[Q] linking only some functions in an object file

[jw c]

Hello, all. 
I have a question. 

For example, when I link test.o test2.o and make
test.elf, all of functions in test.o test2.o gets in
test.elf. 

But I don't want not-used-functions to get in
test.elf. How can I do this? 

Thank you. 

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