/* This is an UDP server which deals with the clients upon receiving a SIGIO
   signal. The signal is generated whenever a datagram arrives.

   For this kind of client handling, three steps have to be done:
	1. Establish signal handler for SIGIO
	2. Set the owner for receiving SIGIO using fcntl()
	3. Set the O_ASYNC flag for socket using fcntl() or use ioctl()
*/

#include <fcntl.h>
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define PORT 9000
#define SA struct sockaddr
#ifdef __linux__
#define IP_RECVDSTADDR IP_PKTINFO
struct in_pktinfo *ip_pkt;
#elif __FreeBSD__
struct in_addr *ip_addr;
#endif

void handle_incoming_data(void);

int sock; /* Global member. */

void handle_sigio(int signo) {
    printf("In handle_sigio() function.\n");
    handle_incoming_data();
    return;
}

void handle_sigaction(int signo, siginfo_t *details, void *addr) {
    printf("In handle_sigaction() function.\n");
    if (details->si_code == SI_SIGIO)
    	printf("File handle: %d; sock = %d\n", details->si_fd, sock);
    else	
	printf("si_code = %d\n", details->si_code);
    handle_incoming_data();
    return;
}

int main(int argc, char **argv) {
    int retn, temp, flags;
    struct sockaddr_in addr;
    char buf[128];
    int optval, optlen;
    struct sigaction act;

    /* Create a socket. */
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0) {
	perror("socket");
	exit(-1);
    }
    printf("Created socket.\n");

    /* Set the socket to return destination address as ancillary data. */
    optval = 1, optlen = sizeof(optval);
    retn = setsockopt(sock, IPPROTO_IP, IP_RECVDSTADDR, &optval, optlen);
    if (retn < 0) {
	perror("IP_PKTINFO");
	close(sock);
	exit(-1);
    }
    printf("Successfully set the option to receive destination address.\n");

    /* 1. Establish signal handler. */
    act.sa_sigaction = &handle_sigaction;
    act.sa_flags = SA_SIGINFO;
    printf("PID: %d\n", getpid());
    sigaction(SIGIO, &act, NULL);
    
    /* 2. Set the SIGIO owner */
    retn = fcntl(sock, F_SETOWN, getpid());
    if (retn < 0) {
	perror("fcntl:F_SETOWN");
	close(sock);
	exit(-3);
    }
    printf("Successfully set the signal handler for SIGIO signal.\n");

    /* 3. Set the ASYNC flag for socket. */
    flags = fcntl(sock, F_GETFL);
    retn = fcntl(sock, F_SETFL, flags | O_ASYNC);
    if (retn < 0) {
	perror("fcntl:F_SETFL");
	close(sock);
	exit(-1);
    }
    printf("Set the O_ASYNC flag for the socket to facilitate SIGIO.\n");

    /* Fill in the address to bind. */
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(PORT);
    addr.sin_addr.s_addr = htons(INADDR_ANY);

    /* Bind the socket. */
    retn = bind(sock, (SA *)&addr, sizeof(SA));
    if (retn < 0) {
	perror("Bind");
	close(sock);
	exit(-2);
    }
    printf("Bound the socket successfully.\n");

    /* Wait for signal and its processing. */
    pause(); 

    close(sock);
    return(0);
}

/* Routine to handle incoming data. */
void handle_incoming_data(void) {
    struct sockaddr_in peer;
    int retn, len;
    char ip[20], mesg[128];
    struct iovec iov[1];
    struct msghdr msg;
    struct cmsghdr *cmsg;

    /* Obtain the client address. Data is also received here. */
    len = sizeof(SA);
    memset(mesg, 0, sizeof(mesg));
    retn = recvfrom(sock, mesg, 128, MSG_PEEK, (SA *)&peer, &len);
    if (retn < 0) {
	perror("recvfrom");
	return;
    }
    printf("Received message from a client.\n");
    printf("  Client-address: %s\n", inet_ntoa(peer.sin_addr));

    /* Now receive the destination address. */
    iov[0].iov_base = mesg;
    iov[0].iov_len = 1;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_control = calloc(1, 100);
    msg.msg_controllen = 100;
    msg.msg_iov = &iov[0];
    msg.msg_iovlen = 1;

    /* Now issue the recvmsg() */
    retn = recvmsg(sock, &msg, 0);
    if (retn < 0) {
	perror("recvmsg");
	return;
    }
    if (! msg.msg_controllen) {
    	printf("  Message: %s\n", mesg);
	printf("No ancillary data present. Returning.\n");
	return;
    }
    
    /* Get the ancillary data using CMSG_* defines. 
	The code becomes platform specific from hereon. */
    cmsg = CMSG_FIRSTHDR(&msg);
    for(;cmsg != NULL; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
	if ((cmsg->cmsg_level == IPPROTO_IP)
	  &&(cmsg->cmsg_type == IP_RECVDSTADDR)) {
#ifdef __linux__
	    ip_pkt = (struct in_pktinfo *)CMSG_DATA(cmsg);
	    printf("  Destination address: %s\n", inet_ntoa(ip_pkt->ipi_addr));
#elif __FreeBSD__
	    ip_addr = (struct in_addr *)CMSG_DATA(cmsg);
	    printf("  Destination address: %s\n", inet_ntoa(*ip_addr));
#endif
	}
    }
    printf("  Message: %s\n", mesg);

    return;
}