Unable to compile XINU source code after adding a new function in assembly language

Question

I have added a file function.S in sys folder in XINU.

   .text
   .globl zfunction

zfunction:
<x86 code>

equivalent C code for this function is

long zfunction(long param)
{

  long mask = 0xfff803dd ;
  param &= mask  ;

  param = (param << 4) & 0xffffffff;

  return param;
}

I need to use the function in zfunction.S in main function. How do I create a header file that links an assembly level code to C program? If I add extern int long zfunction(param) in the header file, make fails with the error

main.o(.text+0x1e): In function `main':
: undefined reference to `zfunction'
make: *** [xinu.elf] Error 1