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       不久前， 我們玩過valgrind定位程式的各類記憶體問題， 當時非常希望valgrind能對一個已經存在的程序進行attach線上除錯， 後來看了valgrind的原理， 發現這樣不可行。 只能讓valgrind重新拉取新程序進行除錯。

       在前面的文章中， 也介紹了kill -6和abort函式， 讓程式core dump來獲取當時棧的值， 其實， 這樣似乎沒有必要。 慢慢來看。

       我們再回憶下strace命令， 可以用strace -p pid， 附著到一個已經在跑的程序上， 實時觀察程序。 那麼， gdb能做到嗎？ 能！ 這就是gdb的attch除錯， 可以獲取當前棧的所有變數值。

       回憶一下， 比如編譯（加-g）後生成了a.out檔案：

       1.  可以直接用gdb a.out來除錯,  其實gdb拉取了新的程序。 類似於valgrind.

       2.  如果有core檔案， 那麼用gdb a.out core,  此時沒有拉取新的程序。 在實際開發中， 這種方式常用。

       很多時候， 在服務端， 程序已經跑起來， 此時不再適合用gdb來拉取新的程序， 而期望用gdb附著到這個已經存在的程序， 如下兩種方式是等價的：

       3. gdb attach pid  （注意， 沒有拉取新的程序）

       4. gdb a.out pid    （注意， 沒有拉取新的程序）

       看程式：

#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來除錯（程序執行在一個視窗， 除錯在另外一個視窗）：

xxxxxx $ ps -aux | grep a.outWarning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html1000      3265  0.0  0.0   2804   876 pts/1    S+   08:18   0:00 ./a.out1000     20400  0.0  0.0   3072   804 pts/0    S+   21:02   0:00 grep a.outxxxxxx $ xxxxxx $ xxxxxx $ xxxxxx $ gdb attach 3265GNU gdb 6.6Copyright (C) 2006 Free Software Foundation, Inc.GDB is free software, covered by the GNU General Public License, and you arewelcome 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 3265Reading symbols from /data/home/xxxxxx/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.soReading symbols from /usr/lib/libstdc++.so.6...done.Loaded symbols for /usr/lib/libstdc++.so.6Reading symbols from /lib/libm.so.6...done.Loaded symbols for /lib/libm.so.6Reading symbols from /lib/libgcc_s.so.1...done.Loaded symbols for /lib/libgcc_s.so.1Reading symbols from /lib/libc.so.6...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/libdl.so.2...done.Loaded symbols for /lib/libdl.so.2Reading symbols from /lib/libpthread.so.0...done.[Thread debugging using libthread_db enabled][New Thread -1211562320 (LWP 3265)]Loaded symbols for /lib/libpthread.so.0Reading symbols from /lib/ld-linux.so.2...done.Loaded symbols for /lib/ld-linux.so.20xb7d5eb0e 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:1818              getchar();(gdb) i localspo = {x = 1, y = 2}distance = 2.23606801(gdb) qThe program is running.  Quit anyway (and detach it)? (y or n) yDetaching from program: /data/home/xxxxxx/a.out, process 3265xxxxxx $ xxxxxx $ xxxxxx $ xxxxxx $ xxxxxx $ xxxxxx $ xxxxxx $ gdb a.out 3265GNU gdb 6.6Copyright (C) 2006 Free Software Foundation, Inc.GDB is free software, covered by the GNU General Public License, and you arewelcome 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/xxxxxx/a.out, process 3265Reading symbols from /lib/libonion.so...done.Loaded symbols for /lib/libonion.soReading symbols from /usr/lib/libstdc++.so.6...done.Loaded symbols for /usr/lib/libstdc++.so.6Reading symbols from /lib/libm.so.6...done.Loaded symbols for /lib/libm.so.6Reading symbols from /lib/libgcc_s.so.1...done.Loaded symbols for /lib/libgcc_s.so.1Reading symbols from /lib/libc.so.6...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/libdl.so.2...done.Loaded symbols for /lib/libdl.so.2Reading symbols from /lib/libpthread.so.0...done.[Thread debugging using libthread_db enabled][New Thread -1211562320 (LWP 3265)]Loaded symbols for /lib/libpthread.so.0Reading symbols from /lib/ld-linux.so.2...done.Loaded symbols for /lib/ld-linux.so.20xb7d5eb0e 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:1818              getchar();(gdb) i localspo = {x = 1, y = 2}distance = 2.23606801(gdb) 

       好， 介紹完畢。 簡單。

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