The XSetDeviceModifierMapping request specifies the KeyCodes of
the keys (if any) that are to be used as modifiers for the
specified device. If it succeeds, the X server generates a
DeviceMappingNotify event, and XSetDeviceModifierMapping
returns MappingSuccess. X permits at most eight modifier keys.
If more than eight are specified in the XModifierKeymap
structure, a BadLength error results.

The modifiermap member of the XModifierKeymap structure
contains eight sets of max_keypermod KeyCodes, one for each
modifier in the order Shift, Lock, Control, Mod1, Mod2, Mod3,
Mod4, and Mod5. Only nonzero KeyCodes have meaning in each set,
and zero KeyCodes are ignored. In addition, all of the nonzero
KeyCodes must be in the range specified by min_keycode and
max_keycode as returned by XListInputDevices, or a BadValue
error results. No KeyCode may appear twice in the entire map,
or a BadValue error results.

An X server can impose restrictions on how modifiers can be
changed, for example, if certain keys do not generate up
transitions in hardware, if auto-repeat cannot be disabled on
certain keys, or if multiple modifier keys are not supported.
If some such restriction is violated, the status reply is
MappingFailed, and none of the modifiers are changed. If the
new KeyCodes specified for a modifier differ from those
currently defined and any (current or new) keys for that
modifier are in the logically down state,
XSetDeviceModifierMapping returns MappingBusy, and none of the
modifiers is changed.

XSetDeviceModifierMapping can generate BadLength, BadDevice,
BadMatch, BadAlloc, and BadValue errors.

The XGetDeviceModifierMapping request returns a pointer to a
newly created XModifierKeymap structure that contains the keys
being used as modifiers. The structure should be freed after
use by calling XFreeModifierMapping . If only zero values
appear in the set for any modifier, that modifier is disabled.

XGetDeviceModifierMapping can generate BadDevice and BadMatch
errors.

The XModifierKeymap structure contains:

typedef struct {
int max_keypermod;
KeyCode *modifiermap;
} XModifierKeymap;

BadLength
       More than eight keys were specified in the
       XModifierKeymap structure.

BadAlloc
       The server failed to allocate the requested resource or
       server memory.

BadDevice
       An invalid device was specified. The specified device
       does not exist or has not been opened by this client via
       XOpenInputDevice. This error may also occur if the
       specified device is the X keyboard or X pointer device.

BadMatch
       This error may occur if an XGetDeviceModifierMapping or
       XChangeDeviceModifierMapping request was made specifying
       a device that has no keys.

BadValue
       Some numeric value falls outside the range of values
       accepted by the request. Unless a specific range is
       specified for an argument, the full range defined by the
       argument's type is accepted. Any argument defined as a
       set of alternatives can generate this error.

XSetDeviceKeyMapping(3), XSetDeviceButtonMapping(3)