Accessing C global variable from assembly file

**limp** · 08-03-2011

Hi all,

I am trying to access a C variable from an assembly file but I am getting an "undefined reference" error in the assembly file's line in which I am trying to access the variable. I am using GCC and GAS by the way.

Code:

test.c

uint8_t test_variable=0;
extern uint8_t test_variable;

Code:

test.S

...
...

movb $0xA5,%al
movb %al,($test_variable)

...
...

The error received is the following:

Code:

undefined reference to `$test_variable'

Any help will be really appreciated as I struggle with this for days.

**~~CommonTater~~** · 08-03-2011

Why do you define the variable then redefine it as extern on the very next line? Most likely the assembler doesn't know where to look...

**limp** · 08-03-2011

Hi,

Originally Posted by CommonTater

Why do you define the variable then redefine it as extern on the very next line? Most likely the assembler doesn't know where to look...

You're right on that, I was just testing one at a time and accidentally posted them both.

I tried to use only

Code:

uint8_t test_variable=0;

and then I tried with

Code:

extern uint8_t test_variable;

but still got the same error in both cases...

**Salem** · 08-03-2011

I would start with gcc -S prog.c to see how the assembler actually names (and refers to) global variables.

Then do the same in your ASM file.

**tabstop** · 08-03-2011

$ is for constants, isn't it?

Anyway, to give an idea, I took this source:

Code:

int main(void) {
    extern int bob;
    bob = 5;
    return 0;
}

compiled it with -S to give assembler, and got back

Code:

        .file   "ext.c"
        .text
.globl main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        movq    %rsp, %rbp
        .cfi_offset 6, -16
        .cfi_def_cfa_register 6
        movl    $5, bob(%rip)
        movl    $0, %eax
        leave
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
        .section        .note.GNU-stack,"",@progbits

I have to admit I hadn't heard of RIP until two minutes ago.

**Fordy** · 08-03-2011

I really dont like GAS syntax, but...

Code:

.globl funcy
.type funcy, @function
funcy:
	push %ebx
	movl foo, %ebx
	addl $10, %ebx
	movl %ebx, foo
	pop %ebx
	ret

Code:

#include <stdio.h>

int foo;

void funcy();

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

Build with "gcc -o <whatever_your_app_is_called> <asm_file_name>.S <c_file_name>.c"

**limp** · 08-03-2011

Hi guys,

Compiling the following C snippet with gcc -S....

Code:

unsigned char test_variable=0;

int main()
{
  test_variable=0xFF;
  return 0;
}

...gave me that:

Code:

	.file	"test_1.c"
.globl _test_variable
	.bss
_test_variable:
	.space 1
	.def	___main;	.scl	2;	.type	32;	.endef
	.text
.globl _main
	.def	_main;	.scl	2;	.type	32;	.endef
_main:
	leal	4(%esp), %ecx
	andl	$-16, %esp
	pushl	-4(%ecx)
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ecx
	subl	$4, %esp
	call	___main
	movb	$-1, _test_variable
	movl	$0, %eax
	addl	$4, %esp
	popl	%ecx
	popl	%ebp
	leal	-4(%ecx), %esp
	ret

Does this help at all?

Thanks.

**tabstop** · 08-03-2011

It doesn't help us much. It should, however, help you immensely, what with it literally telling you how to use the name of your variable right there in the output.

(ETA: And if you want to be extra secure, change your global to an extern (and get rid of the =0 at the end) and see if the assembly is different.)

**limp** · 08-03-2011

Originally Posted by tabstop

(ETA: And if you want to be extra secure, change your global to an extern (and get rid of the =0 at the end) and see if the assembly is different.)

Thanks for the advice. I did that and I got:

Code:

	.file	"test.c"
	.def	___main;	.scl	2;	.type	32;	.endef
	.text
.globl _main
	.def	_main;	.scl	2;	.type	32;	.endef
_main:
	leal	4(%esp), %ecx
	andl	$-16, %esp
	pushl	-4(%ecx)
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ecx
	subl	$4, %esp
	call	___main
	movb	$-1, _test_variable
	movl	$0, %eax
	addl	$4, %esp
	popl	%ecx
	popl	%ebp
	leal	-4(%ecx), %esp
	ret

So basically the only difference is that the "test_variable" variable is not declared in the bss but nothing else really...In my assembly code under which section should I declare the "test_variable"?

Thanks for all the help!

**AndrewHunter** · 08-03-2011

Didn't Fordy post a solution for this? Did that not help you?

**tabstop** · 08-03-2011

Originally Posted by limp

Thanks for the advice. I did that and I got:

Code:

	.file	"test.c"
	.def	___main;	.scl	2;	.type	32;	.endef
	.text
.globl _main
	.def	_main;	.scl	2;	.type	32;	.endef
_main:
	leal	4(%esp), %ecx
	andl	$-16, %esp
	pushl	-4(%ecx)
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ecx
	subl	$4, %esp
	call	___main
	movb	$-1, _test_variable
	movl	$0, %eax
	addl	$4, %esp
	popl	%ecx
	popl	%ebp
	leal	-4(%ecx), %esp
	ret

So basically the only difference is that the "test_variable" variable is not declared in the bss but nothing else really...In my assembly code under which section should I declare the "test_variable"?

Thanks for all the help!

If it's really and truly external, then you shouldn't declare it anywhere I wouldn't think.

**limp** · 08-03-2011

Hi,

Originally Posted by tabstop

If it's really and truly external, then you shouldn't declare it anywhere I wouldn't think.

The fact that it is external doesn't mean that it is not declared. In fact, "extern" keyword is used only into previously declared variables. If you use "extern" in the definition that it is both defined and declared as external at the same time.

What I changed compared to the first version for getting the above assembly code is that I put "extern unsigned char test_variable;" in place of "unsigned char test_variable=0;".

Now, for the generated assembly code I concluded that I need to do something like that:

Code:

test.c

uint8_t test_variable=0;

Code:

test.s

_test_variable:
	.byte test_variable

.text
.code16
...
...

movb $0xA5,%al
movb %al,($_test_variable)

...
...

However this gives me again that:

Code:

undefined reference to `$_test_variable'

and if I try without the "$" I get (although I don't want to write to the address of the "_test_variable" but to the value it points, that's why I use "$"):

Code:

relocation truncated to fit: R_386_16 against `.text' (in the "movb %al,(_test_variable)" line)

Thanks

**AndrewHunter** · 08-03-2011

You don't declare it anywhere in your assembly program. Note the difference is the underscore "_" in front of the variable name. My guess is that for GAS you need to identify C variables as _variable. Then just define your C variable in the global namespace of your c program.

**tabstop** · 08-03-2011

Originally Posted by limp

Hi,

The fact that it is external doesn't mean that it is not declared. In fact, "extern" keyword is used only into previously declared variables. If you use "extern" in the definition that it is both defined and declared as external at the same time.

I meant in the assembly file. When you declared it external in your C program and had your compiler write the assembly, where did it declare the variable in the assembler? Nowhere.

Originally Posted by limp

What I changed compared to the first version for getting the above assembly code is that I put "extern unsigned char test_variable;" in place of "unsigned char test_variable=0;".

and if I try without the "$" I get (although I don't want to write to the address of the "_test_variable" but to the value it points, that's why I use "$"):

I'm not quite sure where you're going with "address of the _test_variable_ but to the value it points". All a variable is, is a piece of memory. You want to take the value 0, and put it in that piece of memory.

Originally Posted by limp

Code:

relocation truncated to fit: R_386_16 against `.text' (in the "movb %al,(_test_variable)" line)

Thanks

I think that means you defined _test_variable to be .text.

**limp** · 08-03-2011

Hi,

Originally Posted by AndrewHunter

You don't declare it anywhere in your assembly program. Note the difference is the underscore "_" in front of the variable name. My guess is that for GAS you need to identify C variables as _variable. Then just define your C variable in the global namespace of your c program.

So you mean having something like that?

Code:

test.c

uint8_t test_variable=0;

Code:

test.s

.extern test_variable

.text
.code16
...
...

movb $0xA5,%al
movb %al,($_test_variable)

...
...

Because I tried that and it also gave "undefined reference to `$_test_variable"

Originally Posted by tabstop

I meant in the assembly file. When you declared it external in your C program and had your compiler write the assembly, where did it declare the variable in the assembler? Nowhere.

From what I understand is that you're saying that in any case I need to put a ".extern test_variable" in the assembly file, right? Please correct me if I understood it wrong.

Thank you both guys.

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