Network-Programming-in-C/examples/udp_server.c

/*
* listener.c -- a datagram sockets "server" demo
*/

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

#define MYPORT "4950"   // the port users will be connecting to

#define MAXBUFLEN 100

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa){
    if( sa->sa_family == AF_INET){
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }

    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char* argv[] ){

    int sockfd;
    struct addrinfo hints, *servinfo, *p;
    int rv;
    int numbytes;
    struct sockaddr_storage their_addr;
    char buf[MAXBUFLEN];
    socklen_t addr_len;
    char s[INET6_ADDRSTRLEN];

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET6; // set to AF_INET to use IPv4
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_flags = AI_PASSIVE;  // use my IP

    if((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0){
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // loop through all the results and bind to the first we can
    for(p = servinfo ; p != NULL ; p = p->ai_next){
        if((sockfd = socket(p->ai_family, p->ai_socktype,
                        p->ai_protocol)) == -1 ){

            perror("listener: socket");
            continue;                    
        }

        if( bind(sockfd, p->ai_addr, p->ai_addrlen) == -1){
            close(sockfd);
            perror("listener: bind");
            continue;
        }

        break;
    }

    if( p == NULL ){
        fprintf(stderr, "listener: failed to bind socket\n");
        return 2;
    }

    freeaddrinfo(servinfo);

    printf("listener: waiting to recvfrom...\n");

    addr_len = sizeof their_addr;
    if((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1, 0,
        (struct sockaddr *)&their_addr, &addr_len)) == -1){
        
        perror("recvfrom");
        exit(1);
    }

    printf("listener: got packet from %s\n",
        inet_ntop(their_addr.ss_family,
        get_in_addr((struct sockaddr *)&their_addr),
        s, sizeof s));

    
    printf("listener: packet is %d bytes long\n", numbytes);
    buf[numbytes] = '\0';
    printf("listener: packet contains \"%s\"\n", buf);

    close(sockfd);
    return 0;
}