在处理网络数据包的时候，往往牵涉到字节顺序的变换。查看linux的相关代码，对这块有很好的实现，源代码贴出来便于查看，今后再慢慢注解。
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#ifndef _LINUX_BYTEORDER_SWAB_H
#define _LINUX_BYTEORDER_SWAB_H
/*
* linux/byteorder/swab.h
* Byte-swapping, independently from CPU endianness
* swabXX[ps]?(foo)
*
* Francois-Rene Rideau 19971205
* separated swab functions from cpu_to_XX,
* to clean up support for bizarre-endian architectures.
*
* See asm-i386/byteorder.h and suches for examples of how to provide
* architecture-dependent optimized versions
*
*/
/* casts are necessary for constants, because we never know how for sure
* how U/UL/ULL map to __u16, __u32, __u64. At least not in a portable way.
*/
#define ___swab16(x) \
({ \
__u16 __x = (x); \
((__u16)( \
(((__u16)(__x) & (__u16)0x00ffU) << 8) | \
(((__u16)(__x) & (__u16)0xff00U) >> 8) )); \
})
#define ___swab24(x) \
({ \
__u32 __x = (x); \
((__u32)( \
((__x & (__u32)0x000000ffUL) << 16) | \
(__x & (__u32)0x0000ff00UL) | \
((__x & (__u32)0x00ff0000UL) >> 16) )); \
})
#define ___swab32(x) \
({ \
__u32 __x = (x); \
((__u32)( \
(((__u32)(__x) & (__u32)0x000000ffUL) << 24) | \
(((__u32)(__x) & (__u32)0x0000ff00UL) << 8) | \
(((__u32)(__x) & (__u32)0x00ff0000UL) >> 8) | \
(((__u32)(__x) & (__u32)0xff000000UL) >> 24) )); \
})
#define ___swab64(x) \
({ \
__u64 __x = (x); \
((__u64)( \
(__u64)(((__u64)(__x) & (__u64)0x00000000000000ffULL) << 56) | \
(__u64)(((__u64)(__x) & (__u64)0x000000000000ff00ULL) << 40) | \
(__u64)(((__u64)(__x) & (__u64)0x0000000000ff0000ULL) << 24) | \
(__u64)(((__u64)(__x) & (__u64)0x00000000ff000000ULL) << 8) | \
(__u64)(((__u64)(__x) & (__u64)0x000000ff00000000ULL) >> 8) | \
(__u64)(((__u64)(__x) & (__u64)0x0000ff0000000000ULL) >> 24) | \
(__u64)(((__u64)(__x) & (__u64)0x00ff000000000000ULL) >> 40) | \
(__u64)(((__u64)(__x) & (__u64)0xff00000000000000ULL) >> 56) )); \
})
#define ___constant_swab16(x) \
((__u16)( \
(((__u16)(x) & (__u16)0x00ffU) << 8) | \
(((__u16)(x) & (__u16)0xff00U) >> 8) ))
#define ___constant_swab24(x) \
((__u32)( \
(((__u32)(x) & (__u32)0x000000ffU) << 16) | \
(((__u32)(x) & (__u32)0x0000ff00U) | \
(((__u32)(x) & (__u32)0x00ff0000U) >> 16) ))
#define ___constant_swab32(x) \
((__u32)( \
(((__u32)(x) & (__u32)0x000000ffUL) << 24) | \
(((__u32)(x) & (__u32)0x0000ff00UL) << 8) | \
(((__u32)(x) & (__u32)0x00ff0000UL) >> 8) | \
(((__u32)(x) & (__u32)0xff000000UL) >> 24) ))
#define ___constant_swab64(x) \
((__u64)( \
(__u64)(((__u64)(x) & (__u64)0x00000000000000ffULL) << 56) | \
(__u64)(((__u64)(x) & (__u64)0x000000000000ff00ULL) << 40) | \
(__u64)(((__u64)(x) & (__u64)0x0000000000ff0000ULL) << 24) | \
(__u64)(((__u64)(x) & (__u64)0x00000000ff000000ULL) << 8) | \
(__u64)(((__u64)(x) & (__u64)0x000000ff00000000ULL) >> 8) | \
(__u64)(((__u64)(x) & (__u64)0x0000ff0000000000ULL) >> 24) | \
(__u64)(((__u64)(x) & (__u64)0x00ff000000000000ULL) >> 40) | \
(__u64)(((__u64)(x) & (__u64)0xff00000000000000ULL) >> 56) ))
/*
* provide defaults when no architecture-specific optimization is detected
*/
#ifndef __arch__swab16
# define __arch__swab16(x) ({ __u16 __tmp = (x) ; ___swab16(__tmp); })
#endif
#ifndef __arch__swab24
# define __arch__swab24(x) ({ __u32 __tmp = (x) ; ___swab24(__tmp); })
#endif
#ifndef __arch__swab32
# define __arch__swab32(x) ({ __u32 __tmp = (x) ; ___swab32(__tmp); })
#endif
#ifndef __arch__swab64
# define __arch__swab64(x) ({ __u64 __tmp = (x) ; ___swab64(__tmp); })
#endif
#ifndef __arch__swab16p
# define __arch__swab16p(x) __arch__swab16(*(x))
#endif
#ifndef __arch__swab24p
# define __arch__swab24p(x) __arch__swab24(*(x))
#endif
#ifndef __arch__swab32p
# define __arch__swab32p(x) __arch__swab32(*(x))
#endif
#ifndef __arch__swab64p
# define __arch__swab64p(x) __arch__swab64(*(x))
#endif
#ifndef __arch__swab16s
# define __arch__swab16s(x) do { *(x) = __arch__swab16p((x)); } while (0)
#endif
#ifndef __arch__swab24s
# define __arch__swab24s(x) do { *(x) = __arch__swab24p((x)); } while (0)
#endif
#ifndef __arch__swab32s
# define __arch__swab32s(x) do { *(x) = __arch__swab32p((x)); } while (0)
#endif
#ifndef __arch__swab64s
# define __arch__swab64s(x) do { *(x) = __arch__swab64p((x)); } while (0)
#endif
/*
* Allow constant folding
*/
#if defined(__GNUC__) && (__GNUC__ >= 2) && defined(__OPTIMIZE__)
# define __swab16(x) \
(__builtin_constant_p((__u16)(x)) ? \
___swab16((x)) : \
__fswab16((x)))
# define __swab24(x) \
(__builtin_constant_p((__u32)(x)) ? \
___swab24((x)) : \
__fswab24((x)))
# define __swab32(x) \
(__builtin_constant_p((__u32)(x)) ? \
___swab32((x)) : \
__fswab32((x)))
# define __swab64(x) \
(__builtin_constant_p((__u64)(x)) ? \
___swab64((x)) : \
__fswab64((x)))
#else
# define __swab16(x) __fswab16(x)
# define __swab24(x) __fswab24(x)
# define __swab32(x) __fswab32(x)
# define __swab64(x) __fswab64(x)
#endif /* OPTIMIZE */
static __inline__ __const__ __u16 __fswab16(__u16 x)
{
return __arch__swab16(x);
}
static __inline__ __u16 __swab16p(__u16 *x)
{
return __arch__swab16p(x);
}
static __inline__ void __swab16s(__u16 *addr)
{
__arch__swab16s(addr);
}
static __inline__ __const__ __u32 __fswab24(__u32 x)
{
return __arch__swab24(x);
}
static __inline__ __u32 __swab24p(__u32 *x)
{
return __arch__swab24p(x);
}
static __inline__ void __swab24s(__u32 *addr)
{
__arch__swab24s(addr);
}
static __inline__ __const__ __u32 __fswab32(__u32 x)
{
return __arch__swab32(x);
}
static __inline__ __u32 __swab32p(__u32 *x)
{
return __arch__swab32p(x);
}
static __inline__ void __swab32s(__u32 *addr)
{
__arch__swab32s(addr);
}
#ifdef __BYTEORDER_HAS_U64__
static __inline__ __const__ __u64 __fswab64(__u64 x)
{
# ifdef __SWAB_64_THRU_32__
__u32 h = x >> 32;
__u32 l = x & ((1ULL<<32)-1);
return (((__u64)__swab32(l)) << 32) | ((__u64)(__swab32(h)));
# else
return __arch__swab64(x);
# endif
}
static __inline__ __u64 __swab64p(__u64 *x)
{
return __arch__swab64p(x);
}
static __inline__ void __swab64s(__u64 *addr)
{
__arch__swab64s(addr);
}
#endif /* __BYTEORDER_HAS_U64__ */
#if defined(__KERNEL__)
#define swab16 __swab16
#define swab24 __swab24
#define swab32 __swab32
#define swab64 __swab64
#define swab16p __swab16p
#define swab24p __swab24p
#define swab32p __swab32p
#define swab64p __swab64p
#define swab16s __swab16s
#define swab24s __swab24s
#define swab32s __swab32s
#define swab64s __swab64s
#endif
#endif /* _LINUX_BYTEORDER_SWAB_H */
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