__xchg 函数的用法

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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

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