c - GDB和汇编: how to examine consts variables defined in heap?

Question

例如下面的代码“justatest”和格式“%s”在堆中定义:

char str[15]="justatest";
int main(){
    printf("%s",str);
    return 0;
}

在GDB中，我在调用printf之前得到了汇编代码:

=> 0x0804841f <+14>:    movl   $0x804a020,0x4(%esp)
   0x08048427 <+22>:    movl   $0x80484d8,(%esp)
   0x0804842e <+29>:    call   0x80482f0 <printf@plt>

我是否必须使用“x/s 0x804a020”和“x/s 0x80484d8”检查参数 1by1

或者堆中是否有一个我可以直接引用的常量表？

谢谢!

Answer 1

您对str驻留在堆上的理解不正确。它的全局变量存储在数据段中。关于打印全局变量，您可以在我的 GNU/Linux 终端上执行以下操作。

$ gcc -g -Wall hello.c
$ gdb -q ./a.out 
Reading symbols from /home/mantosh/practice/a.out...done.
(gdb) break main
Breakpoint 1 at 0x400524: file hello.c, line 6.
(gdb) run
Starting program: /home/mantosh/practice/a.out 

Breakpoint 1, main () at bakwas.c:6
6       printf("%s",str);
(gdb) disassemble main
Dump of assembler code for function main:
   0x0000000000400520 <+0>: push   %rbp
   0x0000000000400521 <+1>: mov    %rsp,%rbp
=> 0x0000000000400524 <+4>: mov    $0x601020,%esi
   0x0000000000400529 <+9>: mov    $0x4005e4,%edi
   0x000000000040052e <+14>:    mov    $0x0,%eax
   0x0000000000400533 <+19>:    callq  0x4003f0 <printf@plt>
   0x0000000000400538 <+24>:    mov    $0x0,%eax
   0x000000000040053d <+29>:    pop    %rbp
   0x000000000040053e <+30>:    retq   
End of assembler dump.

(gdb) p str
$1 = "justatest\000\000\000\000\000"
(gdb) p &str
$2 = (char (*)[15]) 0x601020

// These are addresses of two arguments which would be passed in printf.
// From assembly instruction we can verify that before calling the printf
// these are getting stored into the registers.
(gdb) x/s 0x4005e4
0x4005e4:    "%s"
(gdb) x/s 0x601020
0x601020 <str>:  "justatest