Question

I used netbeans in Windows 7 as my IDE. And below is my assembly code:

/* Atomic exchange (of various sizes) */
inline void *xchg_64(void *ptr, void *x)
{
    __asm__ __volatile__("xchgq %0,%1"
                :"=r" ((unsigned long long) x)
                :"m" (*(volatile long long *)ptr), "0" ((unsigned long long) x)
                :"memory");

    return x;
}

When I compile my project, there’s one error occurs:

tklock.h:29:15: error: lvalue required in asm statement

And line 15 is:

:"memory");

How to fix the problem?

Answer 1

There is no way to directly exchange the values of two memory locations. Intel just hasn’t provided that kind of instructions.

You will have to code it as load-exchange-store using a register as an intermediary.

And in that case you could just as well code it in C…