CODEWITHSUNDEEP
c++linker
Empty Space
Empty Space

Reputation: 751

Trying to understand basic linker behaviour

I have a minimal cpp file with contents,

int main() {}

I then compile and link using

$ g++ -c main.cpp
$ g++ -o main main.o -Wl,-u,foo

Expected Behaviour: Since the linker cannot find the symbol foo, because it is not defined anywhere, I expected the linker to throw an error saying that the symbol foo is not found/resolved.

Actual behaviour: The link step succeeds.

Can someone help me understand this behaviour and how I can force the linker to error out when it cannot find the undefined symbol, possibly using some linker flag?

$ g++ --version
g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-23)

Thanks in advance!!!

EDIT: It is probably related to my misunderstanding about undefined symbols, so let me clarify my understanding of the linker. The linker searches for undefined symbols in the list sequentially through -llib... and foo in this case is one such undefined symbol. I was expecting that at the end of traversing the whole list of libraries, there shouldn't be any undefined symbols left i.e. all symbols must be defined. Am I wrong in thinking this?

Upvotes: 0

Views: 151

Answers (1)

MSalters
MSalters

Reputation: 179779

That's simply not what -Wl,u does. From the ld documentation: "Force symbol to be entered in the output file as an undefined symbol.". So the -u creates a symbol table in your main executable, and adds foo to that.

I.e. the option doesn't affect whether an output file is generated.

You may want to try g++ -c main.cpp -u foo - pass it to the compiler instead of the linker, so the compiler can put foo in the symbol table of main.o. Now the linker will see foo in its inputs, and it will need to act on that.

Upvotes: 1

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