为什么vim -t ctags不够用？我不能跳到我想要的功能

ctags -R dirName, vim -t tags is very powerful, since after these two commands, you can now have the ability to navigate between code of that project, for example you can just jump to the code of certain function using :tag functionName, but this operation still have some pitfalls, as is usual case that one source code could include some function calls such as va_start(), while this code is not included in the dirName directory, so indexs are not generated for this function, and you cannot navigate to the definition of va_start(), but it is needed to navigate to this function? how to find that code? i don't even know where va_start() is defined exactly. How do I generate ctags index for the system functions or function of third party?ctags -R dirName, vim -t tags is very powerful,