path: root/contrib/ntp/sntp/main.c

/*  Copyright (C) 1996, 1997, 2000 N.M. Maclaren
    Copyright (C) 1996, 1997, 2000 The University of Cambridge

This is a complete SNTP implementation, which was easier to write than to port
xntp to a new version of Unix with any hope of maintaining it thereafter.  It
supports the full SNTP (RFC 2030) client- and server-side challenge-response
and broadcast protocols.  It should achieve nearly optimal accuracy with very
few transactions, provided only that a client has access to a trusted server
and that communications are not INVARIABLY slow.  As this is the environment in
which 90-99% of all NTP systems are run ....

The specification of this program is:

    msntp [ --help | -h | -? ] [ -v | -V | -W ]
        [ -B [ period ] | -S | -q [ -f savefile ] |
            [ { -r | -a } [ -P prompt ] [ -l lockfile ] ]
            [ -c count ] [ -e minerr ][ -E maxerr ]
            [ -d delay | -x [ separation ] [ -f savefile ] ]
            [ -4 | -6 ] [ address(es) ] ]

    --help, -h and -? all print the syntax of the command.

    -v indicates that diagnostic messages should be written to standard error,
and -V requests more output for investigating apparently inconsistent
timestamps.  -W requests very verbose debugging output, and will interfere with
the timing when writing to the terminal (because of line buffered output from
C); it is useful only when debugging the source.  Note that the times produced
by -V and -W are the corrections needed, and not the error in the local clock.

    -B indicates that it should behave as a server, broadcasting time packets
at intervals of 'period' minutes.  Acceptable values of 'period' are from 1 to
1440 (a day) and the default is 60.  Naturally, this will work only if the
user has enough privilege.

    -S indicates that it should behave as a server, responding to time requests 
from clients.  Naturally, this will work only if the user has enough privilege.

    -q indicates that it will query a savefile that is being maintained by
it being run in daemon mode.

     The default is that it should behave as a client, and the following options
are then relevant:

    -r indicates that the system clock should be reset by 'settimeofday'.
Naturally, this will work only if the user has enough privilege.

    -a indicates that the system clock should be reset by 'adjtime'.
Naturally, this will work only if the user has enough privilege.

    -x indicates that the program should run as a daemon (i.e. forever), and
allow for clock drift.

    -4 or -6 force dns resolving to ipv4 or ipv6 addresses.

    The default is to write the current date and time to the standard output in
a format like '1996 Oct 15 20:17:25.123 + 4.567 +/- 0.089 secs', indicating the
estimated true (local) time and the error in the local clock.  In daemon mode,
it will add drift information in a format like ' + 1.3 +/- 0.1 ppm', and
display this at roughly 'separation' intervals.

    'minerr' is the maximum ignorable variation between the clocks.  Acceptable
values are from 0.001 to 1, and the default is 0.1 if 'address' is specified
and 0.5 otherwise.

    'maxerr' is the maximum value of various delays that are deemed acceptable.
Acceptable values are from 1 to 60, and the default is 5.  It should sometimes
be increased if there are problems with the network, NTP server or system
clock, but take care.

    'prompt' is the maximum clock change that will be made automatically.
Acceptable values are from 1 to 3600, and the default is 30.  If the program is
being run interactively, larger values will cause a prompt.  The value may also
be 'no', and the change will be made without prompting.

    'count' is the maximum number of NTP packets to require.  Acceptable values
are from 1 to 25 if 'address' is specified and '-x' is not, and from 5 to 25
otherwise; the default is 5.  If the maximum isn't enough, you need a better
consistency algorithm than this program uses.  Don't increase it.

    'delay' is a rough limit on the total running time in seconds.  Acceptable
values are from 1 to 3600, and the default is 15 if 'address' is specified and
300 otherwise.

    'separation' is the time to wait between calls to the server in minutes if
'address' is specified, and the minimum time between broadcast packets if not.
Acceptable values are from 1 to 1440 (a day), and the default is 300.

    'lockfile' may be used in an update mode to ensure that there is only
one copy of msntp running at once.  The default is installation-dependent,
but will usually be /etc/msntp.pid.

    'savefile' may be used in daemon mode to store a record of previous
packets, which may speed up recalculating the drift after msntp has to be
restarted (e.g. because of network or server outages).  The default is
installation-dependent, but will usually be /etc/msntp.state.  Note that
there is no locking of this file, and using it twice may cause chaos.

    'address' is the DNS name or IP number of a host to poll; if no name is
given, the program waits for broadcasts.  Note that a single component numeric
address is not allowed.

For sanity, it is also required that 'minerr' < 'maxerr' < 'delay' (if
listening for broadcasts, 'delay/count' and, in daemon mode, 'separation') and,
for sordid Unixish reasons, that 2*'count' < 'delay'.  The last could be fixed,
but isn't worth it.  Note that none of the above values are closely linked to
the limits described in the NTP protocol (RFC 1305).  Do not increase the
compiled-in bounds excessively, or the code will fail.

The algorithm used to decide whether to accept a correction is whether it would
seem to improve matters.  Unlike the 'xntp' suite, little attempt is made to
handle really knotted scenarios, and diagnostics are written to standard error.
In non-daemon client mode, it is intended to be run as a command or in a 'cron'
job.  Unlike 'ntpdate', its default mode is simply to display the clock error.

It assumes that floating-point arithmetic is tolerably efficient, which is true
for even the cheapest personal computer nowadays.  If, however, you want to
port this to a toaster, you may have problems!

In its terminating modes, its return code is EXIT_SUCCESS if the operation was
completed successfully and EXIT_FAILURE otherwise.

In server or daemon mode, it runs for ever and stops with a return code
EXIT_FAILURE only after a severe error.  Commonly, two server processes will be
run, one with each of the -B and -S options.  In daemon mode, it will fail if
the server is inaccessible for a long time or seriously sick, and will need
manual restarting.


WARNING: this program has reached its 'hack count' and needs restructuring,
badly.  Perhaps the worst code is in run_daemon().  You are advised not to
fiddle unless you really have to. */



#include "header.h"

#include <limits.h>
#include <float.h>
#include <math.h>

#define MAIN
#include "kludges.h"
#undef MAIN



/* NTP definitions.  Note that these assume 8-bit bytes - sigh.  There is
little point in parameterising everything, as it is neither feasible nor
useful.  It would be very useful if more fields could be defined as
unspecified.  The NTP packet-handling routines contain a lot of extra
assumptions. */

#define JAN_1970   2208988800.0        /* 1970 - 1900 in seconds */
#define NTP_SCALE  4294967296.0        /* 2^32, of course! */

#define NTP_PACKET_MIN       48        /* Without authentication */
#define NTP_PACKET_MAX       68        /* With authentication (ignored) */
#define NTP_DISP_FIELD        8        /* Offset of dispersion field */
#define NTP_REFERENCE        16        /* Offset of reference timestamp */
#define NTP_ORIGINATE        24        /* Offset of originate timestamp */
#define NTP_RECEIVE          32        /* Offset of receive timestamp */
#define NTP_TRANSMIT         40        /* Offset of transmit timestamp */

#define NTP_LI_FUDGE          0        /* The current 'status' */
#define NTP_VERSION           3        /* The current version */
#define NTP_VERSION_MAX       4        /* The maximum valid version */
#define NTP_STRATUM          15        /* The current stratum as a server */
#define NTP_STRATUM_MAX      15        /* The maximum valid stratum */
#define NTP_POLLING           8        /* The current 'polling interval' */
#define NTP_PRECISION         0        /* The current 'precision' - 1 sec. */

#define NTP_ACTIVE            1        /* NTP symmetric active request */
#define NTP_PASSIVE           2        /* NTP symmetric passive response */
#define NTP_CLIENT            3        /* NTP client request */
#define NTP_SERVER            4        /* NTP server response */
#define NTP_BROADCAST         5        /* NTP server broadcast */

#define NTP_INSANITY     3600.0        /* Errors beyond this are hopeless */
#define RESET_MIN            15        /* Minimum period between resets */
#define ABSCISSA            3.0        /* Scale factor for standard errors */



/* Local definitions and global variables (mostly options).  These are all of
the quantities that control the main actions of the program.  The first three 
are the only ones that are exported to other modules. */

const char *argv0 = NULL;              /* For diagnostics only - not NULL */
int verbose = 0,                       /* Default = 0, -v = 1, -V = 2, -W = 3 */
    operation = 0;                     /* Defined in header.h - see action */
const char *lockname = NULL;           /* The name of the lock file */

#define COUNT_MAX          25          /* Do NOT increase this! */
#define WEEBLE_FACTOR     1.2          /* See run_server() and run_daemon() */
#define ETHERNET_MAX        5          /* See run_daemon() and run_client() */

#define action_display      1          /* Just display the result */
#define action_reset        2          /* Reset using 'settimeofday' */
#define action_adjust       3          /* Reset using 'adjtime' */
#define action_broadcast    4          /* Behave as a server, broadcasting */
#define action_server       5          /* Behave as a server for clients */
#define action_query        6          /* Query a daemon savefile */

#define save_read_only      1          /* Read the saved state only */
#define save_read_check     2          /* Read and check it */
#define save_write          3          /* Write the saved state */
#define save_clear          4          /* Clear the saved state */

static const char version[] = VERSION; /* For reverse engineering :-) */
static int action = 0,                 /* Defined above - see operation */
    period = 0,                        /* -B value in seconds (broadcast) */
    count = 0,                         /* -c value in seconds */
    delay = 0,                         /* -d or -x value in seconds */
    attempts = 0,                      /* Packets transmitted up to 2*count */
    waiting = 0,                       /* -d/-c except for in daemon mode */
    locked = 0;                        /* set_lock(1) has been called */
static double outgoing[2*COUNT_MAX],   /* Transmission timestamps */
    minerr = 0.0,                      /* -e value in seconds */
    maxerr = 0.0,                      /* -E value in seconds */
    prompt = 0.0,                      /* -p value in seconds */
    dispersion = 0.0;                  /* The source dispersion in seconds */
static FILE *savefile = NULL;          /* Holds the data to restart from */



/* The unpacked NTP data structure, with all the fields even remotely relevant
to SNTP. */

typedef struct NTP_DATA {
    unsigned char status, version, mode, stratum, polling, precision;
    double dispersion, reference, originate, receive, transmit, current;
} ntp_data;



/* The following structure is used to keep a record of packets in daemon mode;
it contains only the information that is actually used for the drift and error
calculations. */

typedef struct {
    double dispersion, weight, when, offset, error;
} data_record;



void fatal (int syserr, const char *message, const char *insert) {

/* Issue a diagnostic and stop.  Be a little paranoid about recursion. */

    int k = errno;
    static int called = 0;

    if (message != NULL) {
        fprintf(stderr,"%s: ",argv0);
        fprintf(stderr,message,insert);
        fprintf(stderr,"\n");
    }
    errno = k;
    if (syserr) perror(argv0);
    if (! called) {
        called = 1;
        if (savefile != NULL && fclose(savefile))
            fatal(1,"unable to close the daemon save file",NULL);
        if (locked) set_lock(0);
    }
    exit(EXIT_FAILURE);
}



void syntax (int halt) {

/* The standard, unfriendly Unix error message.  Some errors are diagnosed more
helpfully.  This is called before any files or sockets are opened. */

    fprintf(stderr,"Syntax: %s [ --help | -h | -? ] [ -v | -V | -W ] \n",argv0);
    fprintf(stderr,"    [ -B period | -S | -q [ -f savefile ] |\n");
    fprintf(stderr,"        [ { -r | -a } [ -P prompt ] [ -l lockfile ] ]\n");
    fprintf(stderr,"            [ -c count ] [ -e minerr ] [ -E maxerr ]\n");
    fprintf(stderr,"            [ -d delay | -x [ separation ] ");
    fprintf(stderr,"[ -f savefile ] ]\n");
    fprintf(stderr,"        [ -4 | -6 ] [ address(es) ] ]\n");
    if (halt) exit(EXIT_FAILURE);
}



void display_data (ntp_data *data) {

/* This formats the essential NTP data, as a debugging aid. */

    fprintf(stderr,"sta=%d ver=%d mod=%d str=%d pol=%d dis=%.6f ref=%.6f\n",
        data->status,data->version,data->mode,data->stratum,data->polling,
        data->dispersion,data->reference);
    fprintf(stderr,"ori=%.6f rec=%.6f\n",data->originate,data->receive);
    fprintf(stderr,"tra=%.6f cur=%.6f\n",data->transmit,data->current);
}



void display_packet (unsigned char *packet, int length) {

/* This formats a possible packet very roughly, as a debugging aid. */

    int i;

    if (length < NTP_PACKET_MIN || length > NTP_PACKET_MAX) return;
    for (i = 0; i < length; ++i) {
        if (i != 0 && i%32 == 0)
            fprintf(stderr,"\n");
         else if (i != 0 && i%4 == 0)
             fprintf(stderr," ");
         fprintf(stderr,"%.2x",packet[i]);
    }
    fprintf(stderr,"\n");
}



void pack_ntp (unsigned char *packet, int length, ntp_data *data) {

/* Pack the essential data into an NTP packet, bypassing struct layout and
endian problems.  Note that it ignores fields irrelevant to SNTP. */

    int i, k;
    double d;

    memset(packet,0,(size_t)length);
    packet[0] = (data->status<<6)|(data->version<<3)|data->mode;
    packet[1] = data->stratum;
    packet[2] = data->polling;
    packet[3] = data->precision;
    d = data->originate/NTP_SCALE;
    for (i = 0; i < 8; ++i) {
        if ((k = (int)(d *= 256.0)) >= 256) k = 255;
        packet[NTP_ORIGINATE+i] = k;
        d -= k;
    }
    d = data->receive/NTP_SCALE;
    for (i = 0; i < 8; ++i) {
        if ((k = (int)(d *= 256.0)) >= 256) k = 255;
        packet[NTP_RECEIVE+i] = k;
        d -= k;
    }
    d = data->transmit/NTP_SCALE;
    for (i = 0; i < 8; ++i) {
        if ((k = (int)(d *= 256.0)) >= 256) k = 255;
        packet[NTP_TRANSMIT+i] = k;
        d -= k;
    }
}



void unpack_ntp (ntp_data *data, unsigned char *packet, int length) {

/* Unpack the essential data from an NTP packet, bypassing struct layout and
endian problems.  Note that it ignores fields irrelevant to SNTP. */

    int i;
    double d;

    data->current = current_time(JAN_1970);    /* Best to come first */
    data->status = (packet[0] >> 6);
    data->version = (packet[0] >> 3)&0x07;
    data->mode = packet[0]&0x07;
    data->stratum = packet[1];
    data->polling = packet[2];
    data->precision = packet[3];
    d = 0.0;
    for (i = 0; i < 4; ++i) d = 256.0*d+packet[NTP_DISP_FIELD+i];
    data->dispersion = d/65536.0;
    d = 0.0;
    for (i = 0; i < 8; ++i) d = 256.0*d+packet[NTP_REFERENCE+i];
    data->reference = d/NTP_SCALE;
    d = 0.0;
    for (i = 0; i < 8; ++i) d = 256.0*d+packet[NTP_ORIGINATE+i];
    data->originate = d/NTP_SCALE;
    d = 0.0;
    for (i = 0; i < 8; ++i) d = 256.0*d+packet[NTP_RECEIVE+i];
    data->receive = d/NTP_SCALE;
    d = 0.0;
    for (i = 0; i < 8; ++i) d = 256.0*d+packet[NTP_TRANSMIT+i];
    data->transmit = d/NTP_SCALE;
}



void make_packet (ntp_data *data, int mode) {

/* Create an outgoing NTP packet, either from scratch or starting from a
request from a client.  Note that it implements the NTP specification, even
when this is clearly misguided, except possibly for the setting of LI.  It
would be easy enough to add a sanity flag, but I am not in the business of
designing an alternative protocol (however much better it might be). */

    data->status = NTP_LI_FUDGE<<6;
    data->stratum = NTP_STRATUM;
    data->reference = data->dispersion = 0.0;
    if (mode == NTP_SERVER) {
        data->mode = (data->mode == NTP_CLIENT ? NTP_SERVER : NTP_PASSIVE);
        data->originate = data->transmit;
        data->receive = data->current;
    } else {
        data->version = NTP_VERSION;
        data->mode = mode;
        data->polling = NTP_POLLING;
        data->precision = NTP_PRECISION;
        data->receive = data->originate = 0.0;
    }
    data->current = data->transmit = current_time(JAN_1970);
}



int read_packet (int which, ntp_data *data, double *off, double *err) {

/* Check the packet and work out the offset and optionally the error.  Note
that this contains more checking than xntp does.  This returns 0 for success, 1
for failure and 2 for an ignored broadcast packet (a kludge for servers).  Note
that it must not change its arguments if it fails. */

    unsigned char receive[NTP_PACKET_MAX+1];
    double delay1, delay2, x, y;
    int response = 0, failed, length, i, k;

/* Read the packet and deal with diagnostics. */

    if ((length = read_socket(which,receive,NTP_PACKET_MAX+1,waiting)) <= 0)
        return 1;
    if (length < NTP_PACKET_MIN || length > NTP_PACKET_MAX) {
        if (verbose)
            fprintf(stderr,"%s: bad length %d for NTP packet on socket %d\n",
                argv0,length,which);
        return 1;
    }
    if (verbose > 2) {
        fprintf(stderr,"Incoming packet on socket %d:\n",which);
        display_packet(receive,length);
    }
    unpack_ntp(data,receive,length);
    if (verbose > 2) display_data(data);

/* Start by checking that the packet looks reasonable.  Be a little paranoid,
but allow for version 1 semantics and sick clients. */

    if (operation == op_server) {
        if (data->mode == NTP_BROADCAST) return 2;
        failed = (data->mode != NTP_CLIENT && data->mode != NTP_ACTIVE);
    } else if (operation == op_listen)
        failed = (data->mode != NTP_BROADCAST);
    else {
        failed = (data->mode != NTP_SERVER && data->mode != NTP_PASSIVE);
        response = 1;
    }
    if (failed || data->status != 0 || data->version < 1 ||
            data->version > NTP_VERSION_MAX ||
            data->stratum > NTP_STRATUM_MAX) {
        if (verbose)
            fprintf(stderr,
                "%s: totally spurious NTP packet rejected on socket %d\n",
                argv0,which);
        return 1;
    }

/* Note that the conventions are very poorly defined in the NTP protocol, so we
have to guess.  Any full NTP server perpetrating completely unsynchronised
packets is an abomination, anyway, so reject it. */

    delay1 = data->transmit-data->receive;
    delay2 = data->current-data->originate;
    failed = ((data->stratum != 0 && data->stratum != NTP_STRATUM_MAX &&
                data->reference == 0.0) ||
            (operation != op_server && data->transmit == 0.0));
    if (response &&
            (data->originate == 0.0 || data->receive == 0.0 ||
                (data->reference != 0.0 && data->receive < data->reference) ||
                delay1 < 0.0 || delay1 > NTP_INSANITY || delay2 < 0.0 ||
                data->dispersion > NTP_INSANITY))
        failed = 1;
    if (failed) {
        if (verbose)
            fprintf(stderr,
                "%s: incomprehensible NTP packet rejected on socket %d\n",
                argv0,which);
        return 1;
    }

/* If it is a response, check that it corresponds to one of our requests and
has got here in a reasonable length of time. */

    if (response) {
        k = 0;
        for (i = 0; i < attempts; ++i)
            if (data->originate == outgoing[i]) {
                outgoing[i] = 0.0;
                ++k;
            }
        if (k != 1 || delay2 > NTP_INSANITY) {
            if (verbose)
                fprintf(stderr,
                    "%s: bad response from NTP server rejected on socket %d\n",
                    argv0,which);
            return 1;
        }
    }

/* Now return the time information.  If it is a server response, it contains
enough information that we can be almost certain that we have not been fooled
too badly.  Heaven help us with broadcasts - make a wild kludge here, and see
elsewhere for other kludges. */

    if (dispersion < data->dispersion) dispersion = data->dispersion;
    if (operation == op_listen || operation == op_server) {
        *off = data->transmit-data->current;
        *err = NTP_INSANITY;
    } else {
        x = data->receive-data->originate;
        y = (data->transmit == 0.0 ? 0.0 : data->transmit-data->current);
        *off = 0.5*(x+y);
        *err = x-y;
        x = data->current-data->originate;
        if (0.5*x > *err) *err = 0.5*x;
    }
    return 0;
}



void format_time (char *text, int length, double offset, double error,
    double drift, double drifterr) {

/* Format the current time into a string, with the extra information as
requested.  Note that the rest of the program uses the correction needed, which
is what is printed for diagnostics, but this formats the error in the local
system for display to users.  So the results from this are the negation of
those printed by the verbose options. */

    int milli, len;
    time_t now;
    struct tm *gmt;
    static const char *months[] = {
        "Jan", "Feb", "Mar", "Apr", "May", "Jun",
        "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
    };

/* Work out and format the current local time.  Note that some semi-ANSI
systems do not set the return value from (s)printf. */

    now = convert_time(current_time(offset),&milli);
    errno = 0;
    if ((gmt = localtime(&now)) == NULL)
        fatal(1,"unable to work out local time",NULL);
    len = 24;
    if (length <= len) fatal(0,"internal error calling format_time",NULL);
    errno = 0;
    sprintf(text,"%.4d %s %.2d %.2d:%.2d:%.2d.%.3d",
            gmt->tm_year+1900,months[gmt->tm_mon],gmt->tm_mday,
            gmt->tm_hour,gmt->tm_min,gmt->tm_sec,milli);
    if (strlen(text) != len)
        fatal(1,"unable to format current local time",NULL);

/* Append the information about the offset, if requested. */

    if (error >= 0.0) {
        if (length < len+30)
            fatal(0,"internal error calling format_time",NULL);
        errno = 0;
        sprintf(&text[len]," %c %.3f +/- %.3f secs",(offset > 0.0 ? '-' : '+'),
                (offset > 0.0 ? offset : -offset),dispersion+error);
        if (strlen(&text[len]) < 22)
            fatal(1,"unable to format clock correction",NULL);
    }

/* Append the information about the drift, if requested. */

    if (drifterr >= 0.0) {
        len = strlen(text);
        if (length < len+25)
            fatal(0,"internal error calling format_time",NULL);
        errno = 0;
        sprintf(&text[len]," %c %.1f +/- %.1f ppm",
                (drift > 0.0 ? '-' : '+'),1.0e6*fabs(drift),
                1.0e6*drifterr);
        if (strlen(&text[len]) < 17)
            fatal(1,"unable to format clock correction",NULL);
    }

/* It would be better to check for field overflow, but it is a lot of code to
trap extremely implausible scenarios.  This will usually stop chaos from
spreading. */

    if (strlen(text) >= length)
        fatal(0,"internal error calling format_time",NULL);
}



double reset_clock (double offset, double error, int daemon) {

/* Reset the clock, if appropriate, and return the correction actually used.
This contains most of the checking for whether changes are worthwhile, except
in daemon mode. */

    double absoff = (offset < 0 ? -offset : offset);
    char text[50];

/* If the correction is large, ask for confirmation before proceeding. */

    if (absoff > prompt) {
        if (! daemon && ftty(stdin) && ftty(stdout)) {
            printf("The time correction is %.3f +/- %.3f+%.3f seconds\n",
                offset,dispersion,error);
            printf("Do you want to correct the time anyway? ");
            fflush(stdout);
            if (toupper(getchar()) != 'Y') {
                printf("OK - quitting\n");
                fatal(0,NULL,NULL);
            }
        } else {
            sprintf(text,"%.3f +/- %.3f+%.3f",offset,dispersion,error);
            fatal(0,"time correction too large: %s seconds",text);
        }
    }

/* See if the correction is reasonably reliable and worth making. */

    if (absoff < (daemon ? 0.5 : 1.0)*minerr) {
        if (daemon ? verbose > 1 : verbose)
            fprintf(stderr,"%s: correction %.3f +/- %.3f+%.3f secs - ignored\n",
                argv0,offset,dispersion,error);
        return 0.0;
    } else if (absoff < 2.0*error) {
        if (daemon ? verbose > 1 : verbose)
            fprintf(stderr,
                "%s: correction %.3f +/- %.3f+%.3f secs - suppressed\n",
                argv0,offset,dispersion,error);
        return 0.0;
    }

/* Make the correction.  Provide some protection against the previous
correction not having completed, but it will rarely help much. */

    adjust_time(offset,(action == action_reset ? 1 : 0),
        (daemon ? 2.0*minerr : 0.0));
    if (daemon ? verbose > 1 : verbose) {
        format_time(text,50,0.0,-1.0,0.0,-1.0);
        fprintf(stderr,
            "%s: time changed by %.3f secs to %s +/- %.3f+%.3f\n",
            argv0,offset,text,dispersion,error);
    }
    return offset;
}



void run_server (void) {

/* Set up a socket, and either broadcast at intervals or wait for requests.
It is quite tricky to get this to fail, and it will usually indicate that the
local system is sick. */

    unsigned char transmit[NTP_PACKET_MIN];
    ntp_data data;
    double started = current_time(JAN_1970), successes = 0.0, failures = 0.0,
        broadcasts = 0.0, weeble = 1.0, x, y;
    int i, j;

    open_socket(0,NULL,delay);
    while (1) {

/* In server mode, provide some tracing of normal running (but not too much,
except when debugging!) */

        if (operation == op_server) {
            x = current_time(JAN_1970)-started;
            if (verbose && x/3600.0+successes+failures >= weeble) {
                weeble *= WEEBLE_FACTOR;
                x -= 3600.0*(i = (int)(x/3600.0));
                x -= 60.0*(j = (int)(x/60.0));
                if (i > 0)
                    fprintf(stderr,"%s: after %d hours %d mins ",argv0,i,j);
                else if (j > 0)
                    fprintf(stderr,"%s: after %d mins %.0f secs ",argv0,j,x);
                else
                    fprintf(stderr,"%s: after %.1f secs ",argv0,x);
                fprintf(stderr,"%.0f acc. %.0f rej. %.0f b'cast\n",
                    successes,failures,broadcasts);
            }

/* Respond to incoming requests or plaster broadcasts over the net.  Note that
we could skip almost all of the decoding, but it provides a healthy amount of
error detection.  We could print some information on incoming packets, but the
code is not structured to do this very helpfully. */

            i = read_packet(0,&data,&x,&y);
            if (i == 2) {
                ++broadcasts;
                continue;
            } else if (i != 0) {
                ++failures;
                continue;
            } else {
                ++successes;
                make_packet(&data,NTP_SERVER);
            }
        } else {
            do_nothing(period);
            make_packet(&data,NTP_BROADCAST);
        }
        if (verbose > 2) {
            fprintf(stderr,"Outgoing packet:\n");
            display_data(&data);
        }
        pack_ntp(transmit,NTP_PACKET_MIN,&data);
        if (verbose > 2) display_packet(transmit,NTP_PACKET_MIN);
        write_socket(0,transmit,NTP_PACKET_MIN);
    }
}



double estimate_stats (int *a_total, int *a_index, data_record *record,
    double correction, double *a_disp, double *a_when, double *a_offset,
    double *a_error, double *a_drift, double *a_drifterr, int *a_wait,
    int update) {

/* This updates the running statistics and returns the best estimate of what to
do now.  It returns the timestamp relevant to the correction.  If broadcasts
are rare and the drift is large, it will fail - you should then use a better
synchronisation method.  It will also fail if something goes severely wrong
(e.g. if the local clock is reset by another process or the transmission errors
are beyond reason).

There is a kludge for synchronisation loss during down time.  If it detects
this, it will update only the history data and return zero; this is then
handled specially in run_daemon().  While it could correct the offset, this
might not always be the right thing to do. */

    double weight, disp, when, offset, error, drift, drifterr,
        now, e, w, x, y, z;
    int total = *a_total, index = *a_index, wait = *a_wait, i;
    char text[50];
 
/* Correct the previous data and store a new entry in the circular buffer. */

    for (i = 0; i < total; ++i) {
        record[i].when += correction;
        record[i].offset -= correction;
    }
    if (update) {
        record[index].dispersion = *a_disp;
        record[index].when = *a_when;
        record[index].offset = *a_offset;
        if (verbose > 1)
            fprintf(stderr,"%s: corr=%.3f when=%.3f disp=%.3f off=%.3f",
                argv0,correction,*a_when,*a_disp,*a_offset); /* See below */
        if (operation == op_listen) {
            if (verbose > 1) fprintf(stderr,"\n");
            record[index].error = minerr;
            record[index].weight = 1.0;
        } else {
            if (verbose > 1) fprintf(stderr," err=%.3f\n",*a_error);
            record[index].error = x = *a_error;
            record[index].weight = 1.0/(x > minerr ? x*x : minerr*minerr);
        }
        if (++index >= count) index = 0;
        *a_index = index;
        if (++total > count) total = count;
        *a_total = total;
        if (verbose > 2)
            fprintf(stderr,"corr=%.6f tot=%d ind=%d\n",correction,total,index);
    }

/* If there is insufficient data yet, use the latest estimates and return
forthwith.  Note that this will not work for broadcasts, but they will be
disabled in run_daemon(). */

    if ((operation == op_listen && total < count && update) || total < 3) {
        *a_drift = 0.0;
        *a_drifterr = -1.0;
        *a_wait = delay;
        return *a_when;
    }

/* Work out the average time, offset, error etc.  Note that the dispersion is
not subject to the central limit theorem.  Unfortunately, the variation in the
source's dispersion is our only indication of how consistent its clock is. */

    disp = weight = when = offset = y = 0.0;
    for (i = 0; i < total; ++i) {
        weight += w = record[i].weight;
        when += w*record[i].when;
        offset += w*record[i].offset;
        y += w*record[i].dispersion;
        if (disp < record[i].dispersion)
            disp = record[i].dispersion;
    }
    when /= weight;
    offset /= weight;
    y /= weight;
    if (verbose > 2)
        fprintf(stderr,"disp=%.6f wgt=%.3f when=%.6f off=%.6f\n",
            disp,weight,when,offset);

/* If there is enough data, estimate the drift and errors by regression.  Note
that it is essential to calculate the mean square error, not the mean error. */

    error = drift = x = z = 0.0;
    for (i = 0; i < total; ++i) {
        w = record[i].weight/weight;
        x += w*(record[i].when-when)*(record[i].when-when);
        drift += w*(record[i].when-when)*(record[i].offset-offset);
        z += w*(record[i].offset-offset)*(record[i].offset-offset);
        error += w*record[i].error*record[i].error+
            2.0*w*(record[i].dispersion-y)*(record[i].dispersion-y);
    }
    if (verbose > 2)
        fprintf(stderr,"X2=%.3f XY=%.6f Y2=%.9f E2=%.9f ",x,drift,z,error);

/* When calculating the errors, add some paranoia mainly to check for coding
errors and complete lunacy, attempting to retry if at all possible.  Because
glitches at this point are so common, log a reset even in non-verbose mode.
There will be more thorough checks later.  Note that we cannot usefully check
the error for broadcasts. */

    z -= drift*drift/x;
    if (verbose > 2) fprintf(stderr,"S2=%.9f\n",z);
    if (! update) {
        if (z > 1.0e6)
            fatal(0,"stored data too unreliable for time estimation",NULL);
    } else if (operation == op_client) {
        e = error+disp*disp+minerr*minerr;
        if (z > e) {
            if (verbose || z >= maxerr*maxerr)
                fprintf(stderr,
                    "%s: excessively high error %.3f > %.3f > %.3f\n",
                    argv0,sqrt(z),sqrt(e),sqrt(error));
            if (total <= 1)
                return 0.0;
            else if (z < maxerr*maxerr) {
                sprintf(text,"resetting on error %.3g > %.3g",
                    sqrt(z),sqrt(e));
                log_message(text);
                return 0.0;
            } else
                fatal(0,"incompatible (i.e. erroneous) timestamps",NULL);
        } else if (z > error && verbose)
            fprintf(stderr,
                "%s: anomalously high error %.3f > %.3f, but < %.3f\n",
                argv0,sqrt(z),sqrt(error),sqrt(e));
    } else {
        if (z > maxerr*maxerr)
            fatal(0,"broadcasts too unreliable for time estimation",NULL);
    }
    drift /= x;
    drifterr = ABSCISSA*sqrt(z/(x*total));
    error = (operation == op_listen ? minerr : 0.0)+ABSCISSA*sqrt(z/total);
    if (verbose > 2)
        fprintf(stderr,"err=%.6f drift=%.6f+/-%.6f\n",error,drift,drifterr);
    if (error+drifterr*delay > NTP_INSANITY)
        fatal(0,"unable to get a reasonable drift estimate",NULL);

/* Estimate the optimal short-loop period, checking it carefully.  Remember to
check that this whole process is likely to be accurate enough and that the
delay function may be inaccurate. */

    wait = delay;
    x = (drift < 0.0 ? -drift : drift);
    if (x*delay < 0.5*minerr) {
        if (verbose > 2) fprintf(stderr,"Drift too small to correct\n");
    } else if (x < 2.0*drifterr) {
        if (verbose > 2)
            fprintf(stderr,"Drift correction suppressed\n");
    } else {
        if ((z = drifterr*delay) < 0.5*minerr) z = 0.5*minerr;
        wait = (x < z/delay ? delay : (int)(z/x+0.5));
        wait = (int)(delay/(int)(delay/(double)wait+0.999)+0.999);
        if (wait > delay)
            fatal(0,"internal error in drift calculation",NULL);
        if (update && (drift*wait > maxerr || wait < RESET_MIN)) {
            sprintf(text,"%.6f+/-%.6f",drift,drifterr);
            fatal(0,"drift correction too large: %s",text);
        }
    }
    if (wait < *a_wait/2) wait = *a_wait/2;
    if (wait > *a_wait*2) wait = *a_wait*2;

/* Now work out what the correction should be, as distinct from what it should
have been, remembering that older times are less certain. */

    now = current_time(JAN_1970);
    x = now-when;
    offset += x*drift;
    error += x*drifterr;
    for (i = 0; i < total; ++i) {
        x = now-record[i].when;
        z = record[i].error+x*drifterr;
        if (z < error) {
            when = record[i].when;
            offset = record[i].offset+x*drift;
            error = z;
        }
    }
    if (verbose > 2)
        fprintf(stderr,"now=%.6f when=%.6f off=%.6f err=%.6f wait=%d\n",
            now,when,offset,error,wait);

/* Finally, return the result. */

    *a_disp = disp;
    *a_when = when;
    *a_offset = offset;
    *a_error = error;
    *a_drift = drift;
    *a_drifterr = drifterr;
    *a_wait = wait;
    return now;
}



double correct_drift (double *a_when, double *a_offset, double drift) {

/* Correct for the drift since the last time it was done, provided that a long
enough time has elapsed.  And do remember to kludge up the time and
discrepancy, when appropriate. */

    double d, x;

    d = current_time(JAN_1970)-*a_when;
    *a_when += d;
    x = *a_offset+d*drift;
    if (verbose > 2)
        fprintf(stderr,"Correction %.6f @ %.6f off=%.6f ",x,*a_when,*a_offset);
    if (d >= waiting && (x < 0.0 ? -x : x) >= 0.5*minerr) {
        if (verbose > 2) fprintf(stderr,"performed\n");
        adjust_time(x,(action == action_reset ? 1 : 0),0.5*minerr);
        *a_offset = 0.0;
        return x;
    } else {
        if (verbose > 2) fprintf(stderr,"ignored\n");
        *a_offset = x;
        return 0.0;
    }
}



void handle_saving (int operation, int *total, int *index, int *cycle,
    data_record *record, double *previous, double *when, double *correction) {

/* This handles the saving and restoring of the state to a file.  While it is
subject to spoofing, this is not a major security problem.  But, out of general
paranoia, check everything in sight when restoring.  Note that this function
has no external effect if something goes wrong. */

    struct {
        data_record record[COUNT_MAX];
        double previous, when, correction;
        int operation, delay, count, total, index, cycle, waiting;
    } buffer;
    double x, y;
    int i, j;

    if (savefile == NULL) return;

/* Read the restart file and print its data in diagnostic mode.  Note that some
care is necessary to avoid introducing a security exposure - but we trust the
C library not to trash the stack on bad numbers! */

    if (operation == save_read_only || operation == save_read_check) {
        if (fread(&buffer,sizeof(buffer),1,savefile) != 1 || ferror(savefile)) {
            if (ferror(savefile))
                fatal(1,"unable to read record from daemon save file",NULL);
            else if (verbose)
                fprintf(stderr,"%s: bad daemon restart information\n",argv0);
            return;
        }
        if (verbose > 2) {
            fprintf(stderr,"Reading prev=%.6f when=%.6f corr=%.6f\n",
                buffer.previous,buffer.when,buffer.correction);
            fprintf(stderr,"op=%d dly=%d cnt=%d tot=%d ind=%d cyc=%d wait=%d\n",
                buffer.operation,buffer.delay,buffer.count,buffer.total,
                buffer.index,buffer.cycle,buffer.waiting);
            if (buffer.total < COUNT_MAX)
                for (i = 0; i < buffer.total; ++i)
                    fprintf(stderr,
                        "disp=%.6f wgt=%.3f when=%.6f off=%.6f err=%.6f\n",
                        buffer.record[i].dispersion,buffer.record[i].weight,
                        buffer.record[i].when,buffer.record[i].offset,
                        buffer.record[i].error);
        }


/* Start checking the data for sanity. */

        if (buffer.operation == 0 && buffer.delay == 0 && buffer.count == 0) {
            if (operation < 0)
                fatal(0,"the daemon save file has been cleared",NULL);
            if (verbose)
                fprintf(stderr,"%s: restarting from a cleared file\n",argv0);
            return;
        }
        if (operation == save_read_check) {
            if (buffer.operation != operation || buffer.delay != delay ||
                    buffer.count != count) {
                if (verbose)
                    fprintf(stderr,"%s: different parameters for restart\n",
                        argv0);
                return;
            }
            if (buffer.total < 1 || buffer.total > count || buffer.index < 0 ||
                    buffer.index >= count || buffer.cycle < 0 ||
                    buffer.cycle >= count || buffer.correction < -maxerr ||
                    buffer.correction > maxerr || buffer.waiting < RESET_MIN ||
                    buffer.waiting > delay || buffer.previous > buffer.when ||
                    buffer.previous < buffer.when-count*delay ||
                    buffer.when >= *when) {
                if (verbose)
                    fprintf(stderr,"%s: corrupted restart information\n",argv0);
                return;
            }

/* Checking the record is even more tedious. */

            x = *when;
            y = 0.0;
            for (i = 0; i < buffer.total; ++i) {
                if (buffer.record[i].dispersion < 0.0 ||
                        buffer.record[i].dispersion > maxerr ||
                        buffer.record[i].weight <= 0.0 ||
                        buffer.record[i].weight > 1.001/(minerr*minerr) ||
                        buffer.record[i].offset < -count*maxerr ||
                        buffer.record[i].offset > count*maxerr ||
                        buffer.record[i].error < 0.0 ||
                        buffer.record[i].error > maxerr) {
                    if (verbose)
                        fprintf(stderr,"%s: corrupted restart record\n",argv0);
                    return;
                }
                if (buffer.record[i].when < x) x = buffer.record[i].when;
                if (buffer.record[i].when > y) y = buffer.record[i].when;
            }

/* Check for consistency and, finally, whether this is too old. */

            if (y > buffer.when || y-x < (buffer.total-1)*delay ||
                    y-x > (buffer.total-1)*count*delay) {
                if (verbose)
                    fprintf(stderr,"%s: corrupted restart times\n",argv0);
                return;
            }
            if (buffer.when < *when-count*delay) {
                if (verbose)
                    fprintf(stderr,"%s: restart information too old\n",argv0);
                return;
            }
        }

/* If we get here, just copy the data back. */

        memcpy(record,buffer.record,sizeof(buffer.record));
        *previous = buffer.previous;
        *when = buffer.when;
        *correction = buffer.correction;
        *total = buffer.total;
        *index = buffer.index;
        *cycle = buffer.cycle;
        waiting = buffer.waiting;
        memset(&buffer,0,sizeof(buffer));

/* Print out the data if requested. */

        if (verbose > 1) {
            fprintf(stderr,"%s: prev=%.3f when=%.3f corr=%.3f\n",
                argv0,*previous,*when,*correction);
            for (i = 0; i < *total; ++i) {
                if ((j = i+*index-*total) < 0) j += *total;
                fprintf(stderr,"%s: when=%.3f disp=%.3f off=%.3f",
                    argv0,record[j].when,record[j].dispersion,record[j].offset);
                if (operation == op_client)
                    fprintf(stderr," err=%.3f\n",record[j].error);
                else
                    fprintf(stderr,"\n");
            }
        }

/* All errors on output are fatal. */

    } else if (operation == save_write) {
        memcpy(buffer.record,record,sizeof(buffer.record));
        buffer.previous = *previous;
        buffer.when = *when;
        buffer.correction = *correction;
        buffer.operation = operation;
        buffer.delay = delay;
        buffer.count = count;
        buffer.total = *total;
        buffer.index = *index;
        buffer.cycle = *cycle;
        buffer.waiting = waiting;
        if (fseek(savefile,0l,SEEK_SET) != 0 ||
                fwrite(&buffer,sizeof(buffer),1,savefile) != 1 ||
                fflush(savefile) != 0 || ferror(savefile))
            fatal(1,"unable to write record to daemon save file",NULL);
        if (verbose > 2) {
            fprintf(stderr,"Writing prev=%.6f when=%.6f corr=%.6f\n",
                *previous,*when,*correction);
            fprintf(stderr,"op=%d dly=%d cnt=%d tot=%d ind=%d cyc=%d wait=%d\n",
                operation,delay,count,*total,*index,*cycle,waiting);
            if (*total < COUNT_MAX)
                for (i = 0; i < *total; ++i)
                    fprintf(stderr,
                        "disp=%.6f wgt=%.3f when=%.6f off=%.6f err=%.6f\n",
                        record[i].dispersion,record[i].weight,
                        record[i].when,record[i].offset,record[i].error);
        }

/* Clearing the save file is similar. */

    } else if (operation == save_clear) {
        if (fseek(savefile,0l,SEEK_SET) != 0 ||
                fwrite(&buffer,sizeof(buffer),1,savefile) != 1 ||
                fflush(savefile) != 0 || ferror(savefile))
            fatal(1,"unable to clear daemon save file",NULL);
    } else
        fatal(0,"internal error in handle_saving",NULL);
}



void query_savefile (void) {

/* This queries a daemon save file. */

    double previous, when, correction = 0.0, offset = 0.0, error = -1.0,
        drift = 0.0, drifterr = -1.0;
    data_record record[COUNT_MAX];
    int total = 0, index = 0, cycle = 0;
    char text[100];

/* This is a few lines stripped out of run_daemon() and slightly hacked. */

    previous = when = current_time(JAN_1970);
    if (verbose > 2) {
        format_time(text,50,0.0,-1.0,0.0,-1.0);
        fprintf(stderr,"Started=%.6f %s\n",when,text);
    }
    handle_saving(save_read_only,&total,&index,&cycle,record,&previous,&when,
        &correction);
    estimate_stats(&total,&index,record,correction,&dispersion,
        &when,&offset,&error,&drift,&drifterr,&waiting,0);
    format_time(text,100,offset,error,drift,drifterr);
    printf("%s\n",text);
    if (fclose(savefile)) fatal(1,"unable to close daemon save file",NULL);
    if (verbose > 2) fprintf(stderr,"Stopped normally\n");
    exit(EXIT_SUCCESS);
}



void run_daemon (char *hostnames[], int nhosts, int initial) {

/* This does not adjust the time between calls to the server, but it does
adjust the time between clock resets.  This function will survive short periods
of server inaccessibility or network glitches, but not long ones, and will then
need restarting manually.

It is far too complex for a single function, but could really only be
simplified by making most of its variables global or by a similarly horrible
trick.  Oh, for nested scopes as in Algol 68! */

    double history[COUNT_MAX], started, previous, when, correction = 0.0,
        weeble = 1.0, accepts = 0.0, rejects = 0.0, flushes = 0.0,
        replicates = 0.0, skips = 0.0, offset = 0.0, error = -1.0,
        drift = 0.0, drifterr = -1.0, maxoff = 0.0, x;
    data_record record[COUNT_MAX];
    int total = 0, index = 0, item = 0, rej_level = 0, rep_level = 0,
        cycle = 0, retry = 1, i, j, k;
    unsigned char transmit[NTP_PACKET_MIN];
    ntp_data data;
    char text[100];

/* After initialising, restore from a previous run if possible.  Note that
only a few of the variables are actually needed to control the operation and
the rest are mainly for diagnostics. */

    started = previous = when = current_time(JAN_1970);
    if (verbose > 2) {
        format_time(text,50,0.0,-1.0,0.0,-1.0);
        fprintf(stderr,"Started=%.6f %s\n",when,text);
    }
    if (initial) {
        handle_saving(save_read_check,&total,&index,&cycle,record,
            &previous,&when,&correction);
        cycle = (nhosts > 0 ? cycle%nhosts : 0);
        if (total > 0 && started-previous < delay) {
            if (verbose > 2) fprintf(stderr,"Last packet too recent\n");
            retry = 0;
        }
        if (verbose > 2)
            fprintf(stderr,"prev=%.6f when=%.6f retry=%d\n",
                previous,when,retry);
        for (i = 0; i < nhosts; ++i) open_socket(i,hostnames[i],delay);
        if (action != action_display) {
            set_lock(1);
            locked = 1;
        }
    }
    dispersion = 0.0;
    attempts = 0;
    for (i = 0; i < count; ++i) history[i] = 0.0;
    while (1) {

/* Print out a reasonable amount of diagnostics, rather like a server.  Note
that it may take a little time, but shouldn't affect the estimates much.  Then
check that we aren't in a failing loop. */

        if (verbose > 2) fprintf(stderr,"item=%d rej=%d\n",item,rej_level);
        x = current_time(JAN_1970)-started;
        if (verbose &&
                x/3600.0+accepts+rejects+flushes+replicates+skips >= weeble) {
            weeble *= WEEBLE_FACTOR;
            x -= 3600.0*(i = (int)(x/3600.0));
            x -= 60.0*(j = (int)(x/60.0));
            if (i > 0)
                fprintf(stderr,"%s: after %d hours %d mins ",argv0,i,j);
            else if (j > 0)
                fprintf(stderr,"%s: after %d mins %.0f secs ",argv0,j,x);
            else
                fprintf(stderr,"%s: after %.1f secs ",argv0,x);
            fprintf(stderr,"acc. %.0f rej. %.0f flush %.0f",
                accepts,rejects,flushes);
            if (operation == op_listen)
                fprintf(stderr," rep. %.0f skip %.0f",replicates,skips);
            fprintf(stderr," max.off. %.3f corr. %.3f\n",maxoff,correction);
            format_time(text,100,offset,error,drift,drifterr);
            fprintf(stderr,"%s: %s\n",argv0,text);
            maxoff = 0.0;
        }
        if (current_time(JAN_1970)-previous > count*delay) {
            if (verbose)
                fprintf(stderr,"%s: no packets in too long a period\n",argv0);
            return;
        }

/* Listen for the next broadcast packet.  This allows up to ETHERNET_MAX
replications per packet, for systems with multiple addresses for receiving
broadcasts; the only reason for a limit is to protect against broken NTP
servers always returning the same time. */

        if (operation == op_listen) {
            flushes += flush_socket(0);
            if (read_packet(0,&data,&offset,&error)) {
                ++rejects;
                if (++rej_level > count)
                    fatal(0,"too many bad or lost packets",NULL);
                if (action != action_display && drifterr >= 0.0) {
                    correction += correct_drift(&when,&offset,drift);
                    handle_saving(save_write,&total,&index,&cycle,record,
                        &previous,&when,&correction);
                }
                continue;
            }
            if ((rej_level -= (count < 5 ? count : 5)) < 0) rej_level = 0;
            x = data.transmit;
            for (i = 0; i < count; ++i)
                if (x == history[i]) {
                    ++replicates;
                    if (++rep_level > ETHERNET_MAX)
                        fatal(0,"too many replicated packets",NULL);
                    goto continue1;
                }
            rep_level = 0;
            history[item] = x;
            if (++item >= count) item = 0;

/* Accept a packet only after a long enough period has elapsed. */

            when = data.current;
            if (! retry && when < previous+delay) {
                if (verbose > 2) fprintf(stderr,"Skipping too recent packet\n");
                ++skips;
                continue;
            }
            retry = 0;
            if (verbose > 2)
                fprintf(stderr,"Offset=%.6f @ %.6f disp=%.6f\n",
                    offset,when,dispersion);

/* Handle the client/server model.  It keeps a record of transmitted times,
mainly out of paranoia.  The waiting time is kludged up to attempt to provide
reasonable resilience against both lost packets and dead servers.  But it
won't handle much of either, and will stop after a while, needing manual
restarting.  Running it under cron is the best approach. */

        } else {
            if (! retry) {
               if (verbose > 2) fprintf(stderr,"Sleeping for %d\n",waiting);
               do_nothing(waiting);
            }
            make_packet(&data,NTP_CLIENT);
            outgoing[item] = data.transmit;
            if (++item >= 2*count) item = 0;
            if (attempts < 2*count) ++attempts;
            if (verbose > 2) {
                fprintf(stderr,"Outgoing packet on socket %d:\n",cycle);
                display_data(&data);
            }
            pack_ntp(transmit,NTP_PACKET_MIN,&data);
            if (verbose > 2) display_packet(transmit,NTP_PACKET_MIN);
            flushes += flush_socket(cycle);
            write_socket(cycle,transmit,NTP_PACKET_MIN);

/* Read the packet and check that it is an appropriate response.  Because this
is rather more numerically sensitive than simple resynchronisation, reject all
very inaccurate packets.  Be careful if you modify this, because the error
handling is rather nasty to avoid replicating code. */

            k = read_packet(cycle,&data,&offset,&error);
            if (++cycle >= nhosts) cycle = 0;
            if (! k)
                when = (data.originate+data.current)/2.0;
            else if (action != action_display && drifterr >= 0.0) {
                correction += correct_drift(&when,&offset,drift);
                handle_saving(save_write,&total,&index,&cycle,record,
                    &previous,&when,&correction);
            }
            if (! k && ! retry && when < previous+delay-2) {
                if (verbose)
                    fprintf(stderr,"%s: packets out of order on socket %d\n",
                        argv0,cycle);
                k = 1;
            }
            if (! k && data.current-data.originate > maxerr) {
                if (verbose)
                    fprintf(stderr,
                        "%s: very slow response rejected on socket %d\n",
                        argv0,cycle);
                k = 1;
            }

/* Count the number of rejected packets and fail if there are too many. */

            if (k) {
                ++rejects;
                if (++rej_level > count)
                    fatal(0,"too many bad or lost packets",NULL);
                else {
                    retry = 1;
                    continue;
                }
            } else
                retry = 0;
            if ((rej_level -= (count < 5 ? count : 5)) < 0) rej_level = 0;
            if (verbose > 2)
                fprintf(stderr,"Offset=%.6f+/-%.6f @ %.6f disp=%.6f\n",
                    offset,error,when,dispersion);
        }
   
/* Calculate the statistics, and display the results or make the initial
correction.  Note that estimate_stats() will return zero if a timestamp
indicates synchronisation loss (usually due to down time or a change of server,
somewhere upstream), and that the recovery operation is unstructured, so great
care should be taken when modifying it.  Also, we want to clear the saved state
is the statistics are bad. */

        handle_saving(save_clear,&total,&index,&cycle,record,&previous,&when,
            &correction);
        ++accepts;
        dispersion = data.dispersion;
        previous = when =
            estimate_stats(&total,&index,record,correction,&dispersion,
                &when,&offset,&error,&drift,&drifterr,&waiting,1);
        if (verbose > 2) {
            fprintf(stderr,"tot=%d ind=%d dis=%.3f when=%.3f off=%.3f ",
                total,index,dispersion,when,offset);
            fprintf(stderr,"err=%.3f wait=%d\n",error,waiting);
        }
        if (when == 0.0) return;
        x = (maxoff < 0.0 ? -maxoff : maxoff);
        if ((offset < 0.0 ? -offset : offset) > x) maxoff = offset;
        correction = 0.0;
        if (operation == op_client || accepts >= count) {
            if (action == action_display) {
                format_time(text,100,offset,error,drift,drifterr);
                printf("%s\n",text);
            } else {
                x = reset_clock(offset,error,1);
                correction += x;
                offset -= x;
            }
        } else
            waiting = delay;
        handle_saving(save_write,&total,&index,&cycle,record,&previous,&when,
            &correction);

/* Now correct the clock for a while, before getting another packet and
updating the statistics. */

        while (when < previous+delay-waiting) {
            do_nothing(waiting);
            if (action == action_display)
                when += waiting;
            else {
                correction += correct_drift(&when,&offset,drift);
                handle_saving(save_write,&total,&index,&cycle,record,
                    &previous,&when,&correction);
            }
        }
continue1: ;
    }
}



void run_client (char *hostnames[], int nhosts) {

/* Get enough responses to do something with; or not, as the case may be.  Note
that it allows for half of the packets to be bad, so may make up to twice as
many attempts as specified by the -c value.  The deadline checking is merely
paranoia, to protect against broken signal handling - it cannot easily be
triggered if the signal handling works. */

    double history[COUNT_MAX], guesses[COUNT_MAX], offset, error, deadline,
        a, b, x, y;
    int accepts = 0, rejects = 0, flushes = 0, replicates = 0, cycle = 0, k;
    unsigned char transmit[NTP_PACKET_MIN];
    ntp_data data;
    char text[100];

    if (verbose > 2) {
        format_time(text,50,0.0,-1.0,0.0,-1.0);
        fprintf(stderr,"Started=%.6f %s\n",current_time(JAN_1970),text);
    }
    for (k = 0; k < nhosts; ++k) open_socket(k,hostnames[k],delay);
    if (action != action_display) {
        set_lock(1);
        locked = 1;
    }
    attempts = 0;
    deadline = current_time(JAN_1970)+delay;

/* Listen to broadcast packets and select the best (i.e. earliest).  This will
be sensitive to a bad NTP broadcaster, but I believe such things are very rare
in practice.  In any case, if you have one, it is probably the only one on your
subnet, so you are knackered!  This allows up to ETHERNET_MAX replications per
packet, for systems with multiple addresses for receiving broadcasts; the only
reason for a limit is to protect against broken NTP servers always returning
the same time. */

    if (operation == op_listen) {
        while (accepts < count) {
            if (current_time(JAN_1970) > deadline)
                fatal(0,"not enough valid broadcasts received in time",NULL);
            flushes += flush_socket(0);
            if (read_packet(0,&data,&x,&y)) {
                if (++rejects > count)
                    fatal(0,"too many bad or lost packets",NULL);
                else
                    continue;
            } else {
                a = data.transmit;
                for (k = 0; k < accepts; ++k)
                    if (a == history[k]) {
                        if (++replicates > ETHERNET_MAX*count)
                            fatal(0,"too many replicated packets",NULL);
                        goto continue1;
                    }
                history[accepts] = a;
                guesses[accepts++] = x;
            }
            if (verbose > 2)
                fprintf(stderr,"Offset=%.6f disp=%.6f\n",x,dispersion);
            else if (verbose > 1)
                fprintf(stderr,"%s: offset=%.3f disp=%.3f\n",
                    argv0,x,dispersion);

/* Note that bubblesort IS a good method for this amount of data.  */

            for (k = accepts-2; k >= 0; --k)
                if (guesses[k] < guesses[k+1])
                    break;
                else {
                    x = guesses[k];
                    guesses[k] = guesses[k+1];
                    guesses[k+1] = x;
                }
continue1:  ;
        }
        offset = guesses[0];
        error = minerr+guesses[count <= 5 ? count-1 : 5]-offset;
        if (verbose > 2)
            fprintf(stderr,"accepts=%d rejects=%d flushes=%d replicates=%d\n",
                accepts,rejects,flushes,replicates);

/* Handle the client/server model.  It keeps a record of transmitted times,
mainly out of paranoia. */

    } else {
        offset = 0.0;
        error = NTP_INSANITY;
        while (accepts < count && attempts < 2*count) {
            if (current_time(JAN_1970) > deadline)
                fatal(0,"not enough valid responses received in time",NULL);
            make_packet(&data,NTP_CLIENT);
            outgoing[attempts++] = data.transmit;
            if (verbose > 2) {
                fprintf(stderr,"Outgoing packet on socket %d:\n",cycle);
                display_data(&data);
            }
            pack_ntp(transmit,NTP_PACKET_MIN,&data);
            if (verbose > 2) display_packet(transmit,NTP_PACKET_MIN);
            flushes += flush_socket(cycle);
            write_socket(cycle,transmit,NTP_PACKET_MIN);
            if (read_packet(cycle,&data,&x,&y)) {
                if (++rejects > count)
                    fatal(0,"too many bad or lost packets",NULL);
                else
                    continue;
            } else
                ++accepts;
            if (++cycle >= nhosts) cycle = 0;

/* Work out the most accurate time, and check that it isn't more accurate than
the results warrant. */

            if (verbose > 2)
                fprintf(stderr,"Offset=%.6f+/-%.6f disp=%.6f\n",x,y,dispersion);
            else if (verbose > 1)
                fprintf(stderr,"%s: offset=%.3f+/-%.3f disp=%.3f\n",
                    argv0,x,y,dispersion);
            if ((a = x-offset) < 0.0) a = -a;
            if (accepts <= 1) a = 0.0;
            b = error+y;
            if (y < error) {
                offset = x;
                error = y;
            }
            if (verbose > 2)
                fprintf(stderr,"best=%.6f+/-%.6f\n",offset,error);
            if (a > b) {
                sprintf(text,"%d",cycle);
                fatal(0,"inconsistent times got from NTP server on socket %s",
                    text);
            }
            if (error <= minerr) break;
        }
        if (verbose > 2)
            fprintf(stderr,"accepts=%d rejects=%d flushes=%d\n",
                accepts,rejects,flushes);
    }

/* Tidy up the socket, issues diagnostics and perform the action. */

    for (k = 0; k < nhosts; ++k) close_socket(k);
    if (accepts == 0) fatal(0,"no acceptable packets received",NULL);
    if (error > NTP_INSANITY)
        fatal(0,"unable to get a reasonable time estimate",NULL);
    if (verbose > 2)
        fprintf(stderr,"Correction: %.6f +/- %.6f disp=%.6f\n",
            offset,error,dispersion);
    if (action == action_display) {
        format_time(text,75,offset,error,0.0,-1.0);
        printf("%s\n",text);
    } else
        (void)reset_clock(offset,error,0);
    if (locked) set_lock(0);
    if (verbose > 2) fprintf(stderr,"Stopped normally\n");
    exit(EXIT_SUCCESS);
}



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

/* This is the entry point and all that.  It decodes the arguments and calls
one of the specialised routines to do the work. */

    char *hostnames[MAX_SOCKETS], *savename = NULL;
    int daemon = 0, nhosts = 0, help = 0, args = argc-1, k;
    char c;

    if (argv[0] == NULL || argv[0][0] == '\0')
        argv0 = "msntp";
    else if ((argv0 = strrchr(argv[0],'/')) != NULL)
        ++argv0;
    else
        argv0 = argv[0];
    setvbuf(stdout,NULL,_IOLBF,BUFSIZ);
    setvbuf(stderr,NULL,_IOLBF,BUFSIZ);
    if (INT_MAX < 2147483647) fatal(0,"msntp requires >= 32-bit ints",NULL);
    if (DBL_EPSILON > 1.0e-13)
        fatal(0,"msntp requires doubles with eps <= 1.0e-13",NULL);
    for (k = 0; k < MAX_SOCKETS; ++k) hostnames[k] = NULL;

/* Decode the arguments. */

    while (argc > 1) {
        k = 1;
	if (strcmp(argv[1],"-4") == 0)
	    preferred_family(PREF_FAM_INET);
	else if (strcmp(argv[1],"-6") == 0)
	    preferred_family(PREF_FAM_INET6);
        else if (strcmp(argv[1],"-B") == 0 && action == 0) {
            action = action_broadcast;
            if (argc > 2) {
                if (sscanf(argv[2],"%d%c",&period,&c) != 1) syntax(1);
                if (period < 1 || period > 1440)
                    fatal(0,"%s option value out of range","-B");
                period *= 60;
                k = 2;
            } else
                period = 60*60;
        } else if (strcmp(argv[1],"-S") == 0 && action == 0)
            action = action_server;
        else if (strcmp(argv[1],"-q") == 0 && action == 0)
            action = action_query;
        else if (strcmp(argv[1],"-r") == 0 && action == 0)
            action = action_reset;
        else if (strcmp(argv[1],"-a") == 0 && action == 0)
            action = action_adjust;
        else if (strcmp(argv[1],"-l") == 0 && lockname == NULL && argc > 2) {
            lockname = argv[2];
            k = 2;
        } else if ((strcmp(argv[1],"-x") == 0) &&
                daemon == 0) {
            if (argc > 2 && sscanf(argv[2],"%d%c",&daemon,&c) == 1) {
                if (daemon < 1 || daemon > 1440)
                    fatal(0,"%s option value out of range",argv[1]);
                k = 2;
            } else
                daemon = 300;
        } else if (strcmp(argv[1],"-f") == 0 && savename == NULL && argc > 2) {
            savename = argv[2];
            k = 2;
        } else if ((strcmp(argv[1],"--help") == 0 ||
                    strcmp(argv[1],"-h") == 0 || strcmp(argv[1],"-?") == 0) &&
                help == 0)
            help = 1;
        else if (strcmp(argv[1],"-v") == 0 && verbose == 0)
            verbose = 1;
        else if (strcmp(argv[1],"-V") == 0 && verbose == 0)
            verbose = 2;
        else if (strcmp(argv[1],"-W") == 0 && verbose == 0)
            verbose = 3;
        else if (strcmp(argv[1],"-e") == 0 && minerr == 0.0 && argc > 2) {
            if (sscanf(argv[2],"%lf%c",&minerr,&c) != 1) syntax(1);
            if (minerr <= 0.000999999 || minerr > 1.0)
                fatal(0,"%s option value out of range","-e");
            k = 2;
        } else if (strcmp(argv[1],"-E") == 0 && maxerr == 0.0 && argc > 2) {
            if (sscanf(argv[2],"%lf%c",&maxerr,&c) != 1) syntax(1);
            if (maxerr < 1.0 || maxerr > 60.0)
                fatal(0,"%s option value out of range","-E");
            k = 2;
        } else if (strcmp(argv[1],"-P") == 0 && prompt == 0.0 && argc > 2) {
            if (strcmp(argv[2],"no") == 0)
                prompt = (double)INT_MAX;
            else {
                if (sscanf(argv[2],"%lf%c",&prompt,&c) != 1) syntax(1);
                if (prompt < 1.0 || prompt > 3600.0)
                    fatal(0,"%s option value out of range","-p");
            }
            k = 2;
        } else if (strcmp(argv[1],"-d") == 0 && delay == 0 && argc > 2) {
            if (sscanf(argv[2],"%d%c",&delay,&c) != 1) syntax(1);
            if (delay < 1 || delay > 3600)
                fatal(0,"%s option value out of range","-d");
            k = 2;
        } else if (strcmp(argv[1],"-c") == 0 && count == 0 && argc > 2) {
            if (sscanf(argv[2],"%d%c",&count,&c) != 1) syntax(1);
            if (count < 1 || count > COUNT_MAX)
                fatal(0,"%s option value out of range","-c");
            k = 2;
        } else
            break;
        argc -= k;
        argv += k;
    }

/* Check the arguments for consistency and set the defaults. */

    if (action == action_broadcast || action == action_server) {
        operation = (action == action_server ? op_server : op_broadcast);
        if (argc != 1 || minerr != 0.0 || maxerr != 0.0 || count != 0 ||
                delay != 0 || daemon != 0 || prompt != 0.0 ||
                lockname != NULL || savename != NULL)
            syntax(1);
    } else if (action == action_query) {
        if (argc != 1 || minerr != 0.0 || maxerr != 0.0 || count != 0 ||
                delay != 0 || daemon != 0 || prompt != 0.0 || lockname != NULL)
            syntax(1);
    } else {
        if (argc < 1 || argc > MAX_SOCKETS || (daemon != 0 && delay != 0))
            syntax(1);
        if ((prompt || lockname != NULL) &&
                action != action_reset && action != action_adjust)
            syntax(1);
        if (count > 0 && count < argc-1)
            fatal(0,"-c value less than number of addresses",NULL);
       if (argc > 1) {
            operation = op_client;
            for (k = 1; k < argc; ++k) {
                if (argv[k][0] == '\0' || argv[k][0] == '-')
                    fatal(0,"invalid Internet address '%s'",argv[k]);
                hostnames[k-1] = argv[k];
            }
            nhosts = argc-1;
        } else {
            operation = op_listen;
            nhosts = 0;
        }
        if (action == 0) action = action_display;
        if (minerr <= 0.0) minerr = (operation == op_listen ? 0.5 : 0.1);
        if (maxerr <= 0.0) maxerr = 5.0;
        if (count == 0) count = (argc-1 < 5 ? 5 : argc-1);
        if ((argc == 1 || (daemon != 0 && action != action_query)) && count < 5)
            fatal(0,"at least 5 packets needed in this mode",NULL);
        if ((action == action_reset || action == action_adjust) &&
                lockname == NULL)
            lockname = LOCKNAME;

/* The '-x' option changes the implications of many other settings, though this
is not usually apparent to the caller.  Most of the time delays are to ensure
that stuck states terminate, and do not affect the result. */

        if (daemon != 0) {
            if (minerr >= maxerr || maxerr >= daemon)
                fatal(0,"values not in order -e < -E < -x",NULL);
            waiting = delay = daemon *= 60;
        } else {
            if (savename != NULL)
                fatal(0,"-f can be specified only with -x",NULL);
            if (delay == 0)
                delay = (operation == op_listen ? 300 :
                        (2*count >= 15 ? 2*count+1 :15));
            if (operation == op_listen) {
                if (minerr >= maxerr || maxerr >= delay/count)
                    fatal(0,"values not in order -e < -E < -d/-c",NULL);
            } else {
                if (minerr >= maxerr || maxerr >= delay)
                    fatal(0,"values not in order -e < -E < -d",NULL);
            }
            if (2*count >= delay) fatal(0,"-c must be less than half -d",NULL);
            waiting = delay/count;
        }
        if (prompt == 0.0) prompt = 30.0;
    }
    if ((daemon || action == action_query) && savename == NULL)
        savename = SAVENAME;

/* Diagnose where we are, if requested, and separate out the classes of 
operation.  The calls do not return. */

    if (help) syntax(args == 1);
    if (verbose) {
        fprintf(stderr,"%s options: a=%d p=%d v=%d e=%.3f E=%.3f P=%.3f\n",
            argv0,action,period,verbose,minerr,maxerr,prompt);
        fprintf(stderr,"    d=%d c=%d %c=%d op=%d l=%s f=%s",
            delay,count,'x',daemon,operation,
            (lockname == NULL ? "" : lockname),
            (savename == NULL ? "" : savename));
        for (k = 0; k < MAX_SOCKETS; ++k)
            if (hostnames[k] != NULL) fprintf(stderr," %s",hostnames[k]);
        fprintf(stderr,"\n");
    }
    if (nhosts == 0) nhosts = 1;    /* Kludge for broadcasts */
    if (operation == op_server || operation == op_broadcast)
        run_server();
    else if (action == action_query) {
        if (savename == NULL || savename[0] == '\0')
            fatal(0,"no daemon save file specified",NULL);
        else if ((savefile = fopen(savename,"rb")) == NULL)
            fatal(0,"unable to open the daemon save file",NULL);
        query_savefile();
    } else if (daemon != 0) {
        if (savename != NULL && savename[0] != '\0' &&
                (savefile = fopen(savename,"rb+")) == NULL &&
                (savefile = fopen(savename,"wb+")) == NULL)
            fatal(0,"unable to open the daemon save file",NULL);
        run_daemon(hostnames,nhosts,1);
        while (1) run_daemon(hostnames,nhosts,0);
    } else
        run_client(hostnames,nhosts);
    fatal(0,"internal error at end of main",NULL);
    return EXIT_FAILURE;
}