XtAppSetFallbackResources man page
XtAppSetFallbackResources — set fallback resources
void XtAppSetFallbackResources(XtAppContext app_context, String* specification_list);
Specifies the application context.
Specifies a NULL-terminated list of resource specifications to pre-load the database, or NULL.
Each entry in specification_list points to a string in the format XrmPutLineResource. following a call to XtAppSetFallbackResources, when a resource database is being created for a particular screen and the Intrinsics are not able to find or read an application-specific class resource file and if specification_list is not NULL the resource specification in specification_list will be merged into the screen resource database in place of the application-specific class resource file. XtAppSetFallbackResources is not required to copy specification_list; the caller must ensure that the contents of the list and the strings addressed by the list remain valid until all displays are initialized or until XtAppSetFallbackResources is called again. The value NULL for specification_list removes any previous fallback resource specification for the application context. The intended use for fallback resources is to provide a minimal number of resources that will make the application usable (or at least terminate with helpful diagnostic messages) when some problem exists in finding and loading the application defaults file.
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface