网络字节序和主机字节序转换函数

 大小端判断

#include <stdio.h>
#include <stdint.h>
int is_little_endian()
{
    union {
        uint32_t num;
        uint8_t c;
    }e;
    e.num = 1;
    return e.c;
}
int main(void)
{
    if(!is_little_endian()){
        printf("big endian\n");
    }else{
        printf("little endian\n");
    }
    return 0;
}

gcc endian.c -o endian

 由执行结果可知，是小端。

$ ./endian
little endian

一 大小端互转相关函数

AME
       htobe16, htole16, be16toh, le16toh, htobe32, htole32, be32toh, le32toh, htobe64, htole64, be64toh, le64toh - convert values between host and big-/little-endian byte order

SYNOPSIS
       #define _BSD_SOURCE             /* See feature_test_macros(7) */
       #include <endian.h>

       uint16_t htobe16(uint16_t host_16bits);
       uint16_t htole16(uint16_t host_16bits);
       uint16_t be16toh(uint16_t big_endian_16bits);
       uint16_t le16toh(uint16_t little_endian_16bits);

       uint32_t htobe32(uint32_t host_32bits);
       uint32_t htole32(uint32_t host_32bits);
       uint32_t be32toh(uint32_t big_endian_32bits);
       uint32_t le32toh(uint32_t little_endian_32bits);

       uint64_t htobe64(uint64_t host_64bits);
       uint64_t htole64(uint64_t host_64bits);
       uint64_t be64toh(uint64_t big_endian_64bits);
       uint64_t le64toh(uint64_t little_endian_64bits);

DESCRIPTION
       These functions convert the byte encoding of integer values from the byte order that the current CPU (the "host") uses, to and from little-endian and big-endian byte order.

       The number, nn, in the name of each function indicates the size of integer handled by the function, either 16, 32, or 64 bits.

       The functions with names of the form "htobenn" convert from host byte order to big-endian order.

       The functions with names of the form "htolenn" convert from host byte order to little-endian order.

       The functions with names of the form "benntoh" convert from big-endian order to host byte order.

       The functions with names of the form "lenntoh" convert from little-endian order to host byte order.

二 转换函数 

进行网络字节序转换的函数有htons()、ntohs()、htonl()、ntohl()等，其中s是short数据类型的意思，l是long数据类型的意思，而h是host，即主机的意思，n是network，即网络的意思。以上4个函数分别如下。

htons()：表示对于short类型的变量，从主机字节序转换为网络字节序。

ntohs()：表示对于short类型的变量，从网络字节序转换为主机字节序。

htonl()：表示对于long类型的变量，从主机字节序转换为网络字节序。

ntohl()：表示对于long类型的变量，从网络字节序转换为主机字节序。

NAME
       htonl, htons, ntohl, ntohs - convert values between host and network byte order

SYNOPSIS
       #include <arpa/inet.h>

       uint32_t htonl(uint32_t hostlong);

       uint16_t htons(uint16_t hostshort);

       uint32_t ntohl(uint32_t netlong);

       uint16_t ntohs(uint16_t netshort);

DESCRIPTION
       The htonl() function converts the unsigned integer hostlong from host byte order to network byte order.

       The htons() function converts the unsigned short integer hostshort from host byte order to network byte order.

       The ntohl() function converts the unsigned integer netlong from network byte order to host byte order.

       The ntohs() function converts the unsigned short integer netshort from network byte order to host byte order.

       On the i386 the host byte order is Least Significant Byte first, whereas the network byte order, as used on the Internet, is Most Significant Byte first.

三 内核中转换相关宏最终实现

#define ___constant_swab16(x) ((__u16)(				\
	(((__u16)(x) & (__u16)0x00ffU) << 8) |			\
	(((__u16)(x) & (__u16)0xff00U) >> 8)))

#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)(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)))

小结

意htons()和ntohs()函数及htonl()和ntohl()函数是对应的转换，两个函数完全可以使用同一套代码，例如：

#define ntohl htonl