gdb 无法访问内存地址错误 [英] gdb can't access memory address error

问题描述

这是我的 disas 代码:

here is my disas code:

   0x0804844d <+0>:     push   %ebp
   0x0804844e <+1>:     mov    %esp,%ebp
   0x08048450 <+3>:     and    $0xfffffff0,%esp
   0x08048453 <+6>:     sub    $0x20,%esp
   0x08048456 <+9>:     movl   $0x8048540,(%esp)
   0x0804845d <+16>:    call   0x8048310 <puts@plt>
   0x08048462 <+21>:    lea    0x1c(%esp),%eax
   0x08048466 <+25>:    mov    %eax,0x4(%esp)
   0x0804846a <+29>:    movl   $0x8048555,(%esp)
   0x08048471 <+36>:    call   0x8048320 <scanf@plt>
   0x08048476 <+41>:    mov    0x1c(%esp),%eax
   0x0804847a <+45>:    cmp    $0x208c,%eax
   0x0804847f <+50>:    jne    0x804848f <main+66>
   0x08048481 <+52>:    movl   $0x8048558,(%esp)
   0x08048488 <+59>:    call   0x8048310 <puts@plt>
   0x0804848d <+64>:    jmp    0x804849b <main+78>
=> 0x0804848f <+66>:    movl   $0x8048569,(%esp)
   0x08048496 <+73>:    call   0x8048310 <puts@plt>
   0x0804849b <+78>:    mov    $0x0,%eax
   0x080484a0 <+83>:    leave  
   0x080484a1 <+84>:    ret 

我要检查的是 $0x208c.当我输入 x/xw 0x208c 时，它返回错误，提示无法访问地址 0x208c 的内存.当我输入 Info registers 并查看 eax 时，它会显示我提供的值.所以基本上这个程序比较两个值，并根据它打印出一些东西.问题是这是大学的作业，我没有代码.希望你能帮忙.谢谢.

what i'm tring to examine is $0x208c. When I type x/xw 0x208c it gives me back error which says Cannot access memory at address 0x208c. When i type Info registers and look at eax it says the value which i provided. So basically this program compares two values and depending on that prints something out.The problem is that this is homework from university and I have not got code. Hope you can help. Thank you.

推荐答案

当我输入 x/xw 0x208c 时，它会返回错误提示 Cannot access memory at address 0x208c

When I type x/xw 0x208c it gives me back error which says Cannot access memory at address 0x208c

你的程序的反汇编表明它做了这样的事情:

The disassembly for your program says that it does something like this:

puts("some string");
int i;
scanf("%d", &i);  // I don't know what the actual format string is.
                  // You can find out with x/s 0x8048555
if (i == 0x208c) { ... } else { ... }

换句话说，0x208c 是您的程序硬编码在其中的值 (8332)，并且 不是指针.因此，GDB 完全正确地告诉您，如果您将 0x208c 解释为指针，则该指针不指向可读内存.

In other words, the 0x208c is a value (8332) that your program has hard-coded in it, and is not a pointer. Therefore, GDB is entirely correct in telling you that if you interpret 0x208c as a pointer, that pointer does not point to readable memory.

我终于想出用 print 语句代替 x/xw

i finally figured out to use print statement instead of x/xw

您似乎不理解 print 和 examine 命令之间的区别.考虑这个例子:

You appear to not understand the difference between print and examine commands. Consider this example:

int foo = 42;
int *pfoo = &foo;

如上，print pfoo 会给你foo 的地址，x pfoo 会给你value 存储在该地址(即 foo 的值).

With above, print pfoo will give you the address of foo, and x pfoo will give you the value stored at that address (i.e. the value of foo).

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