udp.c

//
// To compile:
// gcc -Wall -pedantic -O3 -fomit-frame-pointer -o agent udp.c
//

#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define SERVICE_PORT 9999
#define BUFSIZE 2048

extern int errno;

/**
 * Create and return a socket handler for given port number.
 *
 * Param:
 * port - the port which the socket will be opened. If it equals
 *        to 0, then a random port will be assigned to the socket.
 *
 * Return: a socket handler, or -1 if socket creation fails
 */
int create_udp_socket(const int & port) {
    int sock;
    struct sockaddr_in addr;

    // create an udp socket
    if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("cannot create socket\n");
        return -1;
    }

    // set up our address
    memset(&addr, 0, sizeof(addr));
    addr.sin_len = sizeof(addr);
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = INADDR_ANY;  // bind it to all local addresses

    // bind the server to the address
    if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
        perror("socket binding failed\n");
        return -1;
    }
    return sock;
}

/**
 * Listen any message on given port and then print it to standard output.
 *
 * Param:
 * port - the port which the server will listen to
 */
void serve(const int& port) {
    int sock;
    struct sockaddr_in client_addr;
    socklen_t addrlen = sizeof(client_addr);
    int recvlen;
    unsigned char buf[BUFSIZE];
    char hostname[128];

    // get server's hostname
    gethostname(hostname, 128);

    // create an udp socket
    sock = create_udp_socket(port);
    if (sock == -1) return;

    printf("server started on %s, listening on port %d\n", hostname, port);

    // loop forever, waiting for connection requests and process the message
    while (true) {
        recvlen = recvfrom(sock, buf, BUFSIZE, 0, (struct sockaddr *)&client_addr,
                           &addrlen);
        if (recvlen > 0) {
            buf[recvlen] = 0;
            printf("received message: %s\nfrom:%s port:%d\n",
                    buf,
                    inet_ntoa(client_addr.sin_addr),
                    ntohs(client_addr.sin_port));
        }
    }
}

/**
 * Send a message to given address and port.
 *
 * Param:
 * message - a message to be sent
 * address - destination IP address
 * port    - destination port number
 *
 * Return: 0 if it is successful, otherwise non-zero integer
 */
int send(const char *message, const char *address, const int port) {
    int sock;
    int i;
    struct sockaddr_in server_addr;
    socklen_t addrlen = sizeof(server_addr);
    char buf[BUFSIZE];

    // create an udp socket
    sock = create_udp_socket(0);
    if (sock == -1) return -1;

    // Set up server's address to whom we want to send messages.
    // The address is expressed as a numeric IPv4 address that we will
    // convert to a binary format using inet_aton
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    if (inet_aton(address, &server_addr.sin_addr) == 0) {
        perror("inet_atom() failed\n");
        return 1;
    }

    // now, send the message 5 times
    for (i = 0; i < 5; i++) {
        sprintf(buf, "{\"id\":%d,\"message\":\"%s\"}", i, message);
        printf("sending packet %d to %s port %d\nmessage: %s\n",
                i, address, port, buf);
        if (sendto(sock, buf, strlen(buf), 0, (struct sockaddr *)&server_addr,
                addrlen) < 0) {
            perror("sendto() failed\n");
        }
    }
    close(sock);
    return 0;
}

int main(int argc, char **argv) {
    extern char *optarg;
    extern int optind;
    int c, err = 0;
    int port = SERVICE_PORT;
    char * message = NULL;
    char buf[BUFSIZE];
    char address[20];
    
    sprintf(address, "127.0.0.1");
    while ((c = getopt(argc, argv, "c:a:p:h")) != -1) {
        switch (c) {
        case 'c':
            message = optarg;
            if (strlen(message) <= 0) {
                perror("empty message\n");
                err = 1;
            }
            break;
        case 'a':
            sprintf(address, "%s", optarg);
            if (strlen(address) <= 0) {
                perror("empty address\n");
                err = 2;
            }
            break;
        case 'p':
            port = atoi(optarg);
            if (port < 1024 || port > 65535) {
                sprintf(buf, "invalid port number: %d\n", port);
                perror(buf);
                err = 3;
            }
            break;
        case 'h':
        case '?':
            err = 4;
            break;
        }
    }
    if (err != 0 || (optind < argc)) {
        printf("usage: %s [-c message] [-a address] [-p port]\n", argv[0]);
        return 1;
    }

    if (message == NULL)
        serve(port);
    else
        send(message, address, port);

    return 0;
}