用gdb来调试对stl的空vector进行操作而产生的core

        通过前面的学习， 大家应该很了解gdb调试core了， 今天我们用gdb来调试对stl的空vector进行操作而产生的core.

        先不说core, 而是来学学vector的front方法， 如下：

[taoge@localhost test]$ cat main.cpp #include <iostream>#include <vector>using namespace std;int main(){        vector<int> v;        v.push_back(1);        v.push_back(2);        cout << v.front() << endl;        v.front() -= 10;        cout << v[0] << endl;        return 0;}[taoge@localhost test]$ g++ main.cpp [taoge@localhost test]$ ./a.out 1-9[taoge@localhost test]$ 

        要注意， v.front()是对vector首元素的引用， 所以可以做左值。  但是， 当vector为空的时候， 就有问题了， 产生了core， 如下：

[taoge@localhost test]$ cat main.cpp -n     1  #include <iostream>     2  #include <vector>     3     4  using namespace std;     5     6  int main()     7  {     8          vector<int> v;     9    10          cout << v.front() << endl;    11          v.front() -= 10;    12    13          cout << v[0] << endl;    14    15          return 0;    16  }    17[taoge@localhost test]$ lsmain.cpp[taoge@localhost test]$ g++ main.cpp [taoge@localhost test]$ lsa.out  main.cpp[taoge@localhost test]$ ./a.out Segmentation fault (core dumped)[taoge@localhost test]$ lsa.out  core.2917  main.cpp[taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ gdb a.out core.2917 GNU gdb (GDB) Red Hat Enterprise Linux (7.1-29.el6)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 "i686-redhat-linux-gnu".For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>...Reading symbols from /home/taoge/test/a.out...(no debugging symbols found)...done.[New Thread 2917]Missing separate debuginfo for Try: yum --disablerepo='*' --enablerepo='*-debuginfo' install /usr/lib/debug/.build-id/74/d23352fd770753e375bd0caecf375bd77bded5Reading symbols from /usr/lib/libstdc++.so.6...(no debugging symbols found)...done.Loaded symbols for /usr/lib/libstdc++.so.6Reading symbols from /lib/libm.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libm.so.6Reading symbols from /lib/libgcc_s.so.1...(no debugging symbols found)...done.Loaded symbols for /lib/libgcc_s.so.1Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.Loaded symbols for /lib/ld-linux.so.2Core was generated by `./a.out'.Program terminated with signal 11, Segmentation fault.#0  0x080486f7 in main ()Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.7.el6.i686 libgcc-4.4.4-13.el6.i686 libstdc++-4.4.4-13.el6.i686(gdb) (gdb) (gdb) (gdb) bt#0  0x080486f7 in main ()(gdb) 

        可以看到， 运行程序的时候， 程序core dump了， 产生了core文件， 但是用gdb只能找到出错的地址， 找不到代码行， 为什么呢？ 因为编译的时候没有-g选项， 重新来做一遍， 如下：

[taoge@localhost test]$ cat main.cpp -n     1  #include <iostream>     2  #include <vector>     3     4  using namespace std;     5     6  int main()     7  {     8          vector<int> v;     9    10          cout << v.front() << endl;    11          v.front() -= 10;    12    13          cout << v[0] << endl;    14    15          return 0;    16  }    17[taoge@localhost test]$ g++ main.cpp -g[taoge@localhost test]$ ./a.out Segmentation fault (core dumped)[taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ gdb a.out core.2947 GNU gdb (GDB) Red Hat Enterprise Linux (7.1-29.el6)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 "i686-redhat-linux-gnu".For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>...Reading symbols from /home/taoge/test/a.out...done.[New Thread 2947]Missing separate debuginfo for Try: yum --disablerepo='*' --enablerepo='*-debuginfo' install /usr/lib/debug/.build-id/74/d23352fd770753e375bd0caecf375bd77bded5Reading symbols from /usr/lib/libstdc++.so.6...(no debugging symbols found)...done.Loaded symbols for /usr/lib/libstdc++.so.6Reading symbols from /lib/libm.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libm.so.6Reading symbols from /lib/libgcc_s.so.1...(no debugging symbols found)...done.Loaded symbols for /lib/libgcc_s.so.1Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.Loaded symbols for /lib/ld-linux.so.2Core was generated by `./a.out'.Program terminated with signal 11, Segmentation fault.#0  0x080486f7 in main () at main.cpp:1010              cout << v.front() << endl;Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.7.el6.i686 libgcc-4.4.4-13.el6.i686 libstdc++-4.4.4-13.el6.i686(gdb) (gdb) (gdb) (gdb) bt#0  0x080486f7 in main () at main.cpp:10(gdb) 

        定位到了第10行除了问题。 

         实际上， 单纯的v.front()并不会有错， 错就错在后面用cout用这个东西， 如下再看：

[taoge@localhost test]$ cat main.cpp -n     1  #include <iostream>     2  #include <vector>     3     4  using namespace std;     5     6  int main()     7  {     8          vector<int> v;     9          v.front();    10    11          int a = v.front();    12          cout << v[0] << endl;    13    14          return 0;    15  }    16[taoge@localhost test]$ g++ main.cpp -g[taoge@localhost test]$ lsa.out  main.cpp[taoge@localhost test]$ ./a.out Segmentation fault (core dumped)[taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ gdb a.out core.3099 GNU gdb (GDB) Red Hat Enterprise Linux (7.1-29.el6)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 "i686-redhat-linux-gnu".For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>...Reading symbols from /home/taoge/test/a.out...done.[New Thread 3099]Missing separate debuginfo for Try: yum --disablerepo='*' --enablerepo='*-debuginfo' install /usr/lib/debug/.build-id/74/d23352fd770753e375bd0caecf375bd77bded5Reading symbols from /usr/lib/libstdc++.so.6...(no debugging symbols found)...done.Loaded symbols for /usr/lib/libstdc++.so.6Reading symbols from /lib/libm.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libm.so.6Reading symbols from /lib/libgcc_s.so.1...(no debugging symbols found)...done.Loaded symbols for /lib/libgcc_s.so.1Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.Loaded symbols for /lib/ld-linux.so.2Core was generated by `./a.out'.Program terminated with signal 11, Segmentation fault.#0  0x08048703 in main () at main.cpp:1111              int a = v.front();Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.7.el6.i686 libgcc-4.4.4-13.el6.i686 libstdc++-4.4.4-13.el6.i686(gdb) (gdb) (gdb) (gdb) bt#0  0x08048703 in main () at main.cpp:11(gdb) 

        代码是在第11行core调的， 而不是第9行， 我猜想， 编译器肯定是把第9行优化掉了， 因为它没有什么作用，可以dead code处理。

         本文除了复习gdb调试core, 还有一个重点需要注意： 在操作stl的时候， 必要时一定需要进行合法判断， 这是程序员自己的责任， 而不是stl的责任。 再比如， 在出栈前， 必须对栈进行非空判断， 否则必定core dump.  据我的经验， 很多人在真正项目中就碰到空stl引起的core dump, 最后大费周折。

         再次大声疾呼， 代码质量不是一句空话啊。OK, 本文先讨论到这里。