段错误是什么
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 It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, “segmentation fault” being an example.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
dmesg
-g
nm
ldd
段错误的调试方法
gdb
1、仅当能确定程序一定会发生段错误的情况下使用。
2、当程序的源码可以获得的情况下，使用-g参数编译程序。
3、一般用于测试阶段，生产环境下gdb会有副作用：使程序运行减慢，运行不够稳定，等等。
4、即使在测试阶段，如果程序过于复杂，gdb也不能处理。
使用core文件和gdb
ulimit -c
注意
1、出现段错误时，首先应该想到段错误的定义，从它出发考虑引发错误的原因。
2、在使用指针时，定义了指针后记得初始化指针，在使用的时候记得判断是否为NULL。
3、在使用数组时，注意数组是否被初始化，数组下标是否越界，数组元素是否存在等。
4、在访问变量时，注意变量所占地址空间是否已经被程序释放掉。
5、在处理变量时，注意变量的格式控制是否合理等。