using asm() to execute an atomic subtraction instruction

[jonnyjack]

Thanks! Changing the order of the operands worked, the destination was the
second operand.

On Thu, Sep 27, 2012 at 2:25 PM, Ian Lance Taylor-3 [via gcc] <
ml-node+s1065356n873142h84@n5.nabble.com> wrote:

> On Thu, Sep 27, 2012 at 6:44 AM, jonnyjack <[hidden email]<http://user/SendEmail.jtp?type=node&node=873142&i=0>>
> wrote:
>
> > For a school project, I have to implement a simple atomic_sub() function
> in
> > c.  The implementation must use the asm() function.
> >
> > Right now, my code looks like this
> >
> > void atomic_sub( int * value, int dec_val) {
> >    asm ("LOCK SUB (%0), %1;\n"
> >            :
> >            : "r" (value), "r" (dec_val)
> >            : "memory");
> > }
> >
> > The intention is for "value" to decrement by "dec_val" atomically, but
> when
> > I try to compile I get the following error.
> >
> > "locking.c:28: Error: expecting lockable instruction after 'lock'"
> >
> > This asm() call is on line 28, so this is the instruction the error is
> > referring to.  Does anyone know why this isn't a lockable instruction?
> > According to the documentation from Intel I've read, a sub with a
> > destination of a memory address is lockable.  Any ideas?
>
> You are running into syntax issues.  You are using Intel syntax and
> the assembler is probably expecting AT&T syntax.  You could try the
> -masm=intel option.  Or reverse the operands.
>
> Ian
>
>
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