Questions regarding libevdev

[Callum Oz]

(Sending this email again, since I wasn't on the mailing list and I think
it was blocked the first time).

Peter,

We don't have a requirement to "set" the field more than 16 bits, it's just
a quirk that we noticed.
If I understand correctly the `int` type is larger than 16 bits.

My C isn't the best, but this example should demonstrate why I'm confused:

```c
#include <stdio.h>

int main() {
   int x = 70000;
   printf("x: %d", x); // Not truncated to 16 bits

   short y = 70000;
   printf("y: %d", y); // This IS truncated

  return 0;
}
```

Whereas using the `short` type would accurately display the value as 16
bits.

We're writing bindings for libevdev to Rust, and wanted to get the types
right.
In our tests we noticed that although the API's return type is `int`, it's
being
treated as something smaller and is getting truncated to 16 bits.

Regards,
Cal

On Thu, Mar 21, 2019 at 9:39 AM Callum Oz <acheronfail at gmail.com> wrote:

> Peter,
>
> We don't have a requirement to "set" the field more than 16 bits, it's
> just a quirk that we noticed.
> If I understand correctly the `int` type is larger than 16 bits.
>
> My C isn't the best, but this example should demonstrate why I'm confused:
>
> ```c
> #include <stdio.h>
>
> int main() {
>    int x = 70000;
>    printf("x: %d", x); // Not truncated to 16 bits
>
>    short y = 70000;
>    printf("y: %d", y); // This IS truncated
>
>   return 0;
> }
> ```
>
> Whereas using the `short` type would accurately display the value as 16
> bits.
>
> We're writing bindings for libevdev to Rust, and wanted to get the types
> right.
> In our tests we noticed that although the API's return type is `int`, it's
> being
> treated as something smaller and is getting truncated to 16 bits.
>
> Regards,
> Cal
>
> On Thu, Mar 21, 2019 at 9:24 AM Peter Hutterer <peter.hutterer at who-t.net>
> wrote:
>
>> On Thu, Mar 21, 2019 at 12:14:25AM +0900, Nayan Deshmukh wrote:
>> > Hello,
>> >
>> > We have encountered a quirky behavior of libevdev and wanted to make
>> > sure that it is known and intentional quirk.
>> >
>> > Whenever I set the version for a device through
>> > `libevdev_set_id_version`, the value set is truncated at 65,535 which
>> > is the max value of unsigned short. Any thoughts on this?
>>
>> the id_version sets/gets the struct input_id's version field, from
>> linux/input.h. That's a uint16 so we mirror the kernel API here.
>> The same applies to the other libevdev_set_id_* fields, those are 16 bit
>> only too.
>>
>> Many drivers don't really use that version field at all, so it's largely
>> ignored by userspace anyway. Out of interest - what do you need it be
>> bigger
>> than 16 bits for?
>>
>> Cheers,
>>    Peter
>>
>