XkbKeycodeToKeysym man page

KeySym XkbKeycodeToKeysym (Display *dpy, KeyCode kc, unsigned int group, unsigned int level);

- dpy

connection to X server

- kc

key of interest

- group

group of interest

- level

shift level of interest

XkbKeycodeToKeysym returns the keysym bound to a particular group and shift level for a particular  key on the core keyboard. If kc is not a legal keycode for the core keyboard, or if group or level are out of range for the specified key, XkbKeycodeToKeysym returns NoSymbol.

XStringToKeysym(3).