二月 29, 2012

cygwin 下 gdb 调试段错误

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段错误的常见形式

在编程中以下几类做法容易导致段错误,基本是是错误地使用指针引起的

1)访问系统数据区,尤其是往 系统保护的内存地址写数据最常见就是给一个指针以0地址

2)内存越界(数组越界,变量类型不一致等) 访问到不属于你的内存区域解决方法我们在用C/C++语言写程序的时侯,内存管理的绝大部分工作都是需要我们来做的。实际上,内存管理是一个比较繁琐的工作,无论你多高明,经验多丰富,难 免会在此处犯些小错误,而通常这些错误又是那么的浅显而易于消除。但是手工“除虫”(debug),往往是效率低下且让人厌烦的,本文将就”段错误”这个 内存访问越界的错误谈谈如何快速定位这些”段错误”的语句。

下面将就以下的一个存在段错误的程序介绍几种调试方法:

1 dummy_function (void)
2 {
3 unsigned char *ptr = 0x00;
4 *ptr = 0x00;
5 }
6
7 int main (void)
8 {
9 dummy_function ();
10
11 return 0;
12 }

作为一个熟练的C/C++程序员,以上代码的bug应该是很清楚的,因为它尝试操作地址为0的内存区域,而这个内存区域通常是不可访问的禁区,当然就会出错了。我们尝试编译运行它:

xiaosuo@gentux test $ ./a.out
段错误

出错并退出。

利用gdb逐步查找段错误

这种方法也是被大众所熟知并广泛采用的方法,首先我们需要一个带有调试信息的可执行程序,所以我们加上“-g -rdynamic”的参数进行编译,然后用gdb调试运行这个新编译的程序,具体步骤如下:

xiaosuo@gentux test $ gcc -g -rdynamic d.c
xiaosuo@gentux test $ gdb ./a.out
GNU gdb 6.5
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type “show copying” to see the conditions.
There is absolutely no warranty for GDB. Type “show warranty” for details.
This GDB was configured as “i686-pc-linux-gnu”…Using host libthread_db library “/lib/libthread
(gdb) r
Starting program: /home/xiaosuo/test/a.out
Program received signal SIGSEGV, Segmentation fault.
0x08048524 in dummy_function () at d.c:4
4 *ptr = 0x00;
(gdb)

不用一步步调试我们就找到了出错位置d.c文件的第4行,其实就是如此的简单。

从这里我们还发现进程是由于收到了SIGSEGV信号而结束的。通过进一步的查阅文档(man 7 signal),我们知道SIGSEGV默认handler的动作是打印”段错误”的出错信息,并产生Core文件,由此我们又产生了方法二。

分析Core文件

The default action of certain signals is to cause a process to terminate and produce a core dump file, a disk file containing an image of the process’s memory at the time of termination. A list of the signals which cause a process to dump core can be found in signal(7).

以 上资料摘自man page(man 5 core)。不过奇怪了,我的系统上并没有找到core文件。后来,忆起为了渐少系统上的垃圾文件的数量,禁止了core文件的生成,查看了以下果真如此,将系统的core文件的大小限制在512K大小,再试:

xiaosuo@gentux test $ ulimit -c
0
xiaosuo@gentux test $ ulimit -c 1000
xiaosuo@gentux test $ ulimit -c
1000
xiaosuo@gentux test $ ./a.out
段错误 (core dumped)
xiaosuo@gentux test $ ls
a.out core d.c f.c g.c pango.c test_iconv.c test_regex.c

core文件终于产生了,用gdb调试一下看看吧:

xiaosuo@gentux test $ gdb ./a.out core
GNU gdb 6.5
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type “show copying” to see the conditions.
There is absolutely no warranty for GDB. Type “show warranty” for details.
This GDB was configured as “i686-pc-linux-gnu”…Using host libthread_db library “/lib/libthread.
warning: Can’t read pathname for load map: 输入/输出错误。
Reading symbols from /lib/lib6…done.
Loaded symbols for /lib/li6
Reading symbols from /lib/ld-.2…done.
Loaded symbols for /lib/ld-linux.s2
Core was generated by `./a.out’.
Program terminated with signal 11, Segmentation fault.
#0 0x08048524 in dummy_function () at d.c:4
4 *ptr = 0x00;dfg

利用backtrace和objdump进行分析

[code lang=”c”]

#include <execinfo.h>

#include <stdio.h>

#include <stdlib.h>

#include <signal.h>

/* A dummy function to make the backtrace more interesting. */

void

dummy_function (void)

{

unsigned char *ptr = 0x00;

*ptr = 0x00;

}

void dump(int signo)

{

void *array[10];

size_t size;

char **strings;

size_t i;

size = backtrace (array, 10);

strings = backtrace_symbols (array, size);

printf ("Obtained %zd stack frames.\n", size);

for (i = 0; i < size; i++)

printf ("%s\n", strings[i]);

free (strings);

exit(0);

}

int

main (void)

{

signal(SIGSEGV, &dump);

dummy_function ();

return 0;

}

[/code]

运行结果:xiaosuo@gentux test $ gcc -g -rdynamic g.c

xiaosuo@gentux test $ ./a.out

Obtained 5 stack frames.

./a.out(dump+0x19) [0x80486c2]

[0xffffe420]

./a.out(main+0x35) [0x804876f]

Category : C/C++Linux/Unix

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