Re: How to watch for changes in a location of memory

[Paul Koning <Paul_Koning@dell.com>]

>>>>> "Andreas" == Andreas Schwab <schwab@suse.de> writes:

 Andreas> Eli Zaretskii <eliz@gnu.org> writes:
 >>> From: Andreas Schwab <schwab@suse.de> Cc: gdb Mailing List
 >>> <gdb@sources.redhat.com> Date: Fri, 08 Aug 2008 13:55:47 +0200
 >>> 
 >>> Stefano Sabatini <stefano.sabatini-lala@poste.it> writes:
 >>> 
 >>> > Thank you for the good pointer, yes indeed it seems it does
 >>> what I > want, which is basically: > watch &ctx->foo;
 >>> 
 >>> This is equivalent to `watch ctx', since the address of ctx->foo
 >>> can only change if ctx changes.  Watching an address of something
 >>> is generally not usefull.
 >>  And "watch ctx" is also not generally useful, because most
 >> platforms cannot watch large structures.

 Andreas> In this example, ctx is a pointer (otherwise ctx->foo
 Andreas> wouldn't work), which is small enough on all platforms.

Right.  But from the original note it seems that the requester is
interested in catching changes to field foo of the structure that ctx
points to.

The thing that confuses lots of people when they first use the "watch"
command is that the intuitive meaning is "watch this address" but the
actual meaning is "watch this expression".  So Stefano wrote 
"watch &ctx->foo" but to get the effect that I believe was intended
you'd want "watch ctx->foo".

Similarly, to watch a specific numeric address, you have to say
"watch *(T*)0x123454" as opposed to "watch 0x123454" or something like
that.

	paul