Object files do not contain symbols that should be there

Question

This is the specific error I am getting:

libFoo.so: undefined reference to `IID_IFOOBAR'
collect2: ld returned 1 exit status
make: *** [/home/F.exe] Error 1

when I try to check the symbols in my object file Ao

nm A.obj | grep IID_

I get no symbols listed in my object file of the 'IID_IFOOBAR' that should be there since it is defined in a header file as:

extern const blahType IID_IFOOBAR

Am I missing something? Should IID_IFOOBAR be defined differently somewhere since it is an extern variable? If this question is too specific I will remove it. But perhaps someone's intuition will help me here.

Answer 1

[EDIT] Add the definition of the variable somewhere (without extern ). Extern identifiers without initializers are not definitions - the definition must be somewhere else.

Answer 2

To make an extern ly declared symbol appear in your symbol table as "undefined", you should actually use it from within a library.

If your object file neither contains a definition of symbol (as opposed to declaration) nor uses it somewhere inside its functions, then the symbol won't appear in the symbol table. Why should it?