Byte order (network / host) conversion

Byte order （ The Internet / host ） transformation

Two common conversions ：

Host byte order port <-----------> Network byte order port （uint16_t <----------------> uint16_t）

IPv4 character string <----------------> Network byte order IPv4 （const char * <----------> unsigned int）

/********************************************************* Copyright  2022 Shengkai Liu. All rights reserved. FileName: byte_conversion.c Author: Shengkai Liu Date: 2022-05-30 ***********************************************************/

#include <stdio.h> // printf
#include <arpa/inet.h> // htons, ntohs, inet_pton, inet_ntop
#include <assert.h> // assert
#include <string.h> // strcmp

#define HOST_PORT 12345
#define IP "127.0.0.1"

int main()
{
    
    
    printf("port before conversion: 0x%x \n", HOST_PORT);

    // 1. host to network (unsigned short int)
    uint16_t network_port = htons(HOST_PORT);

    printf("port after conversion: 0x%x\n", network_port);

    // 2. network to host (unsigned short int)
    uint16_t host_port = ntohs(network_port);
    assert(host_port == HOST_PORT);

    
    printf("IPv4 before conversion: %s\n", IP);

    // 3. string of IPv4 to network (unsigned int)
    unsigned int ip_num = 0;
    inet_pton(AF_INET, IP, &ip_num);

    printf("IPv4 after conversion: %d\n", ip_num);

    // 4. network to string of IPv4 (char *)
    char ip_addr[16];
    inet_ntop(AF_INET, &ip_num, ip_addr, sizeof(ip_addr));
    assert(strcmp(IP, ip_addr) == 0);

    return 0;
}