Linker error of unused variable and function

Question

I have written following test program

int proc1();
extern int globalvar;

int func1 ()
{
    return globalvar + 1;
}

int func2()
{
    return proc1()+3;
}

int main()
{
    return 0;
}

As you can see that this program is not doing anything. However, while compiling it, I faced linker error of globalvar and int proc1() despite the facts that these are not going to be referenced from the entry point function main. I faced problem on both Windows(using cl) and Linux(using gcc).

Is there any way to instruct the compiler/linker not to link this unreferenced global variable and function from the entry point (on cl, gcc and clang)?

Exact error message on Windows:

test.obj : error LNK2019: unresolved external symbol "int globalvar" (?globalvar@@3HA) referenced in function "int __cdecl func1(void)" (?func1@@YAHXZ)
test.obj : error LNK2019: unresolved external symbol "int __cdecl proc1(void)" (?proc1@@YAHXZ) referenced in function "int __cdecl func2(void)" (?func2@@YAHXZ)
test.exe : fatal error LNK1120: 2 unresolved externals

`these are not going to be used in the program` - then what do you think `return globalvar + 1;` and `return proc1() + 3;` do? — The Paramagnetic Croissant, Jul 03 '14 at 11:27
@user3477950: See the edit. These are not goint to be referenced from the entry point function main. — doptimusprime, Jul 03 '14 at 11:29
and then what? their callers are not `static`, so **even** if the linker does LTO, it cannot possibly eliminate them... — The Paramagnetic Croissant, Jul 03 '14 at 11:30
@user3477950: Is this because these function can be used in other translation unit? But I have only one translation unit. — doptimusprime, Jul 03 '14 at 11:31
@dbasic i think you got undefined reffernce error. where you defined `proc1()` function. and `globalvar` variable. — Jayesh Bhoi, Jul 03 '14 at 11:31
@user3477950 I'm pretty sure LTO *can* remove these functions. The whole point is that LTO can work with knowledge, not safe assumptions. GCC's `-fwhole-program` can do the same. — ams, Jul 03 '14 at 11:45
Seems very risky to me. Why not implement some abstraction layer between your code and the backend? This would solve your problems without unnecessary compiler "magic"... — Flovdis, Jul 03 '14 at 11:46
@ams I don't think so. What if another program wants to link against the resulting executable? — The Paramagnetic Croissant, Jul 03 '14 at 12:22
@user3477950 then they're breaking the rules of LTO. The whole point of LTO is that it can *know* what's used where, and by what. — ams, Jul 03 '14 at 12:24
@ams consequently, if the rules of LTO are broken, then LTO can't be used. — The Paramagnetic Croissant, Jul 03 '14 at 12:45
@user3477950, "if the rules of LTO are broken, then LTO *mustn't* be used." There, fixed that for you. — ams, Jul 03 '14 at 12:55

score 1 · Accepted Answer · answered Jul 03 '14 at 11:37

1

You can fix this, in gcc, like this:

gcc -ffunction-sections -Wl,--gc-sections test.c

That does two things:

It instructs the compiler to emit each function in its own 'section' in the binary file.
It instructs the linker to discard (garbage collect) sections that are not referenced.

This means that func1 and func2 will be discarded, and therefore there will be not more references to globalvar or proc1.

answered Jul 03 '14 at 11:37

ams

24,923
4
54
75

No idea, you're the one who's actually used those tools. ;-) – ams Jul 03 '14 at 11:41

Linker error of unused variable and function

1 Answers1