On Thu, Sep 27, 2012 at 6:44 AM, jonnyjack <jonnyjack7@xxxxxxxxx> 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
