GDB, DBX, and XDB debuggers.
DDD is an Motif application that besides the "usual" features such as viewing
source texts and breakpoints, DDD provides a _graphical_data_display_, where
data structures are displayed as graphs. A simple mouse click dereferences
pointers or reveals structure contents. Complex data structures can be
explored incrementally and interactively, using automatic layout if
preferred. Each time the program stops, the data display reflects the
current variable values.
