CPU X86 32位

文件：arch\x86\include\asm\cmpxchg_32.h

/*
 * Note: no "lock" prefix even on SMP: xchg always implies lock anyway.
 * Since this is generally used to protect other memory information, we
 * use "asm volatile" and "memory" clobbers to prevent gcc from moving
 * information around.
 */
#define __xchg(x, ptr, size)						\
({									\
	__typeof(*(ptr)) __x = (x);					\
	switch (size) {							\
	case 1:								\
	{								\
		volatile u8 *__ptr = (volatile u8 *)(ptr);		\
		asm volatile("xchgb %0,%1"				\
			     : "=q" (__x), "+m" (*__ptr)		\
			     : "0" (__x)				\
			     : "memory");				\
		break;							\
	}								\
	case 2:								\
	{								\
		volatile u16 *__ptr = (volatile u16 *)(ptr);		\
		asm volatile("xchgw %0,%1"				\
			     : "=r" (__x), "+m" (*__ptr)		\
			     : "0" (__x)				\
			     : "memory");				\
		break;							\
	}								\
	case 4:								\
	{								\
		volatile u32 *__ptr = (volatile u32 *)(ptr);		\
		asm volatile("xchgl %0,%1"				\
			     : "=r" (__x), "+m" (*__ptr)		\
			     : "0" (__x)				\
			     : "memory");				\
		break;							\
	}								\
	default:							\
		__xchg_wrong_size();					\
	}								\
	__x;								\
})

#define xchg(ptr, v)							\
	__xchg((v), (ptr), sizeof(*ptr))

xchg是宏，不是真正的函数，该宏根据不同的字长调用不用的汇编指令操作。

举例：

int i = 10， j = 20；

xchg(&i, j); 返回值10;

//调用该宏后

i = 20, j = 20