Question

I am attempting to write very basic x86 code and call it in a C program. I’m running OSX 10.8.2. Here is the code I have:

start.c:

#include <stdio.h>

void _main();  // inform the compiler that Main is an external function

int main(int argc, char **argv) {
    _main();
    return 0;
}

code.s

.text

.globl _main
_main:
    ret

I run the following commands to attempt compilation:

gcc -c -o code.o code.s
gcc -c -o start.o start.c
gcc -o start start.o code.o

Which then returns this output after the final command:

Undefined symbols for architecture x86_64:
  "__main", referenced from:
      _main in start.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Am I missing something in my compiler calls? Do I need to update something/install something different? I just can’t find a definitive answer anywhere since this is such a general output. Thanks!

Answer 1

You need an extra underscore in your asm _main symbol:

.text

.globl __main
__main:
    ret

C symbols get an underscore prefix when compiled, so your C main is actually _main and an extern C _main actually needs to be defined as __main if you write it in asm.