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

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.