dbXtra and dbxtra

Monitoring the contents of variables

The dbxtra commands when, trace, and stop monitor the contents of a program's variables. The trace and stop commands are not as robust as the when command. For further details see ``Events menu: managing execution time events''.

In our Bridge example, the variable cardsleft is passed to the function Deal(). It may be a good idea to have the value of cardsleft printed out every time the call to Deal(), at line 49, is made. To do this use the when command. At the (dbxtra) prompt, enter:

   (dbxtra) when at 49 {print "at 49: cardsleft = ", cardsleft;}
   [1] when at "deal.c":49 { print "at 49: cardsleft = ",
   deal.deal_hands.cardsleft;}

dbxtra acknowledges your command, and automatically prepends the names of the blocks containing the variable to the variable name and the filename to the line number. Notice the W beside line 49; this indicates a when has been issued for this function.

To verify how a variable is monitored, use the status command:

   (dbxtra) status
   [1] when at "deal.c":49 { print "at 49: cardsleft = ",
   deal.deal_hands.cardsleft;}

There are two things to note here:

The symbol [1] is called an event-id. This is the first command that dbxtra need not execute immediately, but is kept in the background until a particular event occurs.
deal.deal_hands.cardsleft is the qualified name, referring to the cardsleft variable. The qualified name indicates the respective scopes enclosing the variable. In this case, it refers to the cardsleft variable in function deal_hands() in deal.c. It is useful to ensure that the correct variable has been located, since a program can contain many variables with the same name.

Now let's run a.out to see what happens:

   (dbxtra) run
   Dealing 1 hands...
   at 49: cardsleft = 52
   at 49: cardsleft = 39
   at 49: cardsleft = 26
   at 49: cardsleft = 13
   at 49: cardsleft = 0
   Floating point exception in Deal at line 66 in file "deal.c"