gdb attach到已经存在的进程进行在线调试------能获取当前栈的所有变量值！

       不久前， 我们玩过valgrind定位程序的各类内存问题， 当时非常希望valgrind能对一个已经存在的进程进行attach在线调试， 后来看了valgrind的原理， 发现这样不可行。 只能让valgrind重新拉取新进程进行调试。

       在前面的文章中， 也介绍了kill -6和abort函数， 让程序core dump来获取当时栈的值， 其实， 这样似乎没有必要。 慢慢来看。

       我们再回忆下strace命令， 可以用strace -p pid， 附着到一个已经在跑的进程上， 实时观察进程。 那么， gdb能做到吗？ 能！ 这就是gdb的attch调试， 可以获取当前栈的所有变量值。

    

       回忆一下， 比如编译（加-g）后生成了a.out文件：

       1.  可以直接用gdb a.out来调试,  其实gdb拉取了新的进程。 类似于valgrind.

       2.  如果有core文件， 那么用gdb a.out core,  此时没有拉取新的进程。 在实际开发中， 这种方式常用。

       很多时候， 在服务端， 进程已经跑起来， 此时不再适合用gdb来拉取新的进程， 而期望用gdb附着到这个已经存在的进程， 如下两种方式是等价的：

       3. gdb attach pid  （注意， 没有拉取新的进程）

       4. gdb a.out pid    （注意， 没有拉取新的进程）

       看程序：

#include <iostream>
#include <cmath>
using namespace std;

struct Point
{
int x;
int y;
};

int main()
{
Point po;
po.x = 1;
po.y = 2;
float distance = sqrt(po.x * po.x + po.y * po.y);

getchar();
return 0;
}

      看看利用方法3,4来调试（进程运行在一个窗口， 调试在另外一个窗口）：

xxxxxx $ ps -aux | grep a.out
Warning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html 1000      3265  0.0  0.0   2804   876 pts/1    S+   08:18   0:00 ./a.out
1000     20400  0.0  0.0   3072   804 pts/0    S+   21:02   0:00 grep a.out
xxxxxx $
xxxxxx $
xxxxxx $
xxxxxx $ gdb attach 3265
GNU gdb 6.6
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i586-suse-linux"...
attach: No such file or directory.
Attaching to process 3265
Reading symbols from /data/home/erickeyli/a.out...done.
Using host libthread_db library "/lib/libthread_db.so.1".
Reading symbols from /lib/libonion.so...done.
Loaded symbols for /lib/libonion.so
Reading symbols from /usr/lib/libstdc++.so.6...done.
Loaded symbols for /usr/lib/libstdc++.so.6
Reading symbols from /lib/libm.so.6...done.
Loaded symbols for /lib/libm.so.6
Reading symbols from /lib/libgcc_s.so.1...done.
Loaded symbols for /lib/libgcc_s.so.1
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/libdl.so.2...done.
Loaded symbols for /lib/libdl.so.2
Reading symbols from /lib/libpthread.so.0...done.
[Thread debugging using libthread_db enabled]
[New Thread -1211562320 (LWP 3265)]
Loaded symbols for /lib/libpthread.so.0
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
0xb7d5eb0e in __read_nocancel () from /lib/libc.so.6
(gdb) bt
#0  0xb7d5eb0e in __read_nocancel () from /lib/libc.so.6
#1  0xb7d0e0d8 in _IO_file_read_internal () from /lib/libc.so.6
#2  0xb7d0f35b in _IO_new_file_underflow () from /lib/libc.so.6
#3  0xb7d0fa9b in _IO_default_uflow_internal () from /lib/libc.so.6
#4  0xb7d10ddd in __uflow () from /lib/libc.so.6
#5  0xb7d0aebe in getchar () from /lib/libc.so.6
#6  0x0804867c in main () at test.cpp:18
(gdb) f 6
#6  0x0804867c in main () at test.cpp:18
18              getchar();
(gdb) i locals
po = {x = 1, y = 2}
distance = 2.23606801
(gdb) q
The program is running.  Quit anyway (and detach it)? (y or n) y
Detaching from program: /data/home/erickeyli/a.out, process 3265
xxxxxx $
xxxxxx $
xxxxxx $
xxxxxx $
xxxxxx $
xxxxxx $
xxxxxx $ gdb a.out 3265
GNU gdb 6.6
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i586-suse-linux"...
Using host libthread_db library "/lib/libthread_db.so.1".
Attaching to program: /data/home/erickeyli/a.out, process 3265
Reading symbols from /lib/libonion.so...done.
Loaded symbols for /lib/libonion.so
Reading symbols from /usr/lib/libstdc++.so.6...done.
Loaded symbols for /usr/lib/libstdc++.so.6
Reading symbols from /lib/libm.so.6...done.
Loaded symbols for /lib/libm.so.6
Reading symbols from /lib/libgcc_s.so.1...done.
Loaded symbols for /lib/libgcc_s.so.1
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/libdl.so.2...done.
Loaded symbols for /lib/libdl.so.2
Reading symbols from /lib/libpthread.so.0...done.
[Thread debugging using libthread_db enabled]
[New Thread -1211562320 (LWP 3265)]
Loaded symbols for /lib/libpthread.so.0
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
0xb7d5eb0e in __read_nocancel () from /lib/libc.so.6
(gdb) bt
#0  0xb7d5eb0e in __read_nocancel () from /lib/libc.so.6
#1  0xb7d0e0d8 in _IO_file_read_internal () from /lib/libc.so.6
#2  0xb7d0f35b in _IO_new_file_underflow () from /lib/libc.so.6
#3  0xb7d0fa9b in _IO_default_uflow_internal () from /lib/libc.so.6
#4  0xb7d10ddd in __uflow () from /lib/libc.so.6
#5  0xb7d0aebe in getchar () from /lib/libc.so.6
#6  0x0804867c in main () at test.cpp:18
(gdb) f 6
#6  0x0804867c in main () at test.cpp:18
18              getchar();
(gdb) i locals
po = {x = 1, y = 2}
distance = 2.23606801
(gdb)

       好， 介绍完毕。 简单。