API equivalent for 'xmodmap -pm'

[VanLaser]

Hello,

I'm writing a 'touchscreen' keyboard (dockable, sticky etc.) for X11 (for a Linux-based tablet); the key sending method is based on /dev/uinput, so that the X keyboard settings will be taken into account as if the events come from a "real" keyboard device. I'm trying to find out what keys, when pressed, affect the modifiers of the keyboard, so that I can make those keys 1) remain depressed and 2) release automatically after another (non-modifier) key is pressed. Basically, I think, to find out the same info xmodmap -pm offers from the CLI. I may be daft, but I don't see a way to find out this information with xkbcommon API. Any idea? Thanks!

[bluetech]

There is no API which exposes such a thing directly.

What you can do is use an imperative approach. Create a scratch xkb_state, simulate a key press (xkb_state_update_key()), and see if it affected the depressed and/or locked modifiers (enum xkb_state_components in the return value). Note layouts can also be depressed and/or be locked by keys.

If you need more pointers, or if it doesn't work for the use case, let us know.

[yshui]

@bluetech That is a really roundabout way to get information that should be available directly.

It would be really nice if there is a function for this.

[yshui]

And, some of the keycodes, e.g. Meta_L (0xcd), Mode_switch (0xcb), cannot be discovered this way.

[bluetech]

I guess it would be easier to consider if someone who needs this proposes an API that would have been useful for them (implementation not needed, just an API proposal).

[sdilts]

Since xkb_keymap_mod_get_index, xkb_keymap_mod_get_name, and xkb_keymap_num_mods already exist, I guess it would make sense to introduce something similar to this:

/**
 * Get the keycodes associated with a modifier by index.
 * @param[in] keymap The keymap
 * @param[in] idx The index of the modifier
 * @param[out] codes_out A buffer in which the codes should be stored
 * @param[out] codes_size The size of the codes_out buffer
 * 
 * @returns the number of key codes in the keysyms_out array.  
 */
size_t xkb_keymap_mod_get_codes(struct xkb_keymap *keymap,
                                xkb_mod_index_t idx,
                                xkb_keycode_t *codes_out,
                                size_t codes_size);

Although it might be nice (if a bit dangerous) to allow code_size < return_value if the buffer isn't big enough.

[wismill]

I added a feature similar to xmodmap -pm in #352. Note that it is only available as a debug tool, because it exposes internal mapping of modifiers.

About your use case: do you need to know in advance the keys affecting the modifiers, or could it be simplier to just watch the keyboard state and update the key accordingly when you detect a modifier change?