关于字节顺序的转换 -- linux-2.4.26\include\linux\byteorder

在处理网络数据包的时候，牵涉到字节顺序的变换,下边是源代码，很清晰的表示了怎么进行大端小端的字节转换。主要是在编程中要知道这几个宏定义的意义。

#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 <fare@tunes.org> 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 */