Programming

#include <stdio.h>
#include <netdb.h>
#include <sys/socket.h>
#include <arpa/inet.h>

char *GETHOSTBYADDR(const char *INADDR_ANY)
{
    struct sockaddr_in
    {
        short        sin_family;    // e.g. AF_INET, AF_INET6
        unsigned short    sin_port;    // e.g. htons(80)
        struct in_addr    sin_addr;    // see struct in_addr, below
        char        sin_zero[8];    // zero this if you want to
    };

    struct in_addr
    {
        unsigned long s_addr;    // load with inet_pton()
    };

    struct sockaddr_in *SA;

    SA.sin_addr.s_addr = INADDR_ANY; // set the local IP address
    SA.sin_port = htons(80); // set the port number

    static char HOSTNAME[128] = "";

    getnameinfo(SA, sizeof(SA), HOSTNAME, sizeof(HOSTNAME), NULL, 0, NI_NAMEREQD);

    return HOSTNAME;
}

int main(void)
{
    puts(GETHOSTBYADDR("209.85.229.106")); // testing on google's IP

    return 0;
}

sizeof(SA)

NAME
       gethostbyaddr, gethostbyname - network host database functions

SYNOPSIS
       #include <netdb.h>

       struct hostent *gethostbyaddr(const void *addr, socklen_t len,
              int type);
       struct hostent *gethostbyname(const char *name);

char *IPtoHOST(const char *IP) // returns host name from IP address. If failure, returns IP address.
{
	static char BUFFER[65] = "";

	struct sockaddr_in SIN; // create a struct of sockaddr_in type
	memset(&SIN, 0, sizeof(SIN)); // zero SIN struct before use
	SIN.sin_family		= AF_INET; // specify address family
	SIN.sin_addr.s_addr	= inet_addr(IP); // pass the IP
	SIN.sin_port		= 0; // specify port: 0 is chosen by the system. htons(port) is for manual specification (it seems to work specifying any port so I let system choose)

	getnameinfo( (struct sockaddr *)&SIN /* type cast sockaddr_in to sockaddr */, sizeof(SIN), BUFFER, 64, NULL, 0, 0); // If the last parameter (flags) is 0 and if getnameinfo fails, then the IP is written into BUFFER instead of the host name

	return BUFFER;
}