htonl(),htons(),ntohl(),ntons()-- Endianness Conversion Functions

htonl(),htons(),ntohl(),ntons()-- Endianness Conversion Functions

Different machines store variable bytes in different orders internally; some use big-endian mode, while others use little-endian mode. Big-endian mode means that the most significant byte (MSB) is stored at the lowest memory address, and the least significant byte (LSB) is stored at the highest memory address. Little-endian mode means that the least significant byte (LSB) is stored at the lowest memory address, and the most significant byte (MSB) is stored at the highest memory address.

When transmitting data over a network, because the two ends of data transmission may correspond to different hardware platforms and the byte order used for storage may also be inconsistent, the TCP/IP protocol therefore stipulates that network byte order (which is big-endian mode) must be used on the network. Analysis of the endianness storage principles reveals that for char type data, this problem does not exist because it occupies only one byte; this is also one of the reasons why data buffers are generally defined as char type. For non-char type data such as IP addresses and port numbers, it must be converted to big-endian mode before being sent over the network, and then converted back to the receiving host's storage mode after being received.

The Linux system provides four functions for endianness conversion. The function prototypes can be obtained by entering the man byteorder command:

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1. #include <arpa/inet.h> 

2. uint32_t htonl(uint32_t hostlong); 

3. uint16_t htons(uint16_t hostshort); 

4. uint32_t ntohl(uint32_t netlong); 

5. uint16_t ntohs(uint16_t netshort); 

#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);

htons stands for host to network short, and is used to convert host unsigned short type data to network byte order.

htonl stands for host to network long, and is used to convert host unsigned int type data to network byte order. The functions of ntohl and ntohs are the inverse of htonl and htons, respectively.