C语言 GDB：尝试取消引用通用指针

GDB: Attempt to dereference generic pointer

提问by ntc2

How can I make GDB do extra dereferences in a printing function like x/s?

我怎样才能让 GDB 在打印功能中做额外的取消引用 x/s？

When I try explicit dereferences in x/I get the error "Attempt to dereference a generic pointer". Using x/multiple times works, since each use includes an implicit dereference, but this is annoying since I have to copy and paste each intermediate result.

当我在中尝试显式取消引用时，x/出现错误“尝试取消引用通用指针”。x/多次使用是有效的，因为每次使用都包含一个隐式取消引用，但这很烦人，因为我必须复制和粘贴每个中间结果。

Example

例子

Consider the very useful C program, example.c:

考虑非常有用的 C 程序example.c：

#include <stdio.h>
int main(int argc, char **argv) {
  printf("argv[0] = %s\n", argv[0]);
}

If I build it and load it into GDB, I see that argvis stored at 0xc(%ebp), since a double dererence of that is passed as the second argument to printf(i.e. in 0x4(%esp)) on line 26:

如果我构建它并将其加载到 GDB 中，我会看到它argv存储在 0xc(%ebp)，因为它的双重错误作为第二个参数传递给printf（即 in 0x4(%esp)）在第 26 行：

$ gcc -o example example.c
$ gdb example

(gdb) disass main
Dump of assembler code for function main:
   0x080483e4 <+0>:   push   %ebp
   0x080483e5 <+1>:   mov    %esp,%ebp
   0x080483e7 <+3>:   and    
(gdb) break *main + 26
Breakpoint 1 at 0x80483fe

(gdb) run first second
Starting program: /var/tmp/SO-attempt-to-dereference-generic-pointer/example first second
xfffffff0,%esp
   0x080483ea <+6>:   sub    
Breakpoint 1, 0x080483e5 in main ()
(gdb) x/s **(0xc + $ebp)
Attempt to dereference a generic pointer.
x10,%esp
   0x080483ed <+9>:   mov    0xc(%ebp),%eax
   0x080483f0 <+12>:  mov    (%eax),%edx
   0x080483f2 <+14>:  mov    
(gdb) x/xw 0xc + $ebp
0xbfffeba4: 0xbfffec34
(gdb) x/xw 0xbfffec34
0xbfffec34: 0xbfffedc8
(gdb) x/s 0xbfffedc8
0xbfffedc8:  "/var/tmp/SO-attempt-to-dereference-generic-pointer/example"

(gdb) x/xw 0xbfffec34 + 4
0xbfffec38: 0xbfffee03
(gdb) x/s 0xbfffee03
0xbfffee03:  "first"
(gdb) 
x80484e0,%eax
   0x080483f7 <+19>:  mov    %edx,0x4(%esp)
   0x080483fb <+23>:  mov    %eax,(%esp)
   0x080483fe <+26>:  call   0x8048300 <printf@plt>
   0x08048403 <+31>:  leave  
   0x08048404 <+32>:  ret    
End of assembler dump.

I break at printfand run the program with arguments firstand second:

我中断printf并使用参数first和 运行程序second：

(gdb) x/s **((char ***) (0xc + $ebp))
0xbfffedc8:  "/var/tmp/SO-attempt-to-dereference-generic-pointer/example"
(gdb) x/s *(*((char ***) (0xc + $ebp)) + 1)
0xbfffee03:  "first"
(gdb) x/s *(*((char ***) (0xc + $ebp)) + 2)
0xbfffee09:  "second"

I attempt to print argv[0]in GDB, but I get the "generic pointer" error:

我尝试argv[0]在 GDB 中打印，但出现“通用指针”错误：

##代码##

However, by using 'x/xw' to manually dereference a few times, I'm eventually able to print argv[0](and argv[1]):

但是，通过使用 'x/xw' 手动取消引用几次，我最终能够打印argv[0]（和argv[1]）：

##代码##

But this is annoying and indirect (as pointer programming is wont to be?)

但这很烦人而且是间接的（因为指针编程不会是？）

回答by ntc2

The solution is to cast the pointers before dereferencing them.

解决方案是在取消引用它们之前强制转换指针。

For example, picking up where we left off above:

例如，从上面我们离开的地方开始：

##代码##

Note that the stack address 0xc + $ebpis itself a pointer to the contents of that stack location, and so we need char ***and not char **.

请注意，堆栈地址0xc + $ebp本身就是指向该堆栈位置内容的指针，因此我们需要char ***而不是 char **。