Mixing assembly and C...

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Patrick_

Joined: 11 Mar 2006
Posts: 53
Location: 127.0.0.1

Patrick_ 22 Mar 2008, 00:48

I've always coded in pure assembly, but never combined C + assembly. I'm going to be doing so for a project, so I created a small assembly file...

Code:

format COFF

public foo
foo:
   xor eax, eax
   ret

Then I linked it with a C source file:

Code:

#include <stdio.h>

extern void foo(void);

int main(void) {

   foo();
   return 0;
}

But the linker (I'm using gcc/ld) says "undefined reference to 'foo'". What am I doing wrong?

Thanks.

22 Mar 2008, 00:48

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 22 Mar 2008, 01:06

You have to take into account that functions' names get "decorated" by the compiler. For a cdecl function (the default kind in C), this is just an underscore prepended to the name, so a function named "foo" in C source code will be named "_foo" in the object file.

Since fasm does not do name decoration, you have to call the function "_foo".

22 Mar 2008, 01:06

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20522
Location: In your JS exploiting you and your system

revolution 22 Mar 2008, 01:39

You can also use public foo as '_foo'

22 Mar 2008, 01:39

Patrick_

Joined: 11 Mar 2006
Posts: 53
Location: 127.0.0.1

Patrick_ 03 Apr 2008, 23:06

Thanks, that worked. I'm having trouble calling libc functions, though. I have something like:

Code:

format MS COFF

extrn printf
public _main

_main:
     ...

But when I run gcc -o out out.o, it gives me "undefined reference to WinMain@16". I tried "public WinMain@16" and replaced "_main" with "WinMain@16" but no go.

Any ideas? I'm so used to programming in straight assembly...

03 Apr 2008, 23:06

Patrick_

Joined: 11 Mar 2006
Posts: 53
Location: 127.0.0.1

Patrick_ 03 Apr 2008, 23:26

Oops, nevermind. It seems I was linking against the wrong file when I tried publicizing "WinMain@16". It works now. Very Happy

03 Apr 2008, 23:26

asmfan

Joined: 11 Aug 2006
Posts: 392
Location: Russian

asmfan 04 Apr 2008, 06:17

Won't create new thread and ask here
What is the difference of

Quote:

format COFF

and

Quote:

format MS COFF

Could someone explain the plain COFF non MS format?
Underscore is for C calling convention. Can fasm automatically decorate names according keywords in proc directives (stdcall/c)?

And more: why cannot I compile this:

Code:

format  MS COFF
INCLUDE 'win32wxp.inc'

public  MyProc

proc    MyProc stdcall uses ebx esi edi, x:DWORD, y:DWORD
local   .cntr:dword
        mov     eax,[x]
        mov     [.cntr],eax
        mov     eax,[y]
        mov     [.cntr],eax
        ret
endp

_________________
Any offers?

04 Apr 2008, 06:17

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20522
Location: In your JS exploiting you and your system

revolution 04 Apr 2008, 06:25

asmfan wrote:

Could someone explain the plain COFF non MS format

I don't know this, but have you tried searching yet? Even looking at the source code might give you the answer. Worth a try I suppose.

asmfan wrote:

Can fasm automatically decorate names according keywords in proc directives

No it can't, but perhaps you could write a macro.

asmfan wrote:

why cannot I compile this:

Code:

local   .cntr:DWORD

The macro only supports UPPERCASE for the DWORD keyword.

04 Apr 2008, 06:25

asmfan

Joined: 11 Aug 2006
Posts: 392
Location: Russian

asmfan 04 Apr 2008, 09:37

Once again thank you revolution!
And once again i'm stumbling on lower&upper cases of data reservind/declaration directives aka DD vs. dd, dword vs DWORD, etc... and once again i'm asking Thomasz to make additional aliases to allow this cases...
I know what ms coff is (pecoff_v8.doc helped that) but mere coff... maybe googling will help but if there is a few difference then i thought someone could clarify them here on forum and then Tomasz could kindly put them in fasm.pdf for simplicity of understanding.

04 Apr 2008, 09:37

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 04 Apr 2008, 09:53

As we talk here about calling Asm code from C, how to do the inverse ?

here's my code :

Code:


;; Asm code
format MS COFF
extrn print
extrn putchar
extrn itoa

print_float:
mov ebp,esp
push dword [esp+4]
call ftol
push eax
mov ebx,eax ; saves eax
push eax
push 10
push 0
push 0
call itoa
push eax
call print
push '.'
call putchar
sub dword [ebp+4], ebx
fld dword [ebp+4]
.loop:
fmul [ten]
fist dword [ebp+12]
push dword [ebp+12]
push 10
push 0
push 0
call itoa
push eax
fisub dword [esp]
call print
push 0
ficom dword [esp]
fstsw [esp]
fwait
pop eax
sahf
jnz .loop
ret 4

---------

/* C code */

#include <stdarg.h>
#define PREFIX 1
#define SUFFIX 0

void clrscr ();
void scroll ();
void print (const char*);
void printf (const char*, ...);
void putchar (char);
extern void print_float (float);
const char* itoa (int,int,int,int); // PS : it's not a standard C compatible itoa 
extern void get_vendor (char*);
extern long ftol (float);

unsigned int x=0;
unsigned int y=0;

int kernel_main(void)
{
 char buffer[13];
    clrscr();
   get_vendor(buffer);
        printf("Huhu, Kernel is speaking !");
      printf(">CPU Vendor : %s \n",buffer); // works here
       print_float(28.07103); // but not here, and it reboots
      while(1);
   return(0);
}

For the linking, i use :

Code:

fasm float.asm float.o

i586-elf-gcc -ffreestanding -mno-stack-arg-probe -c kernel2.c -o kernel2.o

i586-elf-ld -e kernel_main -Ttext 0x1000 -o kernel2 float.o kernel2.o
i586-elf-objcopy -R .note -R .comment -S -O binary kernel2 kernel2.bin

( the i586-elf is for the cross compiler :p )

If i use for example ( extrn "_print" as print ) instead of ( extrn print ), ld give's me the following error message :

Code:

float.o :: undefined reference to '_print'

But if i use "extrn print" instead, the linking works without any error message, but Bochs reboots on "call itoa" :s

Who can help me please ?

Thanks Smile

04 Apr 2008, 09:53

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20522
Location: In your JS exploiting you and your system

revolution 04 Apr 2008, 10:02

Whenever you call a c-style function then you must use the c-calling convention whereby you must pop the parameters off the stack after the call.

Code:

...
push dword [ebp+12]
push 10
push 0
push 0
call itoa
add esp,4*4 ;pop off the 4 parameters we pushed up there
...

04 Apr 2008, 10:02

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 04 Apr 2008, 10:48

Thanks, but Bochs still reboots, and in the Bochs console i see : "(invalid) : FFFF" :s

04 Apr 2008, 10:48

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20522
Location: In your JS exploiting you and your system

revolution 04 Apr 2008, 10:57

White-spirit wrote:

Thanks, but Bochs still reboots, and in the Bochs console i see : "(invalid) : FFFF" :s

Remember you have to use the c-call for all c-functions you use, including ftol etc. I only posted an example.

04 Apr 2008, 10:57

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 04 Apr 2008, 12:11

Thanks but I prefer do only one C call to see if the issue comes from there :

Code:

push '.'
call putchar
add esp,4

It still reboots :s

04 Apr 2008, 12:11

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20522
Location: In your JS exploiting you and your system

revolution 04 Apr 2008, 13:38

I expect you have a stack imbalance. Post your code where you have only the putchar test.

04 Apr 2008, 13:38

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 04 Apr 2008, 14:13

Code:

;; ASM Code
format ms coff

extrn putchar
public call_test

call_test:
push '.'
call putchar
add esp,4
ret 4

// C Code
void scroll ();
void putchar (char);
extern void call_test ();

unsigned int x=0;
unsigned int y=0;

int kernel_main(void)
{
    call_test();
        while(1);
   return(0); // not very important
}

void scroll()
{
   int i;
      unsigned char *screen = (unsigned char*) 0xB8000;
   for (i = 0*80; i < 24*80; i++)
   {
              screen[i] = screen[i+160];
  }
      for (i = 24*80; i < 25*80; i++) screen[i] = 0;
    y = 24;
}

void putchar( char s ){
      unsigned char* screen = (unsigned char*) (0xB8000+y*160+x*2);
       if (s == 0xA ){ // LF : 0xA
                x=0;
                y++;
                screen=(unsigned char*) (0xB8000+y*160);
    } else {
          *(screen++)=s;
              *(screen)=0x07;
             x++;
        }
      if (x>=79){
         x=0;
                y++;
        }
      if (y>=24){
         scroll();
           y=1;
        }
}

Here's a Bochs screenshot ( notice the '!' after 'Please Visit :' ), and Bochs still reboots Shocked

04 Apr 2008, 14:13

dap

Joined: 01 Dec 2007
Posts: 61
Location: Belgium

dap 04 Apr 2008, 14:24

Try removing the '4' after 'ret'. Also C functions are public by default, you don't need to use 'extern' in their protoypt. If you want to make them private use the 'static' keyword.

04 Apr 2008, 14:24

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 04 Apr 2008, 14:43

I tried, but it's still the same ><

Code:

format ms coff

extrn putchar
public call_test

call_test:
push '.'
call putchar
jmp $ ; To see if it arrives so far
add esp,4
ret

04 Apr 2008, 14:43

edfed

Joined: 20 Feb 2006
Posts: 4354
Location: Now

edfed 04 Apr 2008, 15:02

a char is a byte.
are you sure you push a dword? is it possible it pushes only a word?

push '.' <-- operand size? word or dword?

04 Apr 2008, 15:02

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 04 Apr 2008, 15:43

Thanks edfed, but i don't see any operand size problems, i tested a minimalistic putchar in Asm and it works, so it's a C-call problem :

Code:

format ms coff

public call_test

putchar:
mov al, byte [esp+4]
mov byte [0xB8000], al
mov byte [0xB8001], 0x37
ret 4

call_test:
push '.'
call putchar
jmp $ ; To see if it arrives so far, and yes, it does !
add esp,4
ret

Thanks for help .

04 Apr 2008, 15:43

White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 05 Apr 2008, 15:13

Someone ? :/

05 Apr 2008, 15:13

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