linux源码分析之字节序（5）-- swab.h

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在linux源码分析之字节序（3）、linux源码分析之字节序（4）中都有看到，源码中包含了

#include <linux/swab.h>

该头函数里面介绍了字节交换的具体方法。我们来看看具体代码：

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#ifndef _LINUX_SWAB_H
#define _LINUX_SWAB_H

#include <linux/types.h>

#include <asm/swab.h>

/*
* 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.
*/

//交换_u16类型的字节顺序，如short
#define ___constant_swab16(x) ((__u16)(                         \
        (((__u16)(x) & (__u16)0x00ffU) << 8) |                  \
        (((__u16)(x) & (__u16)0xff00U) >> 8)))
//交换_u32类型的字节顺序
#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)))
//交换_u64类型的字节顺序
#define ___constant_swab64(x) ((__u64)(                         \
        (((__u64)(x) & (__u64)0x00000000000000ffULL) << 56) |   \
        (((__u64)(x) & (__u64)0x000000000000ff00ULL) << 40) |   \
        (((__u64)(x) & (__u64)0x0000000000ff0000ULL) << 24) |   \
        (((__u64)(x) & (__u64)0x00000000ff000000ULL) << 8) |   \
        (((__u64)(x) & (__u64)0x000000ff00000000ULL) >> 8) |   \
        (((__u64)(x) & (__u64)0x0000ff0000000000ULL) >> 24) |   \
        (((__u64)(x) & (__u64)0x00ff000000000000ULL) >> 40) |   \
        (((__u64)(x) & (__u64)0xff00000000000000ULL) >> 56)))

#define ___constant_swahw32(x) ((__u32)(                        \
        (((__u32)(x) & (__u32)0x0000ffffUL) << 16) |            \
        (((__u32)(x) & (__u32)0xffff0000UL) >> 16)))

#define ___constant_swahb32(x) ((__u32)(                        \
        (((__u32)(x) & (__u32)0x00ff00ffUL) << 8) |             \
        (((__u32)(x) & (__u32)0xff00ff00UL) >> 8)))

/*
* Implement the following as inlines, but define the interface using
* macros to allow constant folding when possible:
* ___swab16, ___swab32, ___swab64, ___swahw32, ___swahb32
*/

static __inline__ __u16 __fswab16(__u16 val)
{
#ifdef __arch_swab16
        return __arch_swab16(val);
#else
        return ___constant_swab16(val);
#endif
}

static __inline__ __u32 __fswab32(__u32 val)
{
#ifdef __arch_swab32
        return __arch_swab32(val);
#else
        return ___constant_swab32(val);
#endif
}

static __inline__ __u64 __fswab64(__u64 val)
{
#ifdef __arch_swab64
         return __arch_swab64(val);
#elif defined(__SWAB_64_THRU_32__)
         __u32 h = val >> 32;
         __u32 l = val & ((1ULL << 32) - 1);
         return (((__u64)__fswab32(l)) << 32) | ((__u64)(__fswab32(h)));
#else
         return ___constant_swab64(val);
#endif
}

static __inline__ __u32 __fswahw32(__u32 val)
{
#ifdef __arch_swahw32
        return __arch_swahw32(val);
#else
        return ___constant_swahw32(val);
#endif
}

static __inline__ __u32 __fswahb32(__u32 val)
{
#ifdef __arch_swahb32
        return __arch_swahb32(val);
#else
        return ___constant_swahb32(val);
#endif
}

/**
* __swab16 - return a byteswapped 16-bit value
* @x: value to byteswap
*/
#define __swab16(x)                             \
        (__builtin_constant_p((__u16)(x)) ?     \
        ___constant_swab16(x) :                 \
        __fswab16(x))

/**
* __swab32 - return a byteswapped 32-bit value
* @x: value to byteswap
*/
#define __swab32(x)                             \
        (__builtin_constant_p((__u32)(x)) ?     \
        ___constant_swab32(x) :                 \
        __fswab32(x))

/**
* __swab64 - return a byteswapped 64-bit value
* @x: value to byteswap
*/
#define __swab64(x)                             \
        (__builtin_constant_p((__u64)(x)) ?     \
        ___constant_swab64(x) :                 \
        __fswab64(x))

/**
* __swahw32 - return a word-swapped 32-bit value
* @x: value to wordswap
*
* __swahw32(0x12340000) is 0x00001234
*/
#define __swahw32(x)                            \
        (__builtin_constant_p((__u32)(x)) ?     \
        ___constant_swahw32(x) :                \
        __fswahw32(x))

/**
* __swahb32 - return a high and low byte-swapped 32-bit value
* @x: value to byteswap
*
* __swahb32(0x12345678) is 0x34127856
*/
#define __swahb32(x)                            \
        (__builtin_constant_p((__u32)(x)) ?     \
        ___constant_swahb32(x) :                \
        __fswahb32(x))

/**
* __swab16p - return a byteswapped 16-bit value from a pointer
* @p: pointer to a naturally-aligned 16-bit value
*/
static __inline__ __u16 __swab16p(const __u16 *p)
{
#ifdef __arch_swab16p
        return __arch_swab16p(p);
#else
        return __swab16(*p);
#endif
}

/**
* __swab32p - return a byteswapped 32-bit value from a pointer
* @p: pointer to a naturally-aligned 32-bit value
*/
static __inline__ __u32 __swab32p(const __u32 *p)
{
#ifdef __arch_swab32p
        return __arch_swab32p(p);
#else
        return __swab32(*p);
#endif
}

/**
* __swab64p - return a byteswapped 64-bit value from a pointer
* @p: pointer to a naturally-aligned 64-bit value
*/
static __inline__ __u64 __swab64p(const __u64 *p)
{
#ifdef __arch_swab64p
        return __arch_swab64p(p);
#else
        return __swab64(*p);
#endif
}

/**
* __swahw32p - return a wordswapped 32-bit value from a pointer
* @p: pointer to a naturally-aligned 32-bit value
*
* See __swahw32() for details of wordswapping.
*/
static __inline__ __u32 __swahw32p(const __u32 *p)
{
#ifdef __arch_swahw32p
        return __arch_swahw32p(p);
#else
        return __swahw32(*p);
#endif
}

/**
* __swahb32p - return a high and low byteswapped 32-bit value from a pointe    r
* @p: pointer to a naturally-aligned 32-bit value
*
* See __swahb32() for details of high/low byteswapping.
*/
static __inline__ __u32 __swahb32p(const __u32 *p)
{
#ifdef __arch_swahb32p
         return __arch_swahb32p(p);
#else
         return __swahb32(*p);
#endif
}
/**
* __swab16s - byteswap a 16-bit value in-place215 * @p: pointer to a naturally-aligned 16-bit value
*/
static __inline__ void __swab16s(__u16 *p)
{
#ifdef __arch_swab16s
         __arch_swab16s(p);
#else
         *p = __swab16p(p);
#endif
}
/**
* __swab32s - byteswap a 32-bit value in-place
* @p: pointer to a naturally-aligned 32-bit value
*/
static __inline__ void __swab32s(__u32 *p)
{
#ifdef __arch_swab32s
         __arch_swab32s(p);
#else
         *p = __swab32p(p);
#endif
}

/**
* __swab64s - byteswap a 64-bit value in-place
* @p: pointer to a naturally-aligned 64-bit value
*/
static __inline__ void __swab64s(__u64 *p)
{
#ifdef __arch_swab64s
         __arch_swab64s(p);
#else
         *p = __swab64p(p);
#endif
}

/**
* __swahw32s - wordswap a 32-bit value in-place
* @p: pointer to a naturally-aligned 32-bit value
*
* See __swahw32() for details of wordswapping
*/
static __inline__ void __swahw32s(__u32 *p)
{
#ifdef __arch_swahw32s
         __arch_swahw32s(p);
#else
         *p = __swahw32p(p);
#endif
}

/**
* __swahb32s - high and low byteswap a 32-bit value in-place
* @p: pointer to a naturally-aligned 32-bit value
*
* See __swahb32() for details of high and low byte swapping
*/
static __inline__ void __swahb32s(__u32 *p)
{
#ifdef __arch_swahb32s
         __arch_swahb32s(p);
#else
         *p = __swahb32p(p);
#endif
}

#endif /* _LINUX_SWAB_H */

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小结：

本节主要介绍字节交换的一些方法，包含了整形数据、指针等的字节交换。

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