The debugging aids are intended for use primarily by Xkb implementors and are optional in any implementation. There are two bitmasks that may be used to control debugging. One bitmask controls the output of debugging information, and the other controls behavior. Both bitmasks are initially all zeros. To change the values of any of the debug controls, use XkbSetDebuggingFlags . Bool XkbSetDebuggingFlags ( display, mask, flags, msg, ctrls_mask, ctrls, ret_flags, ret_ctrls ) Display * display ; /* connection to X server */ unsigned int mask ; /* mask selecting debug output flags to change */ unsigned int flags ; /* values for debug output flags selected by mask */ char * msg ; /* message to print right now */ unsigned int ctrls_mask ; /* mask selecting debug controls to change */ unsigned int ctrls ; /* values for debug controls selected by ctrls_mask */ unsigned int * ret_flags ; /* resulting state of all debug output flags */ unsigned int * ret_ctrls ; /* resulting state of all debug controls */ XkbSetDebuggingFlags modifies the debug output flags as specified by mask and flags , modifies the debug controls flags as specified by ctrls_mask and ctrls , prints the message msg , and backfills ret_flags and ret_ctrls with the resulting debug output and debug controls flags. When bits are set in the debug output masks, mask and flags , Xkb prints debug information corresponding to each bit at appropriate points during its processing. The device to which the output is written is implementation-dependent, but is normally the same device to which X server error messages are directed; thus the bits that can be set in mask and flags is implementation-specific. To turn on a debug output selection, set the bit for the output in the mask parameter and set the corresponding bit in the flags parameter. To turn off event selection for an event, set the bit for the output in the mask parameter and do not set the corresponding bit in the flags parameter. When bits are set in the debug controls masks, ctrls_mask and ctrls , Xkb modifies its behavior according to each controls bit. ctrls_mask and ctrls are related in the same way that mask and flags are. The valid controls bits are defined in Table 22.1. Debug Control Mask Value Meaning XkbDF_DisableLocks (1 << 0) Disable actions that lock modifiers XkbSetDebuggingFlags returns True if successful and False otherwise. The only protocol error it may generate is BadAlloc , if for some reason it is unable to allocate storage. XkbSetDebuggingFlags is intended for developer use and may be disabled in production X servers. If it is disabled, XkbSetDebuggingFlags has no effect and does not generate any protocol errors. The message in msg is written immediately. The device to which it is written is implementation dependent but is normally the same device where X server error messages are directed.