总结段错误(Segmentation fault) (转)
段错误(Segmentation fault)
Segmentation fault
From Wikipedia, the free encyclopedia
A segmentation fault (often shortened to segfault ) is a particular error condition that can occur during the operation of computer software . A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed (for example, attempting to write to a read-only location, or to overwrite part of the operating system ).
Segmentation is one approach to memory management and protection in the operating system. It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, "segmentation fault" being an example. Some operating systems still have segmentation at some logical level although paging is used as the main memory management policy.
On Unix-like operating systems, a process that accesses an invalid memory address receives the SIGSEGV signal . On Microsoft Windows , a process that accesses invalid memory receives the STATUS_ACCESS_VIOLATION exception .
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[edit ] Example
Here is an example of ANSI C code that should create a segmentation fault on platforms with memory protection:
int main( void )
{
char * s = "hello world" ;
* s = 'H' ;
}
When the program containing this code is compiled , the string "hello world" is placed in the section of the program binary marked as read-only; when loaded, the operating system places it with other strings and constant data in a read-only segment of memory. When executed, a variable, s , is set to point to the string's location, and an attempt is made to write an H character through the variable into the memory, causing a segmentation fault. Compiling such a program with a compiler that does not check for the assignment of read-only locations at compile time, and running it on a Unix-like operating system produces the following runtime error :
$ gcc segfault.c -g -o segfault
$ ./segfault
Segmentation fault
Backtrace from gdb :
Program received signal SIGSEGV, Segmentation fault.
0x1c0005c2 in main () at segfault.c:6
6 *s = 'H';
The conditions under which segmentation violations occur and how they manifest themselves are specific to an operating system.
Because a very common program error is a null pointer dereference (a read or write through a null pointer, used in C to mean "pointer to no object" and as an error indicator), most operating systems map the null pointer's address such that accessing it causes a segmentation fault.
int * ptr = NULL;
* ptr = 1 ;
This sample code creates a null pointer , and tries to assign a value to its non-existent target. Doing so causes a segmentation fault at runtime on many operating systems.
Another way to cause a segmentation fault is to recurse without a base case, which causes a stack overflow :
int main( void )
{
main( ) ;
}
[edit ] Common causes
A few causes of segmentation fault can be summarized as follows,
- attempt to execute a program that does not compile correctly. Note that most compilers will not output a binary given a compile-time error.
- Buffer overflow
- using uninitialized pointers
- dereference NULL pointers
- attempt to access memory the program does not own.
- attempt to alter memory the program does not own (storage violation ).
Generally, segmentation faults occur because: a pointer is either NULL, or points to random memory (probably never initialized to anything), or points to memory that has been freed/deallocated/"deleted".
e.g.
char * p1 = NULL; // Initialized to null, which is OK,
// (but cannot be dereferenced on many systems).
char * p2; // Not initialized at all.
char * p3 = new char [ 20] ; // Great! it's allocated,
delete [ ] p3; // but now it isn't anymore.
Now, referencing any of these variables could cause a segmentation fault.
我只是把排版弄舒服一点，很好的文章，虽然说是初级篇，但帮助确实很大。
1）往受到系统保护的内存地址写数据
有些内存是内核占用的或者是其他程序正在使用，为了保证系统正常工作，所以会受到系统的保护，而不能任意访问.
#include <stdio.h>
int
main()
{
int i = 0;
scanf ("%d", i); /* should have used &i */
printf ("%d/n", i);
return 0;
}
编译和执行一下, 咋一看，好像没有问题哦，不就是读取一个数据然后给输出来吗？
falcon@falcon:~/temp$ gcc -g -o segerr segerr.c –加-g选项查看调试信息
falcon@falcon:~/temp$ gdb ./segerr
GNU gdb 6.4-debian
Copyright 2005 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 “i486-linux-gnu”…Using host libthread_db library “/ lib/tls/i686/cmov/libthread_db.so.1″.(gdb) l –用l(list)显示我们的源代码
1 #include <stdio.h>
2
3 int
4 main()
5 {
6 int i = 0;
7
8 scanf (”%d”, i); /* should have used &i */
9 printf (”%d/n”, i);
10 return 0;
(gdb) b 8 –用b(break)设置断点
Breakpoint 1 at 0×80483b7: file segerr.c, line 8.
(gdb) p i –用p(print)打印变量i的值[看到没，这里i的值是0哦]
$1 = 0(gdb) r –用r(run)运行，直到断点处
Starting program: /home/falcon/temp/segerrBreakpoint 1, main () at segerr.c:8
8 scanf (”%d”, i); /* should have used &i */ –[试图往地址0处写进一个值]
(gdb) n –用n(next)执行下一步
10Program received signal SIGSEGV, Segmentation fault.
0xb7e9a1ca in _IO_vfscanf () from /lib/tls/i686/cmov/libc.so.6
(gdb) c –在上面我们接收到了SIGSEGV,然后用c(continue)继续执行
Continuing.Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
(gdb) quit –退出gdb
果然
我们“不小心”把&i写成了i
而我们刚开始初始化了i为0,这样我们不是试图向内存地址0存放一个值吗？
[补充：
可以通过man 7 signal查看SIGSEGV的信息。
falcon@falcon:~/temp$ man 7 signal | grep SEGV
Reformatting signal(7), please wait…
SIGSEGV 11 Core Invalid memory reference
例子2：
#include <stdio.h>
int
main()
{
char *p;
p = NULL;
*p = ‘x’;
printf(”%c”, *p);
return 0;
}
很容易发现，这个例子也是试图往内存地址0处写东西。
这里我们通过gdb来查看段错误所在的行
falcon@falcon:~/temp$ gcc -g -o segerr segerr.c
falcon@falcon:~/temp$ gdb ./segerr
GNU gdb 6.4-debian
Copyright 2005 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 “i486-linux-gnu”…Using host libthread_db library “/lib/tls/i686/cmov/libthread_db.so.1″.(gdb) r –直接运行，我们看到抛出段错误以后，自动显示出了出现段错误的行，这就是一个找出段错误的方法
Starting program: /home/falcon/temp/segerrProgram received signal SIGSEGV, Segmentation fault.
0×08048516 in main () at segerr.c:10
10 *p = ‘x’;
(gdb)
2）内存越界(数组越界，变量类型不一致等)
#include <stdio.h>
int
main()
{
char test[1];
printf(”%c”, test[1000000000]);
return 0;
}
这里是比较极端的例子，但是有时候可能是会出现的，是个明显的数组越界的问题
或者是这个地址是根本就不存在的
例子4：
#include <stdio.h>
int
main()
{
int b = 10;
printf(”%s/n”, b);
return 0;
}
我们试图把一个整数按照字符串的方式输出出去，这是什么问题呢？
由于还不熟悉调试动态链接库，所以
我只是找到了printf的源代码的这里
声明部分：
int pos =0 ,cnt_printed_chars =0 ,i ;
unsigned char *chptr ;
va_list ap ;
/* %s格式控制部分：*/
case 's':
chptr =va_arg (ap ,unsigned char *);
i =0 ;
while (chptr [i ])
{...
cnt_printed_chars ++;
putchar (chptr [i ++]);
}
由于我没有仔细分析代码，大致的原因也可能是地址越界的原因？不过我可不确定哦。
如果大家知道怎么调试printf函数，麻烦帮忙找出越界的真正原因吧，这个段错误也可能是
处在va_start和va_arg等函数里头？或者直接看看这个这里的printf源代码的分析，看看是否
可以找出出错的地方：
http://www.wangchao.net.cn/bbsdetail_47325.html
类似的，还有诸如：sprintf等的格式控制问题
比如，试图把char型或者是int的按照%s输出或存放起来，如：
#include <stdio.h>
#include <string.h>
char c=’c';
int i=10;
char buf[100];
printf(”%s”, c); //试图把char型按照字符串格式输出
printf(”%s”, i); //试图把int型按照字符串输出
memset(buf, 0, 100);
sprintf(buf, “%s”, c); //试图把char型按照字符串格式转换
memset(buf, 0, 100);
sprintf(buf, “%s”, i); //试图把int型按照字符串转换
3）其他
其实大概的原因都是一样的，就是段错误的定义。
但是更多的容易出错的地方就要自己不断积累，不段发现，或者吸纳前人已经积累的经验，并且注意避免再次发生。
例如：
<1>定义了指针后记得初始化，在使用的时候记得判断是否为NULL
<2>在使用数组的时候是否被初始化，数组下标是否越界，数组元素是否存在等
<3>在变量处理的时候变量的格式控制是否合理等
一个比较不错的例子：
我在进行一个多线程编程的例子里头，定义了一个线程数组
#define THREAD_MAX_NUM
pthread_t thread[THREAD_MAX_NUM];
用pthread_create创建了各个线程，然后用pthread_join来等待线程的结束
刚开始
我就直接等待，在创建线程都成功的时候，pthread_join能够顺利等待各个线程结束
但是一旦创建线程失败，那用pthread_join来等待那个本不存在的线程时自然会存在访问不存在的内存的情况，从而导致段错误的发生
后来
通过不断调试和思考，并且得到网络上资料的帮助，找到了上面的出错原因和解决办法
解决办法是：
在创建线程之前，先初始化我们的线程数组
在等待线程的结束的时候，判断线程是否为我们的初始值
如果是的话，说明我们的线程并没有创建成功，所以就不能等拉。
上面给出了很常见的几种出现段错误的地方，这样在遇到它们的时候就容易避免拉。
但是人有时候肯定也会有疏忽的，甚至可能还是会经常出现上面的问题或者其他常见的问题
所以对于一些大型一点的程序，如何跟踪并找到程序中的段错误位置就是需要掌握的一门技巧拉。
4。如何发现程序中的段错误？
2" color="black">有个网友对这个做了比较全面的总结，除了感谢他外，我把地址弄了过来。
文章名字叫《段错误bug的调试》
地址是：http://www.cublog.cn/u/5251/showart.php?id=173718
应该说是很全面的。
而我常用的调试方法有：
1）在程序内部的关键部位输出(printf)信息，那样可以跟踪 段错误 在代码中可能的位置
为了方便使用这种调试方法，可以用条件编译指令#ifdef DEBUG和#endif把printf函数给包含起来，编译的时候加上-DDEBUG参数就可以查看调试信息。反之，不加上该参数进行调试就可以。
2）用gdb来调试，在运行到段错误的地方，会自动停下来并显示出错的行和行号
这个应该是很常用的，如果需要用gdb调试，记得在编译的时候加上-g参数，用来显示调试信息
对于这个，网友在《段错误bug的调试》文章里创造性的使用这样的方法，使得我们在执行程序的时候就可以动态扑获段错误可能出现的位置：
通过扑获SIGSEGV信号来触发系统调用gdb来输出调试信息。
如果加上上面提到的条件编译，那我们就可以非常方便的进行段错误的调试拉。
3）还有一个catchsegv命令
通过查看帮助信息，可以看到