From: "clusty" <clusty AT poland DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: inline assembly and c struct
Date: Thu, 8 Feb 2001 19:55:05 +0100
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----- Original Message -----
From: "Nate Eldredge" <neldredge AT hmc DOT edu>
Newsgroups: comp.os.msdos.djgpp
Sent: Saturday, February 03, 2001 1:48 AM
Subject: Re: inline assembly and c struct


> "clusty" <clusty AT poland DOT com> writes:
>
> > I want compiler to print y_offset in my piece of inline assembly code.
> > In above example i will get y_offset in variable and i.e instead of writing
> >
> > asm(...
> >     mov <y_offset>(%%eax),%%edx
> >     ...)
> >
> > I would have to write
> >
> > asm(...
> >     mov _y_offset,%%eax
> >     mov <y_offset>(%%eax),%%edx
> >     ...)
> >
> > see the difference
> >
> > i tried something like this
> >
> > asm(...
> >    "%0(%%eax),%%edx  \n"
> >     ...
> >     ::"m"(*(&some_struct.member-&some_struct)) /*this thing gives*/
> >                                                /*variable at offset of*/
> >                                                /*offset of member */
> >    );
> > but it won't work
>
> I think what you want is (untested)
>
> asm("movl %0(%%eax),%%edx"
>     :: "i" (offsetof(some_struct, member))
>    );
>
> The "i" constraint specifies that this operand is an immediate value,
> and hence will be inserted as a constant.
>
> Note that you should always use the standard `offsetof' macro instead
> of trying to calculate the offseet yourself.
>

ok but i wont be able to do that

> >    "%0(%%eax),%%edx  \n"

because the constraint will be "i".
The constraint must be "m" or  "o" in order to above line to compile.