new flag means updating the size.
> > I assume this is a small issue. It’s an immediate compile error anyway.
> 
> Not if the size is larger that the number of initializers. That would leave
> zeroed elements at the end.
> 
> > > 
> > > New flags are added regularly, so keeping that robust is a good idea IMHO.
> > > 
> > > If it were possible to write:
> > > 
> > > static constexpr std::array<flag_def> selection_targets_def{
> > > 
> > > i.e. without an explicit size, then this would make sense, but C++
> > > doesn't allow this. And std::vector allocates the data on the heap,
> > > which is less efficient as well.
> > But this is possible. With C++17 :). That would necessitate a minimum of GCC6 though.
> 
> How is that done in C++17? I didn't find anything about that.

I believe this is similar to this code:

https://git.linuxtv.org/libcamera.git/tree/src/gstreamer/gstlibcamera-utils.cpp#n15

But there I only used the fact that you can iterate it like any other
collection. This is a recent feature I got told (I'm quite a C++ newby, and I
like static arrays without passing and updating a size).

> 
> Regards,
> 
> 	Hans
> 
> > > 
> > > Let's just keep using normal arrays in this case, they do the job
> > > just fine. Just because you have a hammer, it doesn't mean everything
> > > is now a nail :-)
> > > 
> > > Regards,
> > > 
> > >    Hans
> > > 
> > > > +    flag_def{ V4L2_SEL_TGT_CROP_ACTIVE, "crop" },
> > > > +    flag_def{ V4L2_SEL_TGT_CROP_DEFAULT, "crop_default" },
> > > > +    flag_def{ V4L2_SEL_TGT_CROP_BOUNDS, "crop_bounds" },
> > > > +    flag_def{ V4L2_SEL_TGT_COMPOSE_ACTIVE, "compose" },
> > > > +    flag_def{ V4L2_SEL_TGT_COMPOSE_DEFAULT, "compose_default" },
> > > > +    flag_def{ V4L2_SEL_TGT_COMPOSE_BOUNDS, "compose_bounds" },
> > > > +    flag_def{ V4L2_SEL_TGT_COMPOSE_PADDED, "compose_padded" },
> > > > +    flag_def{ V4L2_SEL_TGT_NATIVE_SIZE, "native_size" },
> > > > };
> > > > 
> > > > bool valid_seltarget_at_idx(unsigned i)
> > > > {
> > > > -    return i < sizeof(selection_targets_def) / sizeof(selection_targets_def[0]) - 1;
> > > > +    return i < selection_targets_def.size();
> > > > }
> > > > 
> > > > unsigned seltarget_at_idx(unsigned i)
> > > > @@ -437,13 +438,9 @@ unsigned seltarget_at_idx(unsigned i)
> > > > 
> > > > std::string seltarget2s(__u32 target)
> > > > {
> > > > -    int i = 0;
> > > > -
> > > > -    while (selection_targets_def[i].str != nullptr) {
> > > > -        if (selection_targets_def[i].flag == target)
> > > > -            return selection_targets_def[i].str;
> > > > -        i++;
> > > > -    }
> > > > +    for (const auto &def : selection_targets_def)
> > > > +        if (def.flag == target)
> > > > +            return def.str;
> > > >    return std::string("Unknown (") + num2s(target) + ")";
> > > > }
> > > > 
> > > > 
> > > 
>