为什么会出现段错误

为什么会出现段错误？
介绍
一句话来说，段错误是指访问的内存超出了系统给这个程序所设定的内存空间，例如访问了不存在的内存地址、访问了系统保护的内存地址、访问了只读的内存地址等等情况。这里贴一个对于“段错误”的准确定义（参考 Answers.com）：

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 It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, "segmentation fault" being an example.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.
段错误产生的原因
访问不存在的内存地址
访问系统保护的内存地址
访问只读的内存地址
栈溢出
段错误信息的获取
程序发生段错误时，提示信息很少，下面有几种查看段错误的发生信息的途径。

dmesg
dmesg 可以在应用程序 crash 掉时，显示内核中保存的相关信息。如下所示，通过 dmesg 命令可以查看发生段错误的程序名称、引起段错误发生的内存地址、指令指针地址、堆栈指针地址、错误代码、错误原因等。

$ dmesg
-g
使用 gcc 编译程序的源码时，加上-g 参数，这样可以使得生成的二进制文件中加入可以用于 gdb 调试的有用信息。

$ gcc -g -o segfault3 segfault3.c
nm
使用 nm 命令列出二进制文件中的符号表，包括符号地址、符号类型、符号名等，这样可以帮助定位在哪里发生了段错误。

$ nm segfault3
ldd
使用 ldd 命令查看二进制程序的共享链接库依赖，包括库的名称、起始地址，这样可以确定段错误到底是发生在了自己的程序中还是依赖的共享库中。

$ ldd ./segfault3