Understand Weak Symbols by Examples

Wikipedia defines the weak symbols: "In computing, a weak symbol is a symbol definition in an object file or dynamic library that may be overridden by other symbol definitions, its value will be zero if no definition found by loader." In other words, we can define a symbol that doesn't need to be resolved at link time. It is a very well-known feature and used a lot in Linux Kernel, Glibc, and so on.