After all of the input files have been read and all symbol resolution is complete, the link-editor searches the internal symbol table for any symbol references that have not been bound to symbol definitions. These symbol references are referred to as undefined symbols. Undefined symbols can affect the link-edit process according to the type of symbol, together with the type of output file being generated.
Usually the errors related to undefined-symbol are mean this:
It means you haven't written a function, or you haven't created a variable, or you haven't linked against the library or object code that contains the missing function or variable.
It means the linker has looked through all the compiled code you told it to, and it still can't find what it's looking for.
