转载：

Then when do we use le32_to_cpu( ) , and when do we use cpu_to_le32( )?

>

> The names tell it. le32_to_cpu is used for convesions from 32bit little

> endian data into CPUs endianness, cpu_to_le32 is used for convesions from

> CPU endianness to little endian 32bit data.

> Actually, both macros do the same thing, but one should make the differences

> clear to make code more readable so that anyone can quickly find out whether

> some data is kept in native endianness or some particular one.

>

code example of big endian and little endian

// 2-byte number

int SHORT_little_endian_TO_big_endian(int i)

{

return ((i>>8)&0xff)+((i << 8)&0xff00);

}

// 4-byte number

int INT_little_endian_TO_big_endian(int i)

{

return((i&0xff)<<24)+((i&0xff00)<<8)+((i&0xff0000)>>8)+((i>>24)&0xff);

}

今天在学习< Understanding Linux Networking Insides >, 字节序的转换也是这几个函数的封装。

所有的协议都要定义使用的字节序，TCP/IP 使用的Big-Endian,

include/linux/byteorder/generic.h

#define ___htonl(x) __cpu_to_be32(x)

#define ___htons(x) __cpu_to_be16(x)

#define ___ntohl(x) __be32_to_cpu(x)

#define ___ntohs(x) __be16_to_cpu(x)

#define htonl(x) ___htonl(x)

#define ntohl(x) ___ntohl(x)

#define htons(x) ___htons(x)

#define ntohs(x) ___ntohs(x)