c - GDB 错误

Question

因此，我正在学习如何用 C 语言编程，并且正在(或者至少，尝试)享受 GDB 带来的乐趣。

所以我写了这个简单的代码:

#include <stdio.h> 

int main (int argc, char *argv[]){

int i;

int n = atoi(argv[2]); 

for (i=0; i<n ; i++){
    printf("%s \n",i+1,argv[1]); // prints the string provided in 
}                                // the arguments for n times
return 0;
}

我正尝试使用 GDB 获取一些信息。所以我用它来尝试从内存地址获取参数，但这是我得到的:

(gdb) break main
Breakpoint 1 at 0x4005d7: file repeat2.c, line 14.
(gdb) break 17
Breakpoint 2 at 0x40062c: file repeat2.c, line 17.
(gdb) run hello 5
Starting program: /root/Scrivania/Programmazione/repeat2 hello 5
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7ffff7ffa000

Breakpoint 1, main (argc=3, argv=0x7fffffffe948) at repeat2.c:14
14      int n = atoi(argv[2]);
(gdb) cont
Continuing.
1    ------>     hello 
2    ------>     hello 
3    ------>     hello 
4    ------>     hello 
5    ------>     hello 

Breakpoint 2, main (argc=3, argv=0x7fffffffe948) at repeat2.c:18
18  return 0;
(gdb) x/3xw 0x7fffffffe948     (I try to read what argv contains)
0x7fffffffe948: 0xffffebbc  0x00007fff  0xffffebe3
(gdb) x/s 0xffffebbc           (I try to read one of the argoments in the array)
0xffffebbc:  <Address 0xffffebbc out of bounds>

为什么我总是收到这个错误？我在 64 位上，我正在使用 Kali Linux

程序如果编译成功，只是我不明白为什么我不能用 GDB 读取这些值。

Answer 1

@DrakaSAN 发现了您程序中的错误。至于你的 gdb 问题:

x/3xw 打印出 3 个 4 字节的单词。 argv 是一个 char * 指针数组。由于您使用的是 64 位系统，指针是 8 个字节，因此您要使用 g(巨型，8 个字节)或 a 而不是 w (地址)，它将自动选择正确的尺寸:

(gdb) break 7
Breakpoint 1 at 0x40058c: file repeat2.c, line 7.
(gdb) run hello 5
Starting program: /tmp/repeat2 hello 5

Breakpoint 1, main (argc=3, argv=0x7fffffffdfe8) at repeat2.c:7
7   int n = atoi(argv[2]); 
(gdb) x/3xg 0x7fffffffdfe8
0x7fffffffdfe8: 0x00007fffffffe365  0x00007fffffffe372
0x7fffffffdff8: 0x00007fffffffe378
(gdb) x/3xa 0x7fffffffdfe8
0x7fffffffdfe8: 0x7fffffffe365  0x7fffffffe372
0x7fffffffdff8: 0x7fffffffe378
(gdb) x/s 0x7fffffffe365
0x7fffffffe365: "/tmp/repeat2"
(gdb) x/s 0x7fffffffe372
0x7fffffffe372: "hello"
(gdb) x/s 0x7fffffffe378
0x7fffffffe378: "5"

感谢@adpeace 建议使用 a 修饰符。