Advanced Topics in C ...

• What you have to type is in bold underlined
• Remember that iterators are transformed into recursions and that it is the transformed code that is traced
• It is recommended to specify a  loop_name when you use an iterator in order to know which iterator is being traced

Start the tracing mode (for the next executions)

[eclipse]: trace.     

Debugger switched on - creep mode

Tune the display so that you can see information about the domains (crucial!)

[eclipse]: true.

  (1) 1 CALL  true   %> ?

  ... gives the list of possible tracing commands (see below)

%> o

current output mode is "QPm", toggle char: D

new output mode is "DQPm".

%> creep

  (1) 1 EXIT  true   %> creep

Yes (0.00s cpu)

Quit the tracing mode (for the next executions)

[eclipse 21]: notrace.

Debugger switched off

Trace information (cf Chapter 14 “Debugging” of Eclipse User manual )

Trace line 

(1)                1           CALL       solve(X, Y, Z, [a,b,c])   %>

(invoc. nb)  depth   port         goal                               prompt

Ports 

• CALL a procedure is called for the first time concerning a particular invocation,
• NEXT the next possibly matching clause of a procedure is tried because unification failed or a sub-goal failed, 
• ELSE the next branch of a disjunction is tried because some goal in the previous branch failed
• EXIT a procedure succeeds,
• FAIL a procedure fails, there is no (other) solution,
• REDO a procedure that already gave a solution is called again for an alternative,
• DELAY a procedure delays,
• RESUME a procedure is woken (the flow enters the procedure box as for a call) because of a unification of a suspending variable,
• LEAVE a procedure is left before having failed or exited because of a call to exit_block/1,

The main tracing commands (Full list accessible by typing “?”, see above)

• h, ? :  help
• N :  tracer off permanently
• <cr> :  creep [once]
• i:  jump to invocation number N
• l:   leap to spypoint [N times]
• n:  nodebug (continue with tracer off)
• q:  jump to the most recent failure's culprit
• s: skip subgoal
• z:  zap to port
• g:  goto ancestor goal (caller)

How to trace breadth-first

• execute one goal
• <creep> to “enter” one level
• <skip> until the exit, fail or leave of a suspicious goal
• remember the invocation number
• <nodebug>
• re-execute same goal
• <i> to jump to the suspicious goal
• iterate

Example

[eclipse 16]: resoudre(X, Y, Z, [AffZ, AffZselonX, AffX, Aff1Y, Aff2Y]).

  (1) 1 CALL  resoudre(X, Y, Z, [AffZ, AffZselonX, AffX, ...])   %> creep

  (2) 2 CALL  var(X, Y, Z, AffZ, AffZselonX, AffX, Aff1Y, Aff2Y)   %> skip

  (2) 2 EXIT  var(X{[femme, homme]}, ...)   %> skip

  (5) 2 CALL  poser_pb(X{[femme, homme]}, Y{[femme, homme]}, ...)   %> skip

  (5) 2 EXIT  poser_pb(X{[femme, homme]}, Y{[femme, homme]}, ...)   %> nodebug? [y]  % accept the default value proposed by the tracer (Here [y] for yes) by simply typing "CR"

[eclipse 17]: resoudre(X, Y, Z, [AffZ, AffZselonX, AffX, Aff1Y, Aff2Y]).

  (1) 1 CALL  resoudre(X, Y, Z, [AffZ, AffZselonX, AffX, ...])   %> i  jump to invoc: [1]? 5

  (5) 2 CALL  poser_pb(X{[femme, homme]}, Y{[femme, homme]}, Z{[femme, homme]}, AffZ{[0, 1]}, AffZselonX{[0, 1]}, AffX{[0, 1]}, Aff1Y{[0, 1]}, Aff2Y{[0, 1]})   %> 

...

How to trace back errors

• <zap> to leave 
• to go where the error occurs
• <G> or <g> to inspect goal stack
• <+> to put a spy point or remember the suspicious ancestor goal invocation number
• reexecute
• <l> to leap to the spy point or <i> to jump to the invocation number

Example

[eclipse 2]: t_tab12.                  % a predicate of yours

instantiation fault in subscript(_197, [1], _200) in module eclipse

Abort

[eclipse 3]: trace.

Debugger switched on - creep mode

[eclipse 4]: t_tab12.

  (1) 1 CALL  t_tab12   %> zap to port: [~ call] leave        % here you do not want the default value, you want the "leave" port

instantiation fault in subscript(_544, [1], _547) in module eclipse

  (8) 4 LEAVE  subscript(..., ..., ..., ...) %> G print all ancestors? [y] 

•  +(0) 0 ....  trace_body(..., ...)
•   (1) 1 ....  t_tab12
•   (2) 2 ....  display_tab2(...)
•   (6) 3 ....  do__7(..., ...)   %> abort? [y] 

Abort

[eclipse 4]: t_tab12.

  (1) 1 CALL  t_tab12   %> i  jump to invoc: [1]? 6           % the goal that you want to debug

   ...