Mixing C and Assembly files

Question

I want to use a naked function in my C++ program using g++. Unfortunately g++, unlike VC++, does not support naked functions and the only way to manage this is to write your own assembly code in a separate file and link with your C++ files. I tried to find some good tutorial for x86 to mix assembly and C/C++ files but couldn't find any good one.

Kindly let me know if you know about any. Note that I'm not asking about inline assembly but linking C and assembly files and ways to declare extern variables of C in assembly and vice versa besides using them in either C or assembly, and also ways to link the C and asm files using Makefile.

Answer 1

In C++ file:

extern "C" void foo(); // Stop name mangling games

int main() {
  foo();
}

in "naked" asm file, for x86:

# modified from http://asm.sourceforge.net/howto/hello.html

.text                   # section declaration
    .global foo

foo:

# write our string to stdout

    movl    $len,%edx   # third argument: message length
    movl    $msg,%ecx   # second argument: pointer to message to write
    movl    $1,%ebx     # first argument: file handle (stdout)
    movl    $4,%eax     # system call number (sys_write)
    int $0x80       # call kernel

# and exit

    movl    $0,%ebx     # first argument: exit code
    movl    $1,%eax     # system call number (sys_exit)
    int $0x80       # call kernel

.data                   # section declaration

msg:
    .ascii  "Hello, world!\n"   # our dear string
    len = . - msg           # length of our dear string

Compile, assemble and link (with g++ rather than ld because it's much easier to do it that way for C++) and run:

ajw@rapunzel:/tmp > g++ -Wall -Wextra test.cc -c -o test.o
ajw@rapunzel:/tmp > as -o asm.o asm.S
ajw@rapunzel:/tmp > g++ test.o asm.o
ajw@rapunzel:/tmp > ./a.out
Hello, world!

If you want to pass arguments to your function or return anything you need to respect the calling conventions.

Answer 2

Here's an example of a trick to achieve the "naked function" effect.

#include <stdio.h>

extern "C" int naked_func ();

static void
dummy ()
{
  __asm__ __volatile__
    (
     "  .global naked_func\n"
     "naked_func:\n"
     "  movq    $3141569, %rax\n"
     "  ret\n"
     );
}

int
main ()
{
  printf ("%d\n", naked_func ());
  return 0;
}