Symbol not found when static linking on MacOSX

I am trying to create a static library and link it on MacOS X (several versions): File foo.c:

char foo[111];

File bar.c:

#include <string.h>

extern char foo[];

int bar(char *src) {
  strcpy(foo, src);
  return strlen(foo);
}

Create a library:

$ cc -c foo.c bar.c
$ ar r libfoobar.a foo.o bar.o
ar: creating archive libfoobar.a
$ ranlib libfoobar.a 
$ nm libfoobar.a 

libfoobar.a(foo.o):
000000000000006f C _foo

libfoobar.a(bar.o):
                 U ___strcpy_chk
0000000000000000 T _bar
                 U _foo
                 U _strlen

Create a small test program:

File main.c:

#include <stdio.h>

int bar(char *);

int main(void) {
  printf("foobarbar = %i\n", bar("123"));
  return 0;
}

Compile and link:

$ cc -c main.c
$ cc -o m main.o -L. -lfoobar
Undefined symbols for architecture x86_64:
  "_foo", referenced from:
      _bar in libfoobar.a(bar.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Why is the symbol not found? It is defined in foo.c? Shouldn't at least ranlib create an index in the library that allows a random order of the files there?

The same code works well under Linux (gcc), and also when the symbol in foo.c is not a char array, but an int.