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.
This question is related to
c++
debugging
gdb
breakpoints
memory-access
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.
Use watch to see when a variable is written to, rwatch when it is read and awatch when it is read/written from/to, as noted above. However, please note that to use this command, you must break the program, and the variable must be in scope when you've broken the program:
Use the watch command. The argument to the watch command is an expression that is evaluated. This implies that the variabel you want to set a watchpoint on must be in the current scope. So, to set a watchpoint on a non-global variable, you must have set a breakpoint that will stop your program when the variable is in scope. You set the watchpoint after the program breaks.
What you're looking for is called a watchpoint.
Usage
(gdb) watch foo
: watch the value of variable foo
(gdb) watch *(int*)0x12345678
: watch the value pointed by an address, casted to whatever type you want
(gdb) watch a*b + c/d
: watch an arbitrarily complex expression, valid in the program's native language
Watchpoints are of three kinds:
You may choose the more appropriate for your needs.
For more information, check this out.
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.
Source: Stackoverflow.com