Find the variable change in GDB - c++

Question

I am working in a project where there are more than 100 files of source code. I am debugging it for find an error.
What I need to find is the time when a particular object assigned to a value. ie. This object is NULL at first, but some other file changes its value, which I don't know.
Are there any method to find when this variable changes its value ?
What I tried upto now is to put a breakpoint on the function where the variable is initilized. I also added a watchpoint. But it does not show any point where the value is changing.

Answer 1

But it does not show any point where the value is changing.
There are two possible explanations:
You've set the watchpoint incorrectly, or
The value changes while the process is in kernel mode (GDB watchpoints are not effective for detecting such change).
Example:
#include <unistd.h>
int global_a;
int global_b;
int foo()
{
global_a = 42;
read(0, &global_b, 1);
return global_a + global_b;
}
int main()
{
return foo();
}
gcc -g -Wall t.c
gdb -q ./a.out
(gdb) start
Temporary breakpoint 1 at 0x400563: file t.c, line 16.
Starting program: /tmp/a.out
Temporary breakpoint 1, main () at t.c:16
16 return foo();
(gdb) watch global_a
Hardware watchpoint 2: global_a
(gdb) watch global_b
Hardware watchpoint 3: global_b
(gdb) c
Continuing.
Hardware watchpoint 2: global_a
Old value = 0
New value = 42
This is a modification of global_a in user-space (via direct assignment), and it triggers watchpoint 2 as expected
foo () at t.c:9
9 read(0, &global_b, 1);
(gdb) c
Continuing.
# Press Enter here
This read 0xA == 10 into global_b.
[Inferior 1 (process 126196) exited with code 064]
Note that the exit code is 064 == 52 == 42+10, but the watchpoint 3 did not fire.
any method to find when this variable changes its value
If you are sure that your "normal" watchpoints are working (e.g. by running above test yourself) and suspect that the variable is being changed via a system call, you can:
Print the address of the variable and
Run your program under strace and look for system calls that could change the value at variable's address.
Using the same example:
(gdb) p &global_b
$1 = (int *) 0x601044 <global_b>
strace -e raw=all ./a.out < /dev/zero
execve(0x7ffe1853e530, 0x7ffe1853f920, 0x7ffe1853f930) = 0
brk(0) = 0x2253000
access(0x7fa66eee48c3, 0) = -1 (errno 2)
mmap(0, 0x2000, 0x3, 0x22, 0xffffffff, 0) = 0x7fa66f0e8000
...
munmap(0x7f57ece26000, 0x203a2) = 0
read(0, 0x601044, 0x1) = 0x1 ### &global_b is "covered" by the buffer being read into
exit_group(42) = ?
+++ exited with 42 +++