Debugger watchpoints in UNIX ?

[Luciano Lavagno]

Has anybody heard of an efficient implementation of watchpoints, i.e. 
breakpoints activated whenever an arbitrary position in memory (e.g. a 
field of a dynamically allocated structure) changes value ?
Both dbx and saber can do something similar, but with horrendous 
performance penalty ("stop address" commands).
I would like something similar to the VMS SET WATCH command... but I
strongly suspect that the UNIX debugging paradigm inherently forbids
such a feature.
Thanks !