weak symbol的设计意图

用nm察看elf文件时，会输出各种符号，其中有两种符号，比较特别

           "V"
           "v" The symbol is a weak object.  When a weak defined symbol is linked with a normal defined
               symbol, the normal defined symbol is used with no error.  When a weak undefined symbol is
               linked and the symbol is not defined, the value of the weak symbol becomes zero with no error.
               On some systems, uppercase indicates that a default value has been specified.

           "W"
           "w" The symbol is a weak symbol that has not been specifically tagged as a weak object symbol.
               When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol
               is used with no error.  When a weak undefined symbol is linked and the symbol is not defined,
               the value of the symbol is determined in a system-specific manner without error.  On some
               systems, uppercase indicates that a default value has been specified.
上面是nm的manpage内容，只描述了what，没有why

在网上搜索许久，只能看到what和how，也没有讲why

不行，我要知其然更知其所以然

于是在wikipedia上找到了答案

简言之，就是一种link-time的overloading

weak symbol的设计意图

原文：http://blog.csdn.net/happen23/article/details/45023609