User`s guide

Chapter 6: Controlling Program Execution

stop [expression|variable] in procedure

Inspects the value at every source line within a given

procedure. Stops if the value has changed.

If the expression is of type pointer, look at the data pointed

to and watch until it changes.

If the expression is not of type pointer, look at the 32 bits at

that address (assume the expression evaluates to an

address).

Using Fast Watchpoints

You can use fast watchpoints with the stop command. A fast watchpoint

watches a speciﬁed variable or memory address without severely impacting

the performance of the program being debugged.

In previous versions of dbx, the debugger had to single-step the process

being debugged and check if the value of a variable had changed after each

instruction. With fast watchpoints, the debugger uses a hardware virtual

memory write protect mechanism to allow the program to run freely until

the variable being watched changes. The program being debugged stops

only when the virtual memory page containing the variable is written to. If

the value of the variable being watched does not changed, dbx continues the

execution of the process. If awrite modiﬁes a watched variable, dbx notiﬁes

you of the change.

Consider a small program that contains a global variable called global:

stop global

This command causes the program to stop if the value of the variable global

changes. The program runs virtually at full speed until global gets assigned

a new value. Similarly, consider the command:

stop 0x100100

This command stops when the 32- bit integer residing at address 0x100100

is modiﬁed, and runs at nearly full speed until the value changes. This form

of the stop command is useful for watching the contents of anonymous

memory, such as the memory returned by malloc().