Local static variable of an inline function in a shared library

C

1. With inline keyword

$ cc --version
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ ls
Makefile   collect.c  collect.h  main.c

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

int main()
{
  printf("%d\n", collect(10));
  printf("%d\n", get_sum());
  return 0;
}

#pragma once

inline int collect(int x)
{
  static int sum = 0;
  sum += x;
  return sum;
}

int get_sum();

#include "collect.h"

int get_sum()
{
  return collect(0);
}

all: libcollect.dylib main

libcollect.dylib: collect.c collect.h
        $(CC) $< -dynamiclib -o $@

main: main.c libcollect.dylib
        $(CC) $< -o $@ -L. -lcollect

$ make
cc collect.c -dynamiclib -o libcollect.dylib 
In file included from collect.c:1:
./collect.h:5:3: warning: non-constant static local variable in inline function may be different in different files [-Wstatic-local-in-inline]
  static int sum = 0;
  ^
./collect.h:3:1: note: use 'static' to give inline function 'collect' internal linkage
inline int collect(int x)
^
static 
1 warning generated.
Undefined symbols for architecture x86_64:
  "_collect", referenced from:
      _get_sum in collect-a79c3b.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [libcollect.dylib] Error 1

2. With static inline keyword

#pragma once

static inline int collect(int x)
{
  static int sum = 0;
  sum += x;
  return sum;
}
int get_sum();

$ make
cc collect.c -dynamiclib -o libcollect.dylib
cc main.c -o main -L. -lcollect
$ ./main
10
0
$

C++

1. With inline keyword

$ c++ --version
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ ls
Makefile    collect.cc  collect.h   main.cc

#include <cstdio>
#include "collect.h"

int main()
{
  printf("%d\n", collect(10));
  printf("%d\n", get_sum());
  return 0;
}

#pragma once

inline int collect(int x)
{
  static int sum = 0;
  sum += x;
  return sum;
}

int get_sum();

#include "collect.h"

int get_sum()
{
  return collect(0);
}

all: libcollect.dylib main

libcollect.dylib: collect.cc collect.h
        $(CXX) $< -dynamiclib -o $@ 

main: main.cc libcollect.dylib
        $(CXX) $< -o $@ -L. -lcollect

$ make
c++ collect.cc -dynamiclib -o libcollect.dylib
c++ main.cc -o main -L. -lcollect
$ ./main
10
10

2. With static inline keyword

#pragma once

static inline int collect(int x)
{
  static int sum = 0;
  sum += x;
  return sum;
}

int get_sum();

$ make
c++ collect.cc -dynamiclib -o libcollect.dylib
c++ main.cc -o main -L. -lcollect
$ ./main
10
0