Does gnu ld link in whole object files or only the needed functions?

Question

We have a library and an executable, that is to be statically linked to the lib. We want to minimize the program space of the final executable.

According to avr-libc's documentation:

the linker links in THE ENTIRE OBJECT MODULE in which the function is located

On the other hand, my colleagues are unanimous on the point that at some pass, the linker throws away any unused functions.

So who is correct or am I misunderstanding something? Is the answer consistent throughout gcc or are we talking only the avr port here?

the linker cannot know if a function will be called, for instance by a function pointer. — CapelliC, Nov 23 '12 at 09:16
I guess if a function name is mentioned in the sources, the function is considered "used", so must be linked in. — Vorac, Nov 23 '12 at 09:21
A "how to do it version": http://stackoverflow.com/questions/1033898/why-do-you-have-to-link-the-math-library-in-c — Ciro Santilli OurBigBook.com, May 15 '15 at 20:02

score 10 · Accepted Answer · answered Nov 23 '12 at 09:18

10

It doesn't perform dead code stripping unless you tell it to. In order to do that, you need to compile everything with:

-fdata-sections -ffunction-sections

in order to mark all data and functions. And when linking with GCC you need to pass:

-Wl,--gc-sections

in order to garbage-collect all unused sections.

answered Nov 23 '12 at 09:18

Nikos C.

So both of my sources were correct and I had not understood everything correct. Just as expected. Thank you for the prompt answer! – Vorac Nov 23 '12 at 09:19

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