Watchpoints

Watchpointing is the capability of dbx to note when the value of a variable or expression has changed.

Stopping Execution When Modified

To stop program execution when the contents of an address is written to:

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(dbx) stop modify &variable

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Keep these points in mind when using stop modify:

• The event occurs when a variable gets written to even if it is the same value.

• The event occurs before the instruction that wrote to the variable is executed, although the new contents of the memory are preset by dbx by emulating the instruction.

• You cannot use addresses of stack variables, for example, auto function local variables.

Stopping Execution When Variables Change

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

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(dbx) stop change variable

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Keep these points in mind when using stop change:

• 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.

  dbx implements stop change by causing automatic single stepping together with a check on the value at each step. Stepping skips over library calls. So, if control flows in the following manner:

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     user_routine calls
        library_routine, which calls
          user_routine2, which changes variable
  

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  dbx does not trace the nested user_ routine2 because tracing skips the library call and the nested call to user_routine2, so 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.

Stopping Execution on a Condition

To stop program execution if a conditional statement evaluates to true:

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(dbx) stop cond condition

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