Linux下的段错误产生的原因及调试方法

叁歲伎倆 2024-04-19 13:53 63阅读 0赞

简而言之,**产生段错误就是访问了错误的内存段**，一般是你没有权限，或者根本就不存在对应的物理内存,尤其常见的是访问0地址.  
  
一般来说, **段错误就是指访问的内存超出了系统所给这个程序的内存空间**，通常这个值是由gdtr来保存的，他是一个**48位的寄存器**，其中的32位是保存由它指向的 gdt表，后13位保存相应于gdt的下标，最后3位包括了程序是否在内存中以及程序的在cpu中的运行级别,指向的**gdt是由以64位为一个单位的表**， 在这张表中就保存着程序运行的**代码段以及数据段的起始地址以及与此相应的段限和页面交换还有程序运行级别还有内存粒度等等的信息**。**一旦一个程序发生了越界 访问，cpu就会产生相应的异常保护，于是segmentation fault就出现了**.  
  
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：

<table> 
 <tbody> 
  <tr> 
   <td> &nbsp;&nbsp;&nbsp;&nbsp; 1&nbsp; dummy_function (void)<br>&nbsp;&nbsp;&nbsp;&nbsp; 2&nbsp; { <br>&nbsp;&nbsp;&nbsp;&nbsp; 3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; unsigned char *ptr = 0x00;<br>&nbsp;&nbsp;&nbsp;&nbsp; 4&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *ptr = 0x00;<br>&nbsp;&nbsp;&nbsp;&nbsp; 5&nbsp; }<br>&nbsp;&nbsp;&nbsp;&nbsp; 6<br>&nbsp;&nbsp;&nbsp;&nbsp; 7&nbsp; int main (void)<br>&nbsp;&nbsp;&nbsp;&nbsp; 8&nbsp; { <br>&nbsp;&nbsp;&nbsp;&nbsp; 9&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; dummy_function ();<br>&nbsp;&nbsp;&nbsp; 10<br>&nbsp;&nbsp;&nbsp; 11&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return 0;<br>&nbsp;&nbsp;&nbsp; 12&nbsp; }<br></td> 
  </tr> 
 </tbody> 
</table>

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:

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ ./a.out<br>段错误<br></td> 
  </tr> 
 </tbody> 
</table>

果然 不出所料，它出错并退出了。  
**1.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:

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ gcc -g -rdynamic d.c<br>xiaosuo@gentux test $ gdb ./a.out<br>GNU gdb 6.5<br>Copyright (C) 2006 Free Software Foundation, Inc.<br>GDB is free software, covered by the GNU General Public License, and you are<br>welcome to change it and/or distribute copies of it under certain conditions.<br>Type "show copying" to see the conditions.<br>There is absolutely no warranty for GDB.&nbsp; Type "show warranty" for details.<br>This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".<br><br>(gdb) r<br>Starting program: /home/xiaosuo/test/a.out<br><br><span style="color:#ff0000;">Program received signal SIGSEGV, Segmentation fault.<br>0x08048524 in dummy_function () at d.c:4<br>4&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *ptr = 0x00;</span><br>(gdb)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <br></td> 
  </tr> 
 </tbody> 
</table>

哦？！ 好像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。  
**2.分析Core文件：**  
Core文件是什么呢？

<table> 
 <tbody> 
  <tr> 
   <td> The&nbsp; 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&nbsp; at the time of termination.&nbsp; A list of the signals which cause a process to dump core can be found in signal(7). </td> 
  </tr> 
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</table>

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拉圾文件的数量（本人有些洁癖，这也是我喜欢Gentoo的原因 之一），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:

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ <span style="color:#ff0000;">ulimit -c</span><br>0&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; (当前值)<br>xiaosuo@gentux test $ <span style="color:#ff0000;">ulimit -c 1000</span><br>xiaosuo@gentux test $ ulimit -c<br>1000 (修改后)<br>xiaosuo@gentux test $ ./a.out<br>段错误 (core dumped)<br>xiaosuo@gentux test $ ls<br>a.out&nbsp; <strong><span style="color:#ff0000;">core&nbsp;</span></strong> d.c&nbsp; f.c&nbsp; g.c&nbsp; pango.c&nbsp; test_iconv.c&nbsp; test_regex.c<br></td> 
  </tr> 
 </tbody> 
</table>

core 文件终于产生了，用gdb调试一下看看吧:

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ gdb ./a.out <span style="color:#ff0000;">core</span><br>GNU gdb 6.5<br>Copyright (C) 2006 Free Software Foundation, Inc.<br>GDB is free software, covered by the GNU General Public License, and you are<br>welcome to change it and/or distribute copies of it under certain conditions.<br>Type "show copying" to see the conditions.<br>There is absolutely no warranty for GDB.&nbsp; Type "show warranty" for details.<br>This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".<br><br><br>warning: Can't read pathname for load map: 输入/输出错误.<br>Reading symbols from /lib/libc.so.6...done.<br>Loaded symbols for /lib/libc.so.6<br>Reading symbols from /lib/ld-linux.so.2...done.<br>Loaded symbols for /lib/ld-linux.so.2<br>Core was generated by `./a.out'.<br>Program terminated with signal 11, Segmentation fault.<br>#0&nbsp; 0x08048524 in dummy_function () at d.c:4<br>4&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *ptr = 0x00;<br></td> 
  </tr> 
 </tbody> 
</table>

哇，好历害，还是一步就定位到了错误所在地，佩服一下 Linux/Unix系统的此类设计。  
接着考虑下去，以前用windows系统下的ie的时侯，有时打开某些网页，会出现“运行时错误”，这个时 侯如果恰好你的机器上又装有windows的编译器的话，他会弹出来一个对话框，问你是否进行调试，如果你选择是，编译器将被打开，并进入调试状态，开始 调试。

Linux下如何做到这些呢？我的大脑飞速地旋转着，有了，让它**在SIGSEGV的handler中调用gdb**，于是第三个方法又诞生了

**3.段错误时启动调试:**

<table> 
 <tbody> 
  <tr> 
   <td> #include &lt;stdio.h&gt;<br>#include &lt;stdlib.h&gt;<br>#include &lt;signal.h&gt;<br>#include &lt;string.h&gt;<br><br>void dump(int signo)<br>{ <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; char buf[1024];<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; char cmd[1024];<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; FILE *fh;<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; snprintf(buf, sizeof(buf), "/proc/%d/cmdline", getpid());<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if(!(fh = fopen(buf, "r")))<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; exit(0);<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if(!fgets(buf, sizeof(buf), fh))<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; exit(0);<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; fclose(fh);<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if(buf[strlen(buf) - 1] == '\n')<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; buf[strlen(buf) - 1] = '\0';<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; snprintf(cmd, sizeof(cmd), "gdb %s %d", buf, getpid());<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; system(cmd);<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; exit(0);<br>}<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; void<br>dummy_function (void)<br>{ <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; unsigned char *ptr = 0x00;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *ptr = 0x00;<br>}<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; int<br>main (void)<br>{ <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; signal(SIGSEGV, &amp;dump);<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; dummy_function ();<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return 0;<br>}<br></td> 
  </tr> 
 </tbody> 
</table>

编译运行效果如下:

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ <span style="color:#ff0000;"><strong>g</strong><strong>cc -g -rdynamic f.c</strong></span><br>xiaosuo@gentux test $ ./a.out<br>GNU gdb 6.5<br>Copyright (C) 2006 Free Software Foundation, Inc.<br>GDB is free software, covered by the GNU General Public License, and you are<br>welcome to change it and/or distribute copies of it under certain conditions.<br>Type "show copying" to see the conditions.<br>There is absolutely no warranty for GDB.&nbsp; Type "show warranty" for details.<br>This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".<br><br>Attaching to program: /home/xiaosuo/test/a.out, process 9563<br>Reading symbols from /lib/libc.so.6...done.<br>Loaded symbols for /lib/libc.so.6<br>Reading symbols from /lib/ld-linux.so.2...done.<br>Loaded symbols for /lib/ld-linux.so.2<br>0xffffe410 in __kernel_vsyscall ()<br>(gdb)<strong><span style="color:#ff0000;"> bt</span></strong><br>#0&nbsp; 0xffffe410 in __kernel_vsyscall ()<br>#1&nbsp; 0xb7ee4b53 in waitpid () from /lib/libc.so.6<br>#2&nbsp; 0xb7e925c9 in strtold_l () from /lib/libc.so.6<br>#3&nbsp; 0x08048830 in dump (signo=11) at f.c:22<br>#4&nbsp; &lt;signal handler called&gt;<br>#5&nbsp; 0x0804884c in dummy_function () at f.c:31<br>#6&nbsp; 0x08048886 in main () at f.c:38<br></td> 
  </tr> 
 </tbody> 
</table>

怎么样？是不是依旧很 酷？  
以上方法都是在系统上有gdb的前提下进行的，如果没有呢？其实**glibc为我们提供了此类能够dump栈内容的函数簇，详见/usr /include/execinfo.h**（这些函数都没有提供man page，难怪我们找不到），另外你也可以通过[**gnu的手册**][gnu]进 行学习。  
**4.利用backtrace和objdump进行分析:**  
重 写的代码如下:

<table> 
 <tbody> 
  <tr> 
   <td> <p>#include &lt;<span style="color:#ff0000;"><strong>execinfo.h</strong></span>&gt;<br>#include &lt;stdio.h&gt;<br>#include &lt;stdlib.h&gt;<br>#include &lt;signal.h&gt;<br><br>/* A dummy function to make the backtrace more interesting. */</p> <p>void&nbsp; dummy_function (void)<br>{ <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; unsigned char *ptr = 0x00;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *ptr = 0x00;<br>}<br><br>void dump(int signo)<br>{ <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; void *array[10];<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; size_t size;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; char **strings;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; size_t i;<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; size = <strong><span style="color:#ff0000;">backtrace</span></strong> (array, 10);<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; strings = <strong><span style="color:#ff0000;">backtrace_symbols</span></strong> (array, size);<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; printf ("Obtained %zd stack frames.\n", size);<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; for (i = 0; i &lt; size; i++)<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; printf ("%s\n", strings[i]);<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; free (strings);<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; exit(0);<br>}<br><br>int&nbsp; main (void)<br>{ <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; signal(SIGSEGV, &amp;dump);<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; dummy_function ();<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return 0;<br>}</p> </td> 
  </tr> 
 </tbody> 
</table>

编译运行结果如下：

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ gcc -g -rdynamic g.c<br>xiaosuo@gentux test $ ./a.out<br>Obtained 5 stack frames.<br><strong><span style="color:#ff0000;">./a.out(dump+0x19) [0x80486c2]<br>[0xffffe420]<br>./a.out(main+0x35) [0x804876f]<br>/lib/libc.so.6(__libc_start_main+0xe6) [0xb7e02866]<br>./a.out [0x8048601]</span></strong><br></td> 
  </tr> 
 </tbody> 
</table>

这次你可能有些失望,似乎没能给出足够的信息来标示错误,不急, 先看看能分析出来什么吧,用objdump反汇编程序,找到地址0x804876f对应的代码位置:

<table> 
 <tbody> 
  <tr> 
   <td> xiaosuo@gentux test $ <span style="color:#ff0000;"><strong>o</strong><strong>bjdump -d a.out</strong></span><br></td> 
  </tr> 
 </tbody> 
</table>

<table> 
 <tbody> 
  <tr> 
   <td> &nbsp;8048765:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; e8 02 fe ff ff&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; call&nbsp;&nbsp; 804856c &lt;signal@plt&gt;<br>&nbsp;804876a:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; e8 25 ff ff ff&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; call&nbsp;&nbsp; 8048694 &lt;dummy_function&gt;<br>&nbsp;<span style="color:#ff0102;">804876f</span>:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; b8 00 00 00 00&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; mov&nbsp;&nbsp;&nbsp; $0x0,%eax<br>&nbsp;8048774:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; c9&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; leave<br></td> 
  </tr> 
 </tbody> 
</table>

我们还是找到了在哪个函数 (dummy\_function)中出错的,信息已然不是很完整,不过有总比没有好的啊!  
**后记:**  
本文给出了分析"段错误"的几种方法,不要认为这是与孔乙己先生的"回"字四种写法一样的哦,因为每种方法都 有其自身的适用范围和适用环境,请酌情使用,或遵医嘱。

转载于:https://my.oschina.net/alphajay/blog/7736

[gnu]: http://www.gnu.org/software/libc/manual/html_node/Backtraces.html