"display" bug

[Yanmeng Ba]

Hello everyone,

I use the GDB 7.0 with python extension to periodically attach GDB to
a running process and output some variable values. The process I am
investigating is quite simple: it prints out the value of a variable
which decrease by 1 every 3 seconds. My method is quite simple and
straightforward:

import sys
import gdb
import time
while True:
    print "attach"
    gdb.execute("attach 31127")
    gdb.execute("break 14")
    gdb.execute("continue")
    gdb.execute("display i")
    gdb.execute("detach")
    time.sleep(6)
end

However, each time I "display" the value of variable i, the number of
value printed is proportional to the number of the run.

Partial of the results displayed in my terminal is as follows:

attach
0xb808b430 in __kernel_vsyscall ()
Breakpoint 4 at 0x804840c: file beer-process.c, line 14.

Breakpoint 1, main () at beer-process.c:14
14                      PrintMessage( i );
3: i = 99961
2: i = 99961
1: i = 99961
attach
0xb808b430 in __kernel_vsyscall ()
Breakpoint 5 at 0x804840c: file beer-process.c, line 14.

Breakpoint 1, main () at beer-process.c:14
14                      PrintMessage( i );
4: i = 99958
3: i = 99958
2: i = 99958
1: i = 99958
attach
0xb808b430 in __kernel_vsyscall ()
Breakpoint 6 at 0x804840c: file beer-process.c, line 14.

Breakpoint 1, main () at beer-process.c:14
14                      PrintMessage( i );
5: i = 99955
4: i = 99955
3: i = 99955
2: i = 99955
1: i = 99955
attach
0xb808b430 in __kernel_vsyscall ()
Breakpoint 7 at 0x804840c: file beer-process.c, line 14.

Breakpoint 1, main () at beer-process.c:14
14                      PrintMessage( i );
6: i = 99952
5: i = 99952
4: i = 99952
3: i = 99952
2: i = 99952
1: i = 99952
attach
0xb808b430 in __kernel_vsyscall ()
Breakpoint 8 at 0x804840c: file beer-process.c, line 14.

Breakpoint 1, main () at beer-process.c:14
14                      PrintMessage( i );
7: i = 99949
6: i = 99949
5: i = 99949
4: i = 99949
3: i = 99949
2: i = 99949
1: i = 99949

It is really weird that it prints more and more values as the the run
goes on. I am not sure whether this is a bug in GDB. If it is not a
bug, could anyone give me some insights on how to fix my problem
here ? Thanks a lot in advance.

best,
Yanmeng Ba