Sun Studio 12: Debugging a Program With dbx

Setting Data Change Breakpoints

You can use data change breakpoints in dbx to note when the value of a variable or expression has changed.

Stopping Execution When an Address Is Accessed

To stop execution when a memory address has been accessed, type:

(dbx) stop access mode address-expression [, byte-size-expression]

mode specifies how the memory was accessed. It can be composed of one or all of the letters:


The memory at the specified address has been read.


The memory has been written to.


The memory has been executed.

mode can also contain either of the following:


Stops the process after the access (default).


Stops the process before the access.

In both cases the program counter will point at the offending instruction. The “before” and “after” refer to the side effect.

address-expression is any expression that can be evaluated to produce an address. If you give a symbolic expression, the size of the region to be watched is automatically deduced; you can override it by specifying byte-size-expression. You can also use nonsymbolic, typeless address expressions; in which case, the size is mandatory.

In the following example, execution will stop execution after the memory address 0x4762 has been read

(dbx) stop access r 0x4762

In this example, execution will stop before the variable speed has be written to:

(dbx) stop access wb &speed

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

For more information on specifying an access event, see access mode address-expression [, byte-size-expression] and stop Command.

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:

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.

Note –

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 a page protection scheme that is much faster.

Stopping Execution on a Condition

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

(dbx) stop cond condition

The program stops executing when the condition occurs.

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

For more information on specifying a condition event, see cond condition-expression and stop Command.