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c++debugginggdbbreakpointsmemory-access

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

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.

Solution

  • 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.