Stopping Execution When Variables Change

To stop program execution if the value of a specified variable has changed, type:

(dbx) stop change variable

Keep these points in mind when using the stop change command:

• dbx stops the program at the line after the line that caused a change in the value of the specified variable.

• If variable is local to a function, the variable is considered to have changed when the function is first entered and storage for variable is allocated. The same is true with respect to parameters.

• The command does not work with multithreaded applications.

For more information on specifying a change event, see change variable and stop Command.

dbx implements stop change by causing automatic single stepping together with a check on the value at each step. Stepping skips over library calls if the library was not compiled with the -g option. So, if control flows in the following manner, dbx does not trace the nested user_routine2 because tracing skips the library call and the nested call to user_routine2.

   user_routine calls
      library_routine, which calls
        user_routine2, which changes variable

The change in the value of variable appears to have occurred after the return from the library call, not in the middle of user_routine2.

dbx cannot set a breakpoint for a change in a block local variable, a variable nested in {}. If you try to set a breakpoint or trace in a block local “nested” variable, dbx issues an error informing you that it cannot perform this operation.

It is faster to watch data changes using the access event than the change event. Instead of automatically single-stepping the program, the access event uses hardware or OS services that are much faster.