Combining c and assembler code - c

Question

This is my C code:
#include <stdio.h>
void sum();
int newAlphabet;
int main(void)
{
sum();
printf("%d\n",newAlphabet);
}
And this is my assembler code:
.globl _sum
_sum:
movq $1, %rax
movq %rax, _newAlphabet
ret
I'm trying to call the sum function, from my main function, to set newAlphabet equal to 1, but when I compile it (gcc -o test assembler.c assembler.s, compiled on a 64-bit OSX laptop) I get the following errors:
32-bit absolute addressing is not supported for x86-64
cannot do signed 4 byte relocation
both caused by the line "movq %rax, _newAlphabet"
I'm sure I'm making a very basic mistake. Can anyone help? Thanks in advance.
EDIT:
Here are the relevant portions of the C code once it has been translated to assembler:
.comm _newAlphabet,4,2
...
movq _newAlphabet#GOTPCREL(%rip), %rax

Answer 1

Mac OS X uses position-independent executables by default, which means your code can't use constant global addresses for variables. Instead you'll need to access globals in an IP-relative way. Just change:
movq %rax, _newAlphabet
to:
mov %eax, _newAlphabet(%rip)
and you'll be set (I changed from 64 to 32 bit registers to match sizeof(int) on Mac OS X. Note that you also need a .globl _newAlphabet in there somewhere. Here's an example I just made based on your code (note that I initialized newAlphabet to prove it works):
example.c:
#include <stdio.h>
void sum(void);
int newAlphabet = 2;
int main(void)
{
printf("%d\n",newAlphabet);
sum();
printf("%d\n",newAlphabet);
return 0;
}
assembly.s:
.globl _sum
.globl _newAlphabet
_sum:
movl $1, _newAlphabet(%rip)
ret
Build & run:
$ cc -c -o example.o example.c
$ cc -c -o assembly.o assembly.s
$ cc -o example example.o assembly.o
$ ./example
2
1