php[6491]: segfault at * rip * rsp * error 6

cxg518

内存不足不会引起段错误,段错误通常出现在访问了非法的地址后，非法地址分为3类：
    1. 访问的地址没有对应的物理内存。这类错误主要出现在越界访问，例如栈越界。比如说当前进程的栈只有5个页和它对应，共20k大小（x86平台），你访问的地址超过了这个范围，就会发生segmentation fault；
    2.对地址的操作与该地址的属性不符合。例如该地址对应的内存是只读的，如文本段，你却试图进行写操作；
    3.低权限访问高权限地址。这类情况发生在用户进程试图访问内核空间。例如x86中，TASK_SIZE以上的地址为内核空间，当用户态进程试图访问这些地址时，就会出现”segmentation fault“了
具体格式如下：

　　解决方法：
　　readelf -s a.out 可以查看符号表，根据rip所指的地址就能找到对应的函数了。
　　
objdump -d a.out > a.s 可以反汇编，不但能找到函数，还能直接定位到指令。
　　注：本人测试没找到map文件（即a.out），故报错的程序和函数并未找到，有找到的博友可以回复告知操作过程，在此谢过！
　　另附上老外的原理解释：
　　

• Well the rip pointer tells you the instruction that caused the crash. You need to look it up in a map file.
  
• 
  
• In the map file you will have a list of functions and their starting address. When you load the application it is loaded to a base address.
  
• The rip pointer - the base address gives you the map file address. If you then search through the map file for a function that starts at
  
• an address slightly lower than your rip pointer and is followed, in the list, by a function with a higher address you have located the function that crashed.
  
• 
  
• From there you need to try and identify what went wrong in your code. Its not much fun but it, at least, gives you a starting point.
  
• 
  
• Edit: The "segfault at" bit is telling you, i'd wager, that you have dereferenced a NULL pointer. The rsp is the current stack pointer.
  
• Alas its probably not all that useful. With a memory dump you "may" be able to figure out more accurately where you'd got to in the function
  
• but it can be really hard to work out, exactly, where you are in an optimised build
  

　　


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