Can I set a breakpoint on 'memory access' in GDB?

Question

I am running an application through gdb and I want to set a breakpoint for any time a specific variable is accessed / changed. Is there a good method for doing this? I would also be interested in other ways to monitor a variable in C/C++ to see if/when it changes.

Answer 1

Supposedly a watchpoint can help you, but my version of gdb on my platform does not support this feature (i.e. I suspect they half-wrote the function so gdb knows what watchpoints are, but they don't work 'yet').

Edit: As others have pointed out, whether or not watchpoints work for you apparently depends on hardware-specific support.

Answer 2

You can use watchpoints, according to this manual.

EDIT: Works for me too, using a very old Pentium IV

Answer 3

I just tried the following:

 $ cat gdbtest.c
 int abc = 43;

 int main()
 {
   abc = 10;
 }
 $ gcc -g -o gdbtest gdbtest.c
 $ gdb gdbtest
 ...
 (gdb) watch abc
 Hardware watchpoint 1: abc
 (gdb) r
 Starting program: /home/mweerden/gdbtest 
 ...

 Old value = 43
 New value = 10
 main () at gdbtest.c:6
 6       }
 (gdb) quit

So it seems possible, but you do appear to need some hardware support.

Answer 4

watch only breaks on write, rwatch let you break on read, and awatch let you break on read/write.

You can set read watchpoints on memory locations:

gdb$ rwatch *0xfeedface
Hardware read watchpoint 2: *0xfeedface

but one limitation applies to the rwatch and awatch commands; you can't use gdb variables in expressions:

gdb$ rwatch $ebx+0xec1a04f
Expression cannot be implemented with read/access watchpoint.

So you have to expand them yourself:

gdb$ print $ebx 
$13 = 0x135700
gdb$ rwatch *0x135700+0xec1a04f
Hardware read watchpoint 3: *0x135700 + 0xec1a04f
gdb$ c
Hardware read watchpoint 3: *0x135700 + 0xec1a04f

Value = 0xec34daf
0x9527d6e7 in objc_msgSend ()

Edit: Oh, and by the way. You need either hardware or software support. Software is obviously much slower. To find out if your OS supports hardware watchpoints you can see the can-use-hw-watchpoints environment setting.

gdb$ show can-use-hw-watchpoints
Debugger's willingness to use watchpoint hardware is 1.

Answer 5

Assuming the first answer is referring to the C-like syntax (char *)(0x135700 +0xec1a04f) then the answer to do rwatch *0x135700+0xec1a04f is incorrect. The correct syntax is rwatch *(0x135700+0xec1a04f).

The lack of ()s there caused me a great deal of pain trying to use watchpoints myself.

Answer 6

Yes you can. http://www.technochakra.com/debugging-types-of-data-breakpoints-in-gdb/ discusses various data breakpoints for gdb.