Strange behavior with gcc and inline

Question

I want to define an inline function in a header file (.h) which can be included by numerous source files (.c). Here is a minimal example with 1 header and 2 source files:

Header file foo.h

int ifunc(int i);

extern inline
int
ifunc(int i)
{
  return i + 1;
}

Source code file: foo.c

#include <stdio.h>
#include "foo.h"

int foo2(int i);

int main()
{
  printf("%d\n", foo2(1));
  return 0;
}

Source code file foo2.c

#include "foo.h"
int foo2(int i)
{
  return ifunc(i);
}

The problem

When I compile with optimization,

gcc -g -Wall -O2 -o foo foo.c foo2.c
$ ./foo
2

everything works fine. However when I turn off optimization, I get this error:

gcc -g -Wall -o foo foo.c foo2.c
/tmp/cc3OrhO9.o: In function `foo2':
foo2.c:5: undefined reference to `ifunc'

Can someone please explain how to fix so that I can run the code with and without -O2? I am using gcc 4.8.5.

Answer 1

if you replace foo.h with

static inline int ifunc(int i)
{
  return i + 1;
}

Both will work. Declaring it extern means it'll be defined somewhere else which in your original example does not happen. And the optimized build doesn't flag as an error because it already optimized it to be inline it but the non-optimized build does not find a definition in any of the .o files (since they were all compiled with ifunc being an extern as defined in foo.h).

Declaring as static inline will ensure that it is local to each file (the downside being that if it does not inline it, you'll end up with each .o that needs it having a local copy, so don't overdo it).