__xchg 函数的用法

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