GDB学习:4

在写程序的时候,最容易遇到的一类问题基本上就是段错误的问题,可以借助gdb来解决这类问题,下面是个例子:

int main(void)
{
    int man = 0;
    scanf("%d", man);
    return 0;

}
运行输入会出现段错误:

root@wc:~/Codes/Learn/learnGDB# ./segfault 
1
段错误 (核心已转储)
下面开始调试:

(gdb) start
Temporary breakpoint 1 at 0x40051e: file segfault.c, line 11.
Starting program: /root/Codes/Learn/learnGDB/segfault 

Temporary breakpoint 1, main () at segfault.c:11
11	    int man = 0;
(gdb) n
12	    scanf("%d", man);
(gdb) n
123

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7a6c742 in _IO_vfscanf_internal (s=<optimized out>, format=<optimized out>, argptr=argptr@entry=0x7fffffffdf28, errp=errp@entry=0x0) at vfscanf.c:1857
1857	vfscanf.c: 没有那个文件或目录.
从上面可以看出,段错误的原因是scanf函数调用产生了一些问题,回到源文件查看会发现scanf的里面少加了一个&符号,所以导致了段错误。


下面在看一个段错误的实例:

#include<stdio.h>
int main(void)
{
    int sum = 0, i = 0;
    char input[5];scanf("%s", input);
    for (i = 0; input[i] != '\0'; i++) {
        if (input[i] < '0' || input[i] > '9') {
            printf("Invalid input!\n");
            sum = -1;
            break;
        }
        sum = sum*10 + input[i] - '0';
    }
    printf("input=%d\n", sum);
    return 0;
}
这个程序输入一个较长的字符串的时候就会出现段错误:

root@wc:~/Codes/Learn/learnGDB# ./segfault1
1234567890987654321234567890987654323456789098765432345678
Invalid input!
input=-1
段错误 (核心已转储)
使用gdb调试查看:

(gdb) n
12345678987654321234567898765432123456789876543212345678987654321
13	    for (i = 0; input[i] != '\0'; i++) {
(gdb) c
Continuing.
Invalid input!
input=-1

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400659 in main () at segfault1.c:25
25	}
(gdb) l
20	        sum = sum*10 + input[i] - '0';
21	
22	    }
23	    printf("input=%d\n", sum);
24	    return 0;
25	}
(gdb) n
12345678987654321234567898765432123456789876543212345678987654321
13	    for (i = 0; input[i] != '\0'; i++) {
(gdb) c
Continuing.
Invalid input!
input=-1

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400659 in main () at segfault1.c:25
25	}
(gdb) l
20	        sum = sum*10 + input[i] - '0';
21	
22	    }
23	    printf("input=%d\n", sum);
24	    return 0;
25	}
程序显示段错误发生在25行,但是这一行什么都没有,这可以算是一条规律,如果某个函数中发生访问越界,很可能并不立即产生段错误,而在函数返回时却产生段错误。这里实际上还是scanf函数存在问题。




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