GCC, inline assembly and constants

[MrA]

I'm using some inline assembly using GCC (eew! AT&T syntax). I want to make use of constants defined in the C source within the assembly bock. However, ld is complaining when I link the binary. Consider the following (incomplete) example:

#define BAR	0

void foo() {
	asm("movl BAR, %eax\n");
}

Now, the error ld is giving me is that the symbol 'BAR' is undefined. BAR is a macro which I expect the preprocessor to inline. So does anyone know how I can use C constants (#defines) in inline assembly?

[dduardo]

You can do something like this

#include <stdio.h>

#define BAR 2

int main( void ) {

		int foo;

		__asm__ __volatile__(
							 "movl %0, %%eax"
						   : "=r"(foo)	   // OUTPUT
						   : "r"(BAR)	   // INPUT
						   );

		printf("EAX=%d\n", foo);

		return 0;
}

This is how the program works:

1) The BAR is replaced by 2 at parsing

2) The 2 is stored in some register (Decided by the compiler for best optimization)

3) The first input argument %0 is moved into the eax register

4) Whatever is in eax is copied into foo

5) foo is printed in order to prove that eax is now 2.

[MrA]

  dduardo said:

You can do something like this

#include <stdio.h>

#define BAR 2

int main( void ) {

		int foo;

		__asm__ __volatile__(
							 "movl %0, %%eax"
						   : "=r"(foo)	   // OUTPUT
						   : "r"(BAR)	   // INPUT
						   );

		printf("EAX=%d\n", foo);

		return 0;
}

This is how the program works:

1) The BAR is replaced by 2 at parsing

2) The 2 is stored in some register (Decided by the compiler for best optimization)

3) The first input argument %0 is moved into the eax register

4) Whatever is in eax is copied into foo

5) foo is printed in order to prove that eax is now 2.

Thanks for the reply. After half an hour of head banging on the table, I managed to figure it out. But I still don't understand why you can't just plug in a constant.

[dduardo]

The reason it doesn't work is because macros don't process stuff within double quotes

For example, lets say you have:

#define HELLO 15

but later in your code you have:

printf("HELLO = %d", HELLO);

Should the preprocessor change the line to:

printf("15 = %d", 15);

That doesn't make sense.

And I don't think there is a way to get around this behavior. If you do find a way I would like to know.

[MrA]

^ Ah, that makes sense. It also explains why I can make a .S file and use macros in there.

[Andareed]

You could do something like:

#define BAR	"0"

void foo() {
	asm("movl " BAR ", %eax\n");
}

[dduardo]

  Andareed said:

You could do something like:

#define BAR	"0"

void foo() {
	asm("movl " BAR ", %eax\n");
}

I just tried it and it doesn't work. Sure it compiles, but then the program segfaults. When I disassembled the code in gdb you get a line like this:

mov 0x2,%eax

This is wrong. The line should be like this:

mov $0x2,%eax

To fix this the code should look like this:

#define BAR	"0"

void foo() {
	asm("movl $" BAR ", %eax\n");
}

This produces the correct assembly. Notice how there is a dollar sign after the movl.

Andareed, almost correct, but it needed a little tweaking.

[Andareed]

I was more commenting on a solution regarding C macros/inling problem that the op had. I'm unfamiliar with this style of asm - I typically work with masm/vc++ style asm.

[MrA]

  Andareed said:

I was more commenting on a solution regarding C macros/inling problem that the op had. I'm unfamiliar with this style of asm - I typically work with masm/vc++ style asm.

Yeah, I'm also used to the Intel syntax that other, less sadistic assemblers use. But I decided to learn how GCC does it. It's kinda neat how GCC can do automatic register assignment with inline assembly. But the syntax is still not what I'm used to. Anywho, thanks, both of you, for your answers.