Xkb maintains a virtual modifier mapping, which lists the virtual modifiers associated with, or bound to, each key. The real modifiers bound to a virtual modifier always include all of the modifiers bound to any of the keys that specify that virtual modifier in their virtual modifier mapping. The server.vmodmap array indicates which virtual modifiers are bound to each key; each entry is a bitmask for the virtual modifier bits. The server.vmodmap array is indexed by keycode.
The vmodmap and vmods members of the server map are the "master" virtual modifier definitions. Xkb automatically propagates any changes to these fields to all other fields that use virtual modifier mappings.
For example, if Mod3 is bound to the Num_Lock key by the core protocol modifier mapping, and the NumLock virtual modifier is bound to they Num_Lock key by the virtual modifier mapping, Mod3 is added to the set of modifiers associated with NumLock.
The virtual modifier mapping is normally updated whenever actions are automatically applied to symbols and few applications should need to change the virtual modifier mapping explicitly.
Use XkbGetMap to get the virtual modifiers from the server or use XkbGetVirtualMods to update a local copy of the virtual modifiers bindings from the server. To set the binding of a virtual modifier to a real modifier, use XkbGetVirtualMods
If the keyboard description defined by xkb includes bindings for virtual modifiers, XkbVirtualModsToReal uses those bindings to determine the set of real modifiers that correspond to the set of virtual modifiers specified in virtual_mask. The virtual_mask parameter is a mask specifying the virtual modifiers to translate; the i-th bit (0 relative) of the mask represents the i-th virtual modifier. If mask_rtrn is non-NULL, XkbVirtualModsToReal backfills it with the resulting real modifier mask. If the keyboard description in xkb does not include virtual modifier bindings, XkbVirtualModsToReal returns False; otherwise, it returns True.
It is possible for a local (client-side) keyboard description (the xkb parameter) to not contain any virtual modifier information (simply because the client has not requested it) while the server's corresponding definition may contain virtual modifier information.