A segmentation fault (often shortened to segfault) is a particular error condition that can occur during the operation of computer software. In short, 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 (e.g., attempts to write to a read-only location, or to overwrite part of the operating system). Systems based on processors like the Motorola 68000 tend to refer to these events as Address or Bus errors.

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 It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, "segmentation fault" being an example.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 invalid memory receives the SIGSEGV signal. On Microsoft Windows, a process that accesses invalid memory receives the STATUS_ACCESS_VIOLATION exception.

所谓的段错误 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

写程序好多年了，Segment fault 是许多C程序员头疼的提示。指针是好东西，但是随着指针的使用却诞生了这个同样威力巨大的恶魔。

Segment fault 之所以能够流行于世，是与Glibc库中基本所有的函数都默认型参指针为非空有着密切关系的。

不知道什么时候才可以有能够处理NULL的glibc库诞生啊！

不得已，我现在为好多的函数做了衣服，避免glibc的函数被NULL给感染，导致我的Mem访问错误，而我还不知道NULL这个病毒已经在侵蚀我的身体了。

Segment fault 永远的痛......

sudo apt-get install gcc-4.0 libc6-dev
sudo apt-get install gdb

$ gcc -o segerr segerr.c
$ ./segerr
10
段错误

$ gcc -g -o segerr segerr.c        --加-g选项查看调试信息
$ gdb ./segerr
(gdb) l                    --用l(list)显示我们的源代码
1       ＃i nclude <stdio.h>
2
3       int
4       main()
5       {
6               int i = 0;
7
8               scanf ("%d", i); 
9               printf ("%d\n", i);
10              return 0;
(gdb) b 8                --用b(break)设置断点
Breakpoint 1 at 0x80483b7: 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/segerr

Breakpoint 1, main () at segerr.c:8
8               scanf ("%d", i);  --[试图往地址0处写进一个值]
(gdb) n                    --用n(next)执行下一步
10

Program 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

$ man 7 signal | grep SEGV
Reformatting signal(7), please wait...
       SIGSEGV      11       Core    Invalid memory reference

$ gcc -g -o segerr segerr.c
$ gdb ./segerr
(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/segerr

Program received signal SIGSEGV, Segmentation fault.
0x08048516 in main () at segerr.c:10
10              *p = 'x';
(gdb)

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 ++]);
　　}

Catch segmentation faults in programs