GDB:尝试取消引用通用指针 [英] GDB: Attempt to dereference generic pointer
问题描述
我怎样才能让GDB在打印功能像做额外的指针引用 X / S
?
How can I make GDB do extra dereferences in a printing function like
x/s
?
当我尝试在 X /
我得到的错误尝试明确的指针引用
解引用一个普通指针。使用 X /
多次工作,因为
每次使用包括一个隐含解引用,但是这是恼人的,因为
我有复制和每个中间结果粘贴。
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.
考虑非常有用的C程序, example.c
:
Consider the very useful C program, example.c
:
#include <stdio.h>
int main(int argc, char **argv) {
printf("argv[0] = %s\n", argv[0]);
}
如果我构建它并将其加载到GDB,我看到的argv
存储在位于0xC(%EBP)
,因为一个双dererence作为第二传递
参数的printf
(即为0x4(%ESP)
)的行26:
If I build it and load it into GDB, I see that argv
is 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:
$ 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 $0xfffffff0,%esp
0x080483ea <+6>: sub $0x10,%esp
0x080483ed <+9>: mov 0xc(%ebp),%eax
0x080483f0 <+12>: mov (%eax),%edx
0x080483f2 <+14>: mov $0x80484e0,%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.
我打破在的printf
键,带参数运行程序第一
和第二
:
I break at printf
and run the program with arguments first
and
second
:
(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
我试图打印的argv [0]
广发行,但我得到了通用指针
错误:
I attempt to print argv[0]
in GDB, but I get the "generic pointer"
error:
Breakpoint 1, 0x080483e5 in main ()
(gdb) x/s **(0xc + $ebp)
Attempt to dereference a generic pointer.
然而,通过使用X / XW'手动提领了几下,我
最终能够打印的argv [0]
(和的argv [1]
)
(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)
但是,这是恼人的和间接的(如指针编程是不会要?)
But this is annoying and indirect (as pointer programming is wont to be?)
推荐答案
解决方案是取消引用之前投指针。
The solution is to cast the pointers before dereferencing them.
例如,拿起我们不放过上面:
For example, picking up where we left off above:
(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"
请注意堆栈地址位于0xC + $ EBP
本身就是一个指针
该堆栈单元的内容,所以我们需要字符***
而不是的char **
。
Note that the stack address 0xc + $ebp
is itself a pointer to the
contents of that stack location, and so we need char ***
and not
char **
.
这篇关于GDB:尝试取消引用通用指针的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!