togl_flythrough.c reads:
--cut: http://svn.osgeo.org/grass/grass/trunk/visualization/nviz/src/togl_flythrough.c--
901 int Nget_viewdir_cmd(Nv_data * data, Tcl_Interp * interp, /* Current interpreter. */
902 int argc, /* Number of arguments. */
903 char **argv /* Argument strings. */
904 )
905 {
...
920 Tcl_SetResult(interp, Tcl_Merge(3, list), TCL_VOLATILE);
921
922 return (TCL_OK);
923 }
--cut: http://svn.osgeo.org/grass/grass/trunk/visualization/nviz/src/togl_flythrough.c--
However:
--cut: Tcl_Merge(3tcl)--
Tcl_Merge is the inverse of Tcl_SplitList: it takes a collection
of strings given by argc and argv and generates a result string
that has proper list structure. [...]
The result string is dynamically allocated using Tcl_Alloc; the
caller must eventually release the space using Tcl_Free.
--cut: Tcl_Merge(3tcl)--
Hence, it seems that TCL_DYNAMIC is to be used instead of the
current TCL_VOLATILE.