gdb硬件断点-----watch使用方法

硬件断点使用watch监测，可以监测栈变量和堆变量值的变化，当被监测变量值发生变化时，程序被停住。

1.    栈变量
测试代码(文件名为1.c，比较low，哈哈)：

#include <string.h>

void test(void)
{
        int iA, iB, iC;

        iA = 1;
        iB = 2;
        iC = 3;

        iB += iA + iC;
        printf("iB = %d\n", iB);
        iB = 0;
        printf("iB = %d\n", iB);
        iB *= iC;
        printf("iB = %d\n", iB);

        return;
}

void main(void)
{
        test();

        return;
}

测试过程(监测变量iB值在代码中发生变化的位置)：

[root@ceph181 test]# vim 1.c
[root@ceph181 test]# gcc -g 1.c
[root@ceph181 test]# ls
1.c  a.out debug.c  log.c  mmap.c  sscanf.c  sync_fetch.c  time.c  unlikely.c
[root@ceph181 test]# gdb  a.out
GNU gdb (GDB) Red Hat Enterprise Linux (7.2-60.el6_4.1)
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/work/test/a.out...done.
(gdb) b test
Breakpoint 1 at 0x4004cc: file 1.c, line 8.
(gdb) r
Starting program: /home/work/test/a.out

Breakpoint 1, test () at 1.c:8
8        iA = 1;
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.132.el6.x86_64
(gdb) watch iB
Hardware watchpoint 2: iB
(gdb) c
Continuing.
Hardware watchpoint 2: iB

Old value = 4195296
New value = 2
test () at 1.c:10
10        iC = 3;
(gdb) c
Continuing.
Hardware watchpoint 2: iB

Old value = 2
New value = 6
test () at 1.c:13
13        printf("iB = %d\n", iB);
(gdb) c
Continuing.
iB = 6
Hardware watchpoint 2: iB

Old value = 6
New value = 0
test () at 1.c:15
15        printf("iB = %d\n", iB);
(gdb) c
Continuing.
iB = 0
iB = 0

Watchpoint 2 deleted because the program has left the block in
which its expression is valid.
main () at 1.c:26
26        return;
(gdb) c
Continuing.

Program exited with code 07.
(gdb)

注意： 栈变量的生命周期有限，因此，使用watch监测其硬件断点时，要注意生命周期

2.    堆变量
int *piA = malloc(sizeof(int));
监测堆变量piA值的变化时，
    1）    在gdb中打印出变量piA的地址： print  &piA，记为piB；
    2）    watch *piB
    3）    continue
如下例所示，要监测指针connection->session的值在什么时候被修改：

Breakpoint 1, xio_connection_destroy (connection=0x7ffff0009240) at ../common/xio_connection.c:2364
2364        int            retval = 0;
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.132.el6.x86_64 libibverbs-1.1.8mlnx1-OFED.3.1.1.0.0.x86_64 libmlx4-1.0.6mlnx1-OFED.3.1.1.0.0.x86_64 libmlx5-1.0.2mlnx1-OFED.3.1.1.0.3.x86_64 libnl-1.1.4-2.el6.x86_64 librdmacm-1.0.21mlnx-OFED.3.0.1.5.2.x86_64 numactl-2.0.7-8.el6.x86_64
(gdb) p connection->session
$1 = (struct xio_session *) 0x6120b0
(gdb) p &connection->session
$2 = (struct xio_session **) 0x7ffff0009298
(gdb) watch *($2)
Hardware watchpoint 2: *($2)
(gdb) disable 1
(gdb) c
Continuing.
Hardware watchpoint 2: *($2)

Old value = (struct xio_session *) 0x6120b0
New value = (struct xio_session *) 0x0
0x0000003917805e94 in rdma_get_cm_event () from /usr/lib64/librdmacm.so.1
(gdb)