/* dgramsrvr.c: *  * Example datagram server : */#include <stdio.h>#include <unistd.h>#include <stdlib.h>#include <errno.h>#include <string.h>#include <time.h>#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>/* * This function reports the error and * exits back to the shell : */static voidbail(const char *on_what) {    fputs(strerror(errno),stderr);    fputs(": ",stderr);    fputs(on_what,stderr);    fputc('\n',stderr);    exit(1);}intmain(int argc,char **argv) {    int z;    char *srvr_addr = NULL;    struct sockaddr_in adr_inet;/* AF_INET */    struct sockaddr_in adr_clnt;/* AF_INET */    int len_inet;               /* length  */    int s;                       /* Socket */    char dgram[512];        /* Recv buffer */    char dtfmt[512];   /* Date/Time Result */    time_t td;    /* Current Time and Date */    struct tm tm;      /* Date time values */    /*     * Use a server address from the command     * line, if one has been provided.     * Otherwise, this program will default     * to using the arbitrary address     * 127.0.0.23 :     */    if ( argc >= 2 ) {        /* Addr on cmdline: */        srvr_addr = argv[1];    } else {        /* Use default address: */        srvr_addr = "127.0.0.23";    }    /*     * Create a UDP socket to use :     */    s = socket(AF_INET,SOCK_DGRAM,0);    if ( s == -1 )        bail("socket()");    /*     * Create a socket address, for use     * with bind(2) :     */    memset(&adr_inet,0,sizeof adr_inet);    adr_inet.sin_family = AF_INET;    adr_inet.sin_port = htons(9090);    adr_inet.sin_addr.s_addr =        inet_addr(srvr_addr);    if ( adr_inet.sin_addr.s_addr == INADDR_NONE )        bail("bad address.");    len_inet = sizeof adr_inet;    /*     * Bind a address to our socket, so that     * client programs can contact this     * server:     */    z = bind(s,        (struct sockaddr *)&adr_inet,        len_inet);    if ( z == -1 )        bail("bind()");    /*     * Now wait for requests:     */    for (;;) {        /*         * Block until the program receives a         * datagram at our address and port:         */        len_inet = sizeof adr_clnt;        z = recvfrom(s,                /* Socket */            dgram,           /* Receiving buffer */            sizeof dgram,   /* Max recv buf size */            0,              /* Flags: no options */            (struct sockaddr *)&adr_clnt,/* Addr */            &len_inet);    /* Addr len, in & out */        if ( z < 0 )            bail("recvfrom(2)");        /*         * Process the request :         */        dgram[z] = 0;          /* null terminate */        if ( !strcasecmp(dgram,"QUIT") )            break;                /* Quit server */        /*         * Get the current date and time:         */        time(&td);    /* Get current time & date */        tm = *localtime(&td);  /* Get components */        /*         * Format a new date and time string,         * based upon the input format string:         */        strftime(dtfmt,      /* Formatted result */            sizeof dtfmt,     /* Max result size */            dgram,     /* Input date/time format */            &tm);      /* Input date/time values */        /*         * Send the formatted result back to the         * client program:         */        z = sendto(s,   /* Socket to send result */            dtfmt, /* The datagram result to snd */            strlen(dtfmt), /* The datagram lngth */            0,              /* Flags: no options */            (struct sockaddr *)&adr_clnt,/* addr */            len_inet);  /* Client address length */        if ( z < 0 )            bail("sendto(2)");    }    /*     * Close the socket and exit:     */    close(s);    return 0;}