Range of accepted screen values does not match

[wysiwys]

In src/keymap.h, the type of the screen member in the xkb_switch_screen_action struct is int8_t. This also matches the type specified in the X Keyboard Extension Protocol Specification for SA_SwitchScreen. However, during the validation step in src/xkbcomp/action.c, in the function HandleSwitchScreen(), all values in the range [-255, +255] are accepted.

Should the validation step instead check that the value is in [-128, +127], in order to fit inside the range of int8_t?

[wismill]

I would say the range should be -128..+127 indeed, but we parse as X11 xkbcomp does.

[wysiwys]

Thanks! Is this a situation where libxkbcommon's xkbcomp tries to match the X11 version as closely as possible?

[wismill]

We try to follow X11 xkbcomp behavior as much as possible, but if it is buggy we may diverge. Here I have not analyzed much the impact, since libxkbcommon does not offer an API to handle this type of action, although we parse and serialize it.

[wysiwys]

As far as I can tell, the impact would be quite limited on modern systems. This is because there is generally only one screen index (0) available on modern systems that use the X server, with all monitors being combined into one conceptual 'screen'. However, there may be some other configurations I am not aware of, which could be relevant.

For the situation where the parsed screen index exceeds the range, would it make sense to add a log message? This could be a way to notify the user of how the screens are handled without changing the actual behavior of the parser.