-split a fullpath into the volume, path, (base) name and extension
-(all of the pointers can be NULL)
+This function splits a full file name into components: the volume (with the
+first version) path (including the volume in the second version), the base name
+and the extension. Any of the output parameters ({\it volume}, {\it path},
+{\it name} or {\it ext}) may be {\tt NULL} if you are not interested in the
+value of a particular component. Also, {\it fullpath} may be empty on entry.
+
+On return, {\it path} contains the file path (without the trailing separator),
+{\it name} contains the file name and {\it ext} contains the file extension
+without leading dot. All three of them may be empty if the corresponding
+component is. The old contents of the strings pointed to by these parameters
+will be overwritten in any case (if the pointers are not {\tt NULL}).