TotalView Users Guide - CI Wiki

Using Watchpoints
location. This can occur when your watchpoints are monitoring adjacent
memory locations and a single store instruction modifies these locations.
For example, suppose that you have two 1-byte watchpoints, one on location
0x10000 and the other on location 0x10001. Also suppose that your
program uses a single instruction to store a 2-byte value at locations
0x10000 and 0x10001. If the 2-byte storage operation modifies both bytes,
the watchpoint for location 0x10000 triggers. The watchpoint for location
0x10001 does not trigger.
Here’s a second example. Suppose that you have a 4-byte integer that uses
storage locations 0x10000 through 0x10003, and you set a watchpoint on
this integer. If a process modifies location 0x10002, TotalView triggers the
watchpoint. Now suppose that you’re watching two adjacent 4-byte integers
that are stored in locations 0x10000 through 0x10007. If a process
writes to locations 0x10003 and 0x10004 (that is, one byte in each),
TotalView triggers the watchpoint associated with location 0x10003. The
watchpoint associated with location 0x10004 does not trigger.
Copying Previous Data Values
TotalView keeps an internal copy of data in the watched memory locations
for each process that shares the watchpoint. If you create watchpoints that
cover a large area of memory or if your program has a large number or processes,
you increase TotalView’s virtual memory requirements. Furthermore,
TotalView refetches data for each memory location whenever it continues
the process or thread. This can affect performance.
Using Conditional Watchpoints
If you associate an expression with a watchpoint (by selecting the
Conditional button in the Watchpoint Properties dialog box entering an
expression), TotalView evaluates the expression after the watchpoint triggers.
The programming statements that you can use are identical to those
used when you create an eval point, except that you can’t call functions
from a watchpoint expression.
The variables used in watchpoint expressions must be global. This is
because the watchpoint can be triggered from any procedure or scope in
your program.
Fortran does not have global variables. Consequently, you can’t directly refer to your
program’s variables.
TotalView has two variables that are used exclusively with conditional
watchpoint expressions:
$oldval The value of the memory locations before a change is
made.
$newval The value of the memory locations after a change is
made.
378 Chapter 16: Setting Action Points