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

[Rosen Penev <rosenp@gmail.com>]

> On Apr 21, 2021, at 02:33, Hans Verkuil <hverkuil@xs4all.nl> wrote:
> 
>> 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.
Ah yes that is a real problem. I’ve been bitten by it in the past.
> 
>>> 
>>> 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.
C++17 has class template argument deduction. It allows putting just std::array or std::vector or any other container really without the <>. Of course you have to allow it to deduce the arguments by putting them in the initializers. This is actually what I did in the previous cec-tuner patch.
> 
> 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) + ")";
>>>> }
>>>> 
>>>> 
>>> 
>