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

[Rosen Penev <rosenp@gmail.com>]

> 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.
> 
>> +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.
> 
> 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.
> 
> 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) + ")";
>> }
>> 
>> 
>