I've run into a tough "undefined reference" error when trying to build a module in Linux with C++. I'm going to describe it at a high level, and post code later if necessary (it's proprietary, so posting it would require some name changing). Some details: Module A (a library) has a class that we'll call Foo with a method called Bar. Module A builds just fine, and using nm to look at the object file shows that the constructor and Bar are both defined (they show up as 'T'). Module B (a library) contains a class that uses module A, with references to Foo::Foo, Foo::~Foo, and Foo::Bar. Its makefile includes -L/path/to/Foo and -lFoo. This module also builds just fine. However, when I run nm on Module B's object file, the calls to module Foo::Foo, Foo::~Foo, and Foo::Bar are undefined (they show up as 'U'). Why it builds is beyond me. Module C - whose output is an executable - contains references to module B. When I try to build Module C, then it yells at me for the undefined reference from Module B to Module A's Foo and Bar methods. Why does module B build if a reference is undefined? Why is the error only reported once we get to Module C? EDIT: I should mention that Module C's makefile also has -L/path/to/Foo and -lFoo, but it still fails. Any high-level guesses as to anything I should try? I have a feeling I'm going to have to post some code...

Reputation: 275

Tricky Undefined Reference Error

I've run into a tough "undefined reference" error when trying to build a module in Linux with C++. I'm going to describe it at a high level, and post code later if necessary (it's proprietary, so posting it would require some name changing). Some details:

Module A (a library) has a class that we'll call Foo with a method called Bar. Module A builds just fine, and using nm to look at the object file shows that the constructor and Bar are both defined (they show up as 'T').
Module B (a library) contains a class that uses module A, with references to Foo::Foo, Foo::~Foo, and Foo::Bar. Its makefile includes -L/path/to/Foo and -lFoo. This module also builds just fine. However, when I run nm on Module B's object file, the calls to module Foo::Foo, Foo::~Foo, and Foo::Bar are undefined (they show up as 'U'). Why it builds is beyond me.
Module C - whose output is an executable - contains references to module B. When I try to build Module C, then it yells at me for the undefined reference from Module B to Module A's Foo and Bar methods.
1. Why does module B build if a reference is undefined?
2. Why is the error only reported once we get to Module C?

EDIT:

I should mention that Module C's makefile also has -L/path/to/Foo and -lFoo, but it still fails. Any high-level guesses as to anything I should try? I have a feeling I'm going to have to post some code...

Upvotes: 2

Answers (3)

automatom

Reputation: 275

I figured out why it wasn't building. It's the same problem as was had here:

undefined reference to symbol even when nm indicates that this symbol is present in the shared library

Upvotes: 1

tp1

Reputation: 1207

When you create shared library, all the symbols do not need to be defined. This behaviour can be changed with compiler commandline switches(linker option --no-allow-shlib-undefined) if needed.

Upvotes: 0

Code-Apprentice

Reputation: 83577

Does module B build or just compile? Are you actually linking module B to module A before you try to build Module C? I would be surprised if you are. In the compiling stage, the compiler only checks that all names are declared. The compiler doesn't look for an definitions (i.e. implementations). The linking stage takes care of this detail. In the linking stage, you need to specify all modules needed to build the project (in this case A, B, and C.)

Upvotes: 0

Tricky Undefined Reference Error

Answers (3)

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