Question on mixing C++ and x86 assembly

[RideFire]

Here is my question that I can't manage to google and solve. I wrote a simple C++ program that adds two numbers. However, how do I call an assembly code to do this addition. It will be easier to show what I have so here it is.

Here is my file called Adding.cpp

//============================================================================
// Name        : Adding.cpp
// Author      : Casey
// Version     : Eclipse Galileo
// Copyright   : 
// Description : 
//============================================================================

#include <iostream>
using namespace std;

int main() {
	int x;
	extern int sum(int a1, a2);
	cout << "Enter Numbers\n";
	cin >> a1, a2;
	x = sum(a1, a2);

	cout << x << endl;


	return 0;

Now here is my assembly language code in a different file called Add.s

//
//Add.s
//Created on: Sep 14, 2011
//Author: Casey

Sum:

pushl %ebp,
movl %ebp, %esp

movl 8(%ebp), %eax
movl 12(%ebp), %ecx
addl %eax, %ecx

pop %ebp
ret

I have multiple errors on the lines 'extern' and 'cin' and I'm not 100% sure I wrote the code correctly. How do I put these two together?

[NicAx64]

you have to use extern "C" in the file scope.
like this.

extern "C" {
int sum(int a1, int a2);
}

and also as I could remember you need to use global directive to label Sum.
like

.global _Sub

_Sum:

The underscore is added by the C compiler when it naming.so you have to
_Sum instead of _Sum,

[RideFire]

No it didn't work. "expected unqualified-id before string constant."
This is what I did.

#include <iostream>
using namespace std;

int main() {
	int x;
	extern "C" {
	int Sum(int a1, a2);
	}
	cout << "Enter Numbers\n";
	cin >> Sum(a1, a2);
	x = Sum(a1, a2);

	cout << x << endl;


	return 0;
}

and

//
//Add.s
//Created on: Sep 14, 2011
//Author: Casey
.global _Sum
Sum:

pushl %ebp,
movl %ebp, %esp

movl 8(%ebp), %eax
movl 12(%ebp), %ecx
addl %eax, %ecx

pop %ebp
ret

[gerard4143]

Here's an example that works on my AMD/64 bit machine.

testit.cpp

#include <iostream>

extern "C" int sum(int, int);//to avoid name mangling

int main()
{
	std::cout << sum(3, 4) << std::endl;
	return 0;
}

asm_file.s

.section .data

.section .bss

.section .text
	.global sum
sum:
	pushq	%rbp
	movq	%rsp, %rbp

	xorq	%rax, %rax
	addl	%edi, %eax
	addl	%esi, %eax

	movq	%rbp, %rsp
	popq	%rbp
	ret

objdump -D testit.o>testfile

testfile

testit.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	be 04 00 00 00       	mov    $0x4,%esi //the values and registers we want
   9:	bf 03 00 00 00       	mov    $0x3,%edi //the values and registers we want
   e:	e8 00 00 00 00       	callq  13 <main+0x13>
  13:	89 c6                	mov    %eax,%esi
  15:	bf 00 00 00 00       	mov    $0x0,%edi
  1a:	e8 00 00 00 00       	callq  1f <main+0x1f>
  1f:	be 00 00 00 00       	mov    $0x0,%esi
  24:	48 89 c7             	mov    %rax,%rdi
  27:	e8 00 00 00 00       	callq  2c <main+0x2c>
  2c:	b8 00 00 00 00       	mov    $0x0,%eax
  31:	c9                   	leaveq 
  32:	c3                   	retq

And the compile lines.

g++ testit.cpp -c
as asm_file.s -o asm_file.o
g++ asm_file.o testit.o -o testit

[RideFire]

Hmm... Your code I copied/pasted and it still didn't work. On line 7 of your code,I get the error 'undefined reference to sum'.

[gerard4143]

Hmm... Your code I copied/pasted and it still didn't work. On line 7 of your code,I get the error 'undefined reference to sum'.

How did you compile it?

[RideFire]

Im using Eclipse Galileo and I clicked run which is pretty much what I do for every program I make. I have build all automatically selected.

[raptr_dflo]

In case your compiler is fussy, try changing line 3 to:

extern "C" {
  int sum(int, int);//to avoid name mangling
}

Are your .cpp and .s files both included as dependencies of your executable?

[RideFire]

Are your .cpp and .s files both included as dependencies of your executable?

Not sure how to quote here but to answer you probably not. what do I do to do that?

[raptr_dflo]

Since you say:

I have build all automatically selected.

I imagine you have to go into the portion of the interface which deals with setting up your build. I'm not familiar with the Galileo Eclipse IDE, so I can't help with any details, but there should be a way to add additional source files to your build, and (if necessary) a way to specify how to assemble your .s file into a .o file.