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From: Aravinda <aravindakidambi@gmail.com>
Date: Thu, 24 Dec 2009 20:47:00 -0000
Message-ID: <26eb53620912241247i718992d6i422451f4b7f9931@mail.gmail.com>
Subject: Problem with manual watchpoints
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Hi,

Im facing a very wierd problem. Its nothing to do with GDB, but i was
just wondering if theres something about 'hardware watchpoints' im
missing here.

I have a kernel module that can add watchpoints for a process. The
module can be invoked by __add_watchpoint(pid, address).
I wrote the program below to test the module. In the below program, I
have allocated a buffer 'a' and access 'a[0], a[1] .... a[25]'. I have
the address of a[20] loaded in one of the debug register.

a = malloc(20);
__add_watchpoint(getpid(), &a[20]);

for (i = 0; i < 25; i ++) {
	/* getc(stdin); ----> without this, no SIGTRAP is getting generated */
	printf("Accessing now %x\n", &a[i]);
	a[i]++;
}

While I expect the program to be interrupted by a tracepoint exception
on accessing a[20], it actually does not. It runs to completion
without catching any trap exception. However, if I just add a
'getc(stdin)' before accessing every element, it does get the
exception on accessing a[20].

Does anyone know what could be going on ?

Thanks,
Aravinda