VBCC - Relative pointer to string?

[LIV2]

I'm trying to write this simple device in a fashion that it is position independent so it may be loaded into an expansion rom for example

I'm running into a problem where a couple of pointers to some strings end up requiring relocation which I can work around with some inline assembly but I'm wondering if there is a more correct way to do this? I'm a noob when it comes to programming especially C so I'm sorry if this is a really stupid question!

I am using both the small code and small data options with vbcc.

i.e the example code would be here where devname uses some inline assembly but devid doesn't - to illustrate the problem

const char devname[] = "example.device";
const char devid[] = "example.device - 1.23 - (Feb 2022)";
struct testres *Init(__reg("d0") struct testres *me, __reg("a6") struct ExecBase *SysBase)
{

    APTR PCRelDevName() = "\tlea _devname(PC),a1\n\tmove.l a1,d0";
    struct CurrentBinding *cb = NULL;
    me->SysBase = SysBase;
    me->lib.lib_Node.ln_Name = PCRelDevName();
    me->lib.lib_Node.ln_Type = NT_RESOURCE;
    me->lib.lib_Flags = LIBF_SUMUSED | LIBF_CHANGED;
    me->lib.lib_Version = 69;
    me->lib.lib_Revision = 0;
    me->lib.lib_IdString = (APTR)devid;

    return me;
}

which emits:

_Init
	movem.l	l3,-(a7)
	move.l	a6,a3
	move.l	d0,a2
	move.l	#0,a5
	move.l	a3,(34,a2)
	lea _devname(PC),a1
	move.l a1,d0
	move.l	d0,(10,a2)
	move.b	#8,(8,a2)
	move.b	#6,(14,a2)
	move.w	#69,(20,a2)
	move.w	#0,(22,a2)
	move.l	#_devid,(24,a2)
	move.l	a2,d0

[phx]

As you already noticed, the small-code mode doesn't really help you to generate position independant code. It just makes all function calls PC-relative.

There is no PIC option for the m68k backend yet (a few other backends have that), so there is also no real solution now. I doubt such an option will make it into the upcoming V0.9h release, but it certainly would be a good thing to have.

A workaround to avoid assembler inlines could be to call a normal function with your constant string pointer, which writes it to the target structure. That makes sure vbccm68k generates a LEA (for register arguments) or PEA (for stack arguments), which is optimized to PC-relative by vasm.

You can also add the

-pic

option to vasmm68k_mot in your config file, so any non-position-independant access is immediately reported by the assembler.

[LIV2]

Ok I'll do that then thanks for your help!