TotalView Users Guide - CI Wiki

Using Watchpoints
stored, TotalView doesn’t trigger the watchpoint because the location’s value
did not change.
For example, if location 0x10000 has a value of 0 and your program writes a
value of 0 to this location, TotalView doesn’t trigger the watchpoint, even
though your program wrote data to the memory location. See “Triggering
Watchpoints” on page 377 for more details on when watchpoints trigger.
You can also create conditional watchpoints. A conditional watchpoint is similar
to a conditional breakpoint in that TotalView evaluates the expression when
the value in the watched memory location changes. You can use conditional
watchpoints for a number of purposes. For example, you can use one to test
whether a value changes its sign—that is, it becomes positive or negative—or
whether a value moves above or below some threshold value.
Using Watchpoints on Different Architectures
The number of watchpoints, and their size and alignment restrictions, differ
from platform to platform. This is because TotalView relies on the operating
system and its hardware to implement watchpoints.
Watchpoints are not available on Macintosh computers running OS X, IBM PowerPC
computers running Linux Power, and Hewlett Packard (HP) computers running HP-UX
(PA-RISC).
The following list describes constraints that exist on each platform:
Computer Constraints
HP Alpha Tru64 Tru64 places no limitations on the number of watchpoints that
you can create, and no alignment or size constraints. However,
watchpoints can’t overlap, and you can’t create a watchpoint on
an already write-protected page.
Watchpoints use a page-protection scheme. Because the page
size is 8,192 bytes, watchpoints can degrade performance if your
program frequently writes to pages that contains watchpoints
IRIX6 MIPS Watchpoints are implemented on IRIX 6.2 and later operating
systems. These systems let you create approximately 100
watchpoints. There are no alignment or size constraints.
However, watchpoints can’t overlap.
IBM AIX You can create one watchpoint on AIX 4.3.3.0-2 (AIX 4.3R) or later
systems running 64-bit chips. These are Power3 and Power4
systems. (AIX 4.3R is available as APAR IY06844.) A watchpoint
cannot be longer than 8 bytes, and you must align it within an 8byte
boundary. If your watchpoint is less than 8 bytes and it
doesn’t span an 8-byte boundary, TotalView figures out what to
do.
You can create compiled conditional watchpoints when you use
this system. When watchpoints are compiled, they are evaluated
by the process rather than having to be evaluated in TotalView
where all evaluations are single-threaded and must be sent from
separately executing processes. Only systems having fast traps
can have compiled watchpoints.
374 Chapter 16: Setting Action Points