Network byte order

Network byte order description

TCP/IP Provisions of the agreement , Network data stream should adopt big end byte order , That is, low address, high byte . For example, in the previous section UDP Segment format , Address 0-1 yes 16 Bit source port number , If the port number is 1000（0x3e8）, Address 0 yes 0x03, Address 1 yes 0xe8, That is, the first 0x03, Reissue 0xe8, this 16 The bit in the buffer of the sending host should also be a low address memory 0x03, Highland site storage 0xe8. however , If the sending host is small endian , this 16 Bits are interpreted as 0xe803, instead of 1000. therefore , Send the host to 1000 Byte order conversion is required before filling in the send buffer . similarly , If the receiving host is in small end byte order , Receive 16 The source port number of bit also needs to be converted into byte order . If the host is big endian , No conversion is required for sending and receiving . Empathy ,32 Bit IP The address should also consider the network byte order and host byte order .
In order to make the network program portable , Make the same C The code can run normally after compiling on both big end and small end computers , You can call the following library functions to do network byte order and host byte order conversion .

Network big end , Small end of host

Network and host byte order conversion function – Data are all addresses

#include <arpa/inet.h>

uint32_t htonl(uint32_t hostlong); Host to network , Long
uint16_t htons(uint16_t hostshort); Network to host , short
uint32_t ntohl(uint32_t netlong); Network to host , Long
uint16_t ntohs(uint16_t netshort); Network to host , short
h Express host,n Express network,l Express 32 Bit long integers ,s Express 16 Bit short integer .
If the address of the host is small endian , These functions do the corresponding large and small conversion of parameters, and then return , If the host is big endian , These functions don't convert , Return the parameters as they are .

Dot decimal string conversion function

#include <arpa/inet.h>
int inet_pton(int af, const char *src, void *dst);// Hexadecimal string to big end

const char *inet_ntop(int af, const void *src, char *dst, socklen_t size);// Big end to hexadecimal string 

Support IPv4 and IPv6
Reentrant function
among inet_pton and inet_ntop Not only can it be converted IPv4 Of in_addr, You can also convert IPv6 Of in6_addr.
So the function interface is void *addrptr.

Code

#include <stdio.h>

#include <arpa/inet.h>
int main(int argc, char *argv[])
{
    

	char buf[]="192.168.1.4";
	unsigned int num=0;
	inet_pton(AF_INET,buf,&num);
	unsigned char * p = (unsigned char *)&num;
	printf("%d %d %d %d\n",*p,*(p+1),*(p+2),*(p+3));
	char ip[16]="";
	printf("%s\n",inet_ntop(AF_INET,&num,ip,16));
	return 0;
}