让gdb再次搜索符号文件

Question

我的发行版 (Debian) 在单独的包中提供调试文件。所以经常发生的事情是，我运行一个程序 gdb 直到它崩溃，以便为错误报告获得可用的回溯。但是bt有点没用，缺少符号信息——因为我没有安装对应的-dbg包。

如果我现在安装这个包，有没有办法让gdb再次搜索符号文件，而不丢失我当前的回溯？

Answer 1

我将建议使用 gdb gcore 命令的替代方法，可能适合您。

这是 gcore 描述：

(gdb) help gcore
Save a core file with the current state of the debugged process.
Argument is optional filename.  Default filename is 'core.<process_id>'

所以我有一个导致崩溃的程序：

include <iostream>

int f()
{
  time_t curr_ts = time(0);
  std::cout << "Before crash " << curr_ts << std::endl;
  int * ptr = 0;
  *ptr = *ptr +1 ;
  std::cout << "After crash " << curr_ts << std::endl;
  return *ptr;
}

int main()
{

  std::cout << "Before f() " << std::endl;
  f();
  std::cout << "After f() "  << std::endl;
  return 0;
}

我用调试信息编译了它。但是我将带有调试信息的可执行文件放在存档中，测试使用剥离版本。

因此它在 gdb 下崩溃：

$ gdb ./a.out                
Reading symbols from ./a.out...(no debugging symbols found)...done.
(gdb) r
Starting program: /home/crash/a.out 
Before f() 
Before crash 1435322344

Program received signal SIGSEGV, Segmentation fault.
0x000000000040097d in ?? ()
(gdb) bt
#0  0x000000000040097d in ?? ()
#1  0x00000000004009e0 in ?? ()
#2  0x000000314981ed1d in __libc_start_main () from /lib64/libc.so.6
#3  0x00000000004007f9 in ?? ()
#4  0x00007fffffffde58 in ?? ()
#5  0x000000000000001c in ?? ()
#6  0x0000000000000001 in ?? ()
#7  0x00007fffffffe1a9 in ?? ()
#8  0x0000000000000000 in ?? ()
(gdb) gcore crash2.core
Saved corefile crash2.core

我简单地用gcore生成核心文件然后离开gdb。然后我从存档中得到带有调试符号的版本，我可以看到所有符号：

$ gdb ./a.out ./crash2.core
Reading symbols from ./a.out...done.

warning: exec file is newer than core file.
[New LWP 15215]
Core was generated by `/home/crash/a.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000000000040097d in f () at main.cpp:8
8     *ptr = *ptr +1 ;
(gdb) bt
#0  0x000000000040097d in f () at main.cpp:8
#1  0x00000000004009e0 in main () at main.cpp:17

(gdb) info locals 
curr_ts = 1435322344
ptr = 0x0

更新

如果你 set backtrace past-main on 你至少会看到这个 __libc_start_main。如果你只分析核心文件（甚至可能没有保存在那里）保存机智 gcore:

，则上面 __libc_start_main 的内容不会被打印出来

$ gdb ./a.out crash2.core
Reading symbols from ./a.out...done.

warning: exec file is newer than core file.
[New LWP 15215]
Core was generated by `/home/crash/a.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000000000040097d in f () at main.cpp:8
8     *ptr = *ptr +1 ;
(gdb) set backtrace past-main on
(gdb) bt
#0  0x000000000040097d in f () at main.cpp:8
#1  0x00000000004009e0 in main () at main.cpp:17
#2  0x000000314981ed1d in __libc_start_main () from /lib64/libc.so.6
Backtrace stopped: Cannot access memory at address 0x4007d0
(gdb)

但是如果我用我的测试程序（其中包含调试信息）在 gdb 下重现崩溃，我可以看到所有内容（参见 set backtrace past-main on && set backtrace past-entry on）：

$ gdb ./a.out 
Reading symbols from ./a.out...done.
(gdb) r
Starting program: /home/crash/a.out 
Before f() 
Before crash 1435328858

Program received signal SIGSEGV, Segmentation fault.
0x000000000040097d in f () at main.cpp:8
8     *ptr = *ptr +1 ;
(gdb) bt
#0  0x000000000040097d in f () at main.cpp:8
#1  0x00000000004009e0 in main () at main.cpp:17
(gdb) set backtrace past-main on
(gdb) set backtrace past-entry on
(gdb) bt
#0  0x000000000040097d in f () at main.cpp:8
#1  0x00000000004009e0 in main () at main.cpp:17
#2  0x000000314981ed1d in __libc_start_main () from /lib64/libc.so.6
#3  0x00000000004007f9 in _start ()
#4  0x00007fffffffde58 in ?? ()
#5  0x000000000000001c in ?? ()
#6  0x0000000000000001 in ?? ()
#7  0x00007fffffffe1a9 in ?? ()
#8  0x0000000000000000 in ?? ()
(gdb)

Answer 2

有一个技巧可以让 gdb 再次尝试读取符号文件：

(gdb) nosharedlibrary
(gdb) sharedlibrary

第一个命令告诉它忘记它拥有的所有符号信息，第二个命令告诉它重新读取它。

让gdb再次搜索符号文件

Make gdb search for symbol files again

debugging

gdb

debug-symbols