Object linking order trouble - c

Question

I am facing linking problem. I'll illustrate it:
a.c:
extern void b(void);
int main() {
a();
return 0;
}
void a() {
b();
}
b.S:
.extern a
b:
jmp a
No matter if I'll link
gcc a.o b.o -o c
or
gcc b.o a.o -o c
I'll get unresolved symbols. How do I link these files? I can't merge them. Example may be nonsensical, but that illustrates point, what do i try to archive.

Answer 1

Initial investigation:
a.c
extern void b(void);
void a(void);
int main() {
a();
return 0;
}
void a() {
b();
}
b.S
.extern a
b:
jmp a
b.c
void a(void);
void b(void)
{
a();
}
Output
$ gcc -c a.c
$ gcc -c b.c -o b_gcc.o
$ as b.S -o b_as.o
$ gcc a.o b_gcc.o -o test_gcc
$ gcc a.o b_as.o -o test_as
a.o: In function `a':
a.c:(.text+0x15): undefined reference to `b'
collect2: error: ld returned 1 exit status
So what gives? Why is it okay with GCC but not GAS?
$ objdump -t b_gcc.o > syms_gcc
$ objdump -t b_as.o > syms_as
$ diff syms_gcc syms_as
2c2
< b_gcc.o: file format elf64-x86-64
---
> b_as.o: file format elf64-x86-64
5d4
< 0000000000000000 l df *ABS* 0000000000000000 b.c
9,12c8
< 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack
< 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame
< 0000000000000000 l d .comment 0000000000000000 .comment
< 0000000000000000 g F .text 000000000000000b b
---
> 0000000000000000 l .text 0000000000000000 b
Okay, so gcc makes b a global symbol. Lets try .global b in b.S:
$ as b.S -o b_as2.o
$ gcc a.o b_as2.o
$
Success. So gcc/ld will do multi-pass symbol resolution for anything that is not in a static library. But it only looks for global symbols. Here's the final b.S:
.extern a
.global b
b:
jmp a