dump1090/net_io.c

// Part of dump1090, a Mode S message decoder for RTLSDR devices.
//
// net_io.c: network handling.
//
// Copyright (c) 2014,2015 Oliver Jowett <oliver@mutability.co.uk>
//
// This file is free software: you may copy, redistribute and/or modify it  
// under the terms of the GNU General Public License as published by the
// Free Software Foundation, either version 2 of the License, or (at your  
// option) any later version.  
//
// This file is distributed in the hope that it will be useful, but  
// WITHOUT ANY WARRANTY; without even the implied warranty of  
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License  
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

// This file incorporates work covered by the following copyright and  
// permission notice:
//
//   Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
//
//   All rights reserved.
//
//   Redistribution and use in source and binary forms, with or without
//   modification, are permitted provided that the following conditions are
//   met:
//
//    *  Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//
//    *  Redistributions in binary form must reproduce the above copyright
//       notice, this list of conditions and the following disclaimer in the
//       documentation and/or other materials provided with the distribution.
//
//   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
//   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
//   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
//   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
//   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
//   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
//   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
//   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
//   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
//   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#include "dump1090.h"

#include <assert.h>

//
// ============================= Networking =============================
//
// Note: here we disregard any kind of good coding practice in favor of
// extreme simplicity, that is:
//
// 1) We only rely on the kernel buffers for our I/O without any kind of
//    user space buffering.
// 2) We don't register any kind of event handler, from time to time a
//    function gets called and we accept new connections. All the rest is
//    handled via non-blocking I/O and manually polling clients to see if
//    they have something new to share with us when reading is needed.
//
//=========================================================================
//
// Networking "stack" initialization
//
struct service {
	char *descr;
	int *socket;
	struct net_writer *writer;
	int port;
	int enabled;
};

struct service services[MODES_NET_SERVICES_NUM];

void modesInitNet(void) {
    int j;

    struct service svc[MODES_NET_SERVICES_NUM] = {
	    {"Raw TCP output", &Modes.raw_out.socket, &Modes.raw_out, Modes.net_output_raw_port, 1},
	    {"Raw TCP input", &Modes.ris, NULL, Modes.net_input_raw_port, 1},
	    {"Beast TCP output", &Modes.beast_out.socket, &Modes.beast_out, Modes.net_output_beast_port, 1},
	    {"Beast TCP input", &Modes.bis, NULL, Modes.net_input_beast_port, 1},
	    {"HTTP server", &Modes.https, NULL, Modes.net_http_port, 1},
	    {"Basestation TCP output", &Modes.sbs_out.socket, &Modes.sbs_out, Modes.net_output_sbs_port, 1},
	    {"FlightAware TSV output", &Modes.fatsv_out.socket, &Modes.fatsv_out, Modes.net_fatsv_port, 1}
    };

	memcpy(&services, &svc, sizeof(svc));//services = svc;

    Modes.clients = NULL;

#ifdef _WIN32
    if ( (!Modes.wsaData.wVersion) 
      && (!Modes.wsaData.wHighVersion) ) {
      // Try to start the windows socket support
      if (WSAStartup(MAKEWORD(2,1),&Modes.wsaData) != 0) 
        {
        fprintf(stderr, "WSAStartup returned Error\n");
        }
      }
#endif

    for (j = 0; j < MODES_NET_SERVICES_NUM; j++) {
		services[j].enabled = (services[j].port != 0);
		if (services[j].enabled) {
			int s = anetTcpServer(Modes.aneterr, services[j].port, Modes.net_bind_address);
			if (s == -1) {
				fprintf(stderr, "Error opening the listening port %d (%s): %s\n",
					services[j].port, services[j].descr, Modes.aneterr);
				exit(1);
			}
			anetNonBlock(Modes.aneterr, s);
			*services[j].socket = s;

                        if (services[j].writer) {
                            if (! (services[j].writer->data = malloc(MODES_OUT_BUF_SIZE)) ) {
                                fprintf(stderr, "Out of memory allocating output buffer for service %s\n", services[j].descr);
                                exit(1);
                            }

                            services[j].writer->socket = s;
                            services[j].writer->connections = 0;
                            services[j].writer->dataUsed = 0;
                            services[j].writer->lastWrite = mstime();
                        }
		} else {
			if (Modes.debug & MODES_DEBUG_NET) printf("%s port is disabled\n", services[j].descr);
		}
    }

#ifndef _WIN32
    signal(SIGPIPE, SIG_IGN);
#endif
}
//
//=========================================================================
//
// This function gets called from time to time when the decoding thread is
// awakened by new data arriving. This usually happens a few times every second
//
struct client * modesAcceptClients(void) {
    int fd, port;
    unsigned int j;
    struct client *c;

    for (j = 0; j < MODES_NET_SERVICES_NUM; j++) {
		if (services[j].enabled) {
			fd = anetTcpAccept(Modes.aneterr, *services[j].socket, NULL, &port);
			if (fd == -1) continue;

			anetNonBlock(Modes.aneterr, fd);
			c = (struct client *) malloc(sizeof(*c));
			c->service    = *services[j].socket;
			c->next       = Modes.clients;
			c->fd         = fd;
			c->buflen     = 0;
			Modes.clients = c;
			anetSetSendBuffer(Modes.aneterr,fd, (MODES_NET_SNDBUF_SIZE << Modes.net_sndbuf_size));

			if (services[j].writer) {
				if (++ services[j].writer->connections == 1) {
                                    services[j].writer->lastWrite = mstime(); // suppress heartbeat initially
				}
			}

			j--; // Try again with the same listening port

			if (Modes.debug & MODES_DEBUG_NET)
				printf("Created new client %d\n", fd);
		}
    }
    return Modes.clients;
}
//
//=========================================================================
//
// On error free the client, collect the structure, adjust maxfd if needed.
//
void modesCloseClient(struct client *c) {
    int j;

    // Clean up, but defer removing from the list until modesNetCleanup().
    // This is because there may be stackframes still pointing at this
    // client (unpredictably: reading from client A may cause client B to
    // be freed)

    close(c->fd);

    for (j = 0; j < MODES_NET_SERVICES_NUM; j++) {
        if (c->service == *services[j].socket) {
            if (services[j].writer)
                services[j].writer->connections--;
	    break;
        }
    }

    if (Modes.debug & MODES_DEBUG_NET)
        printf("Closing client %d\n", c->fd);

    // mark it as inactive and ready to be freed
    c->fd = -1;
    c->service = -1;
}
//
//=========================================================================
//
// Send the write buffer for the specified writer to all connected clients
//
static void flushWrites(struct net_writer *writer) {
    struct client *c;

    for (c = Modes.clients; c; c = c->next) {
        if (c->service == writer->socket) {
#ifndef _WIN32
            int nwritten = write(c->fd, writer->data, writer->dataUsed);
#else
            int nwritten = send(c->fd, writer->data, writer->dataUsed, 0 );
#endif
            if (nwritten != writer->dataUsed) {
                modesCloseClient(c);
            }
        }
    }

    writer->dataUsed = 0;
    writer->lastWrite = mstime();
}

// Prepare to write up to 'len' bytes to the given net_writer.
// Returns a pointer to write to, or NULL to skip this write.
static void *prepareWrite(struct net_writer *writer, int len) {
	if (!writer ||
	    !writer->connections ||
	    !writer->data)
		return NULL;

	if (len > MODES_OUT_BUF_SIZE)
		return NULL;

	if (writer->dataUsed + len >= MODES_OUT_BUF_SIZE) {
		// Flush now to free some space
		flushWrites(writer);
	}

	return writer->data + writer->dataUsed;
}

// Complete a write previously begun by prepareWrite.
// endptr should point one byte past the last byte written
// to the buffer returned from prepareWrite.
static void completeWrite(struct net_writer *writer, void *endptr) {
	writer->dataUsed = endptr - writer->data;

	if (writer->dataUsed >= Modes.net_output_flush_size) {
		flushWrites(writer);
	}
}

//
//=========================================================================
//
// Write raw output in Beast Binary format with Timestamp to TCP clients
//
void modesSendBeastOutput(struct modesMessage *mm) {
    int  msgLen = mm->msgbits / 8;
    char *p = prepareWrite(&Modes.beast_out, 2 + 2 * (7 + msgLen));
    char * pTimeStamp;
    char ch;
    int  j;
    unsigned char *msg = (Modes.net_verbatim ? mm->verbatim : mm->msg);

    if (!p)
        return;

    *p++ = 0x1a;
    if      (msgLen == MODES_SHORT_MSG_BYTES)
      {*p++ = '2';}
    else if (msgLen == MODES_LONG_MSG_BYTES)
      {*p++ = '3';}
    else if (msgLen == MODEAC_MSG_BYTES)
      {*p++ = '1';}
    else
      {return;}

    pTimeStamp = (char *) &mm->timestampMsg;
    for (j = 5; j >= 0; j--) {
        *p++ = (ch = pTimeStamp[j]);
        if (0x1A == ch) {*p++ = ch; }
    }

    *p++ = ch = (char) round(sqrt(mm->signalLevel) * 255);
    if (0x1A == ch) {*p++ = ch; }

    for (j = 0; j < msgLen; j++) {
        *p++ = (ch = msg[j]);
        if (0x1A == ch) {*p++ = ch; }
    }

    completeWrite(&Modes.beast_out, p);
}

//
//=========================================================================
//
// Write raw output to TCP clients
//
void modesSendRawOutput(struct modesMessage *mm) {
    int  msgLen = mm->msgbits / 8;
    char *p = prepareWrite(&Modes.raw_out, msgLen*2 + 15);
    int j;
    unsigned char * pTimeStamp;
    unsigned char *msg = (Modes.net_verbatim ? mm->verbatim : mm->msg);

    if (!p)
        return;

    if (Modes.mlat && mm->timestampMsg) {
        *p++ = '@';
        pTimeStamp = (unsigned char *) &mm->timestampMsg;
        for (j = 5; j >= 0; j--) {
            sprintf(p, "%02X", pTimeStamp[j]);
            p += 2;
        }
    } else
        *p++ = '*';

    for (j = 0; j < msgLen; j++) {
        sprintf(p, "%02X", msg[j]);
        p += 2;
    }

    *p++ = ';';
    *p++ = '\n';

    completeWrite(&Modes.raw_out, p);
}
//
//=========================================================================
//
// Write SBS output to TCP clients
// The message structure mm->bFlags tells us what has been updated by this message
//
void modesSendSBSOutput(struct modesMessage *mm) {
    char *p;
    struct timespec now;
    struct tm    stTime_receive, stTime_now;
    int          msgType;

    // For now, suppress non-ICAO addresses
    if (mm->addr & MODES_NON_ICAO_ADDRESS)
        return;

    p = prepareWrite(&Modes.sbs_out, 200);
    if (!p)
        return;

    //
    // SBS BS style output checked against the following reference
    // http://www.homepages.mcb.net/bones/SBS/Article/Barebones42_Socket_Data.htm - seems comprehensive
    //

    if (mm->msgtype == -1) {
        // heartbeat
        p += sprintf(p, "\r\n");
        completeWrite(&Modes.sbs_out, p);
        return;
    }

    // Decide on the basic SBS Message Type
    if        ((mm->msgtype ==  4) || (mm->msgtype == 20)) {
        msgType = 5;
    } else if ((mm->msgtype ==  5) || (mm->msgtype == 21)) {
        msgType = 6;
    } else if ((mm->msgtype ==  0) || (mm->msgtype == 16)) {
        msgType = 7;
    } else if  (mm->msgtype == 11) {
        msgType = 8;
    } else if ((mm->msgtype != 17) && (mm->msgtype != 18)) {
        return;
    } else if ((mm->metype >= 1) && (mm->metype <=  4)) {
        msgType = 1;
    } else if ((mm->metype >= 5) && (mm->metype <=  8)) {
        if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID)
            {msgType = 2;}
        else
            {msgType = 7;}
    } else if ((mm->metype >= 9) && (mm->metype <= 18)) {
        if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID)
            {msgType = 3;}
        else
            {msgType = 7;}
    } else if (mm->metype !=  19) {
        return;
    } else if ((mm->mesub == 1) || (mm->mesub == 2)) {
        msgType = 4;
    } else {
        return;
    }

    // Fields 1 to 6 : SBS message type and ICAO address of the aircraft and some other stuff
    p += sprintf(p, "MSG,%d,111,11111,%06X,111111,", msgType, mm->addr); 

    // Find current system time
    clock_gettime(CLOCK_REALTIME, &now);
    localtime_r(&now.tv_sec, &stTime_now);

    // Find message reception time
    localtime_r(&mm->sysTimestampMsg.tv_sec, &stTime_receive);

    // Fields 7 & 8 are the message reception time and date
    p += sprintf(p, "%04d/%02d/%02d,", (stTime_receive.tm_year+1900),(stTime_receive.tm_mon+1), stTime_receive.tm_mday);
    p += sprintf(p, "%02d:%02d:%02d.%03u,", stTime_receive.tm_hour, stTime_receive.tm_min, stTime_receive.tm_sec, (unsigned) (mm->sysTimestampMsg.tv_nsec / 1000000U));

    // Fields 9 & 10 are the current time and date
    p += sprintf(p, "%04d/%02d/%02d,", (stTime_now.tm_year+1900),(stTime_now.tm_mon+1), stTime_now.tm_mday);
    p += sprintf(p, "%02d:%02d:%02d.%03u", stTime_now.tm_hour, stTime_now.tm_min, stTime_now.tm_sec, (unsigned) (now.tv_nsec / 1000000U));

    // Field 11 is the callsign (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_CALLSIGN_VALID) {p += sprintf(p, ",%s", mm->flight);}
    else                                           {p += sprintf(p, ",");}

    // Field 12 is the altitude (if we have it) - force to zero if we're on the ground
    if ((mm->bFlags & MODES_ACFLAGS_AOG_GROUND) == MODES_ACFLAGS_AOG_GROUND) {
        p += sprintf(p, ",0");
    } else if (mm->bFlags & MODES_ACFLAGS_ALTITUDE_VALID) {
        p += sprintf(p, ",%d", mm->altitude);
    } else {
        p += sprintf(p, ",");
    }

    // Field 13 is the ground Speed (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_SPEED_VALID) {
        p += sprintf(p, ",%d", mm->velocity);
    } else {
        p += sprintf(p, ","); 
    }

    // Field 14 is the ground Heading (if we have it)       
    if (mm->bFlags & MODES_ACFLAGS_HEADING_VALID) {
        p += sprintf(p, ",%d", mm->heading);
    } else {
        p += sprintf(p, ",");
    }

    // Fields 15 and 16 are the Lat/Lon (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID) {p += sprintf(p, ",%1.5f,%1.5f", mm->fLat, mm->fLon);}
    else                                         {p += sprintf(p, ",,");}

    // Field 17 is the VerticalRate (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) {p += sprintf(p, ",%d", mm->vert_rate);}
    else                                           {p += sprintf(p, ",");}

    // Field 18 is  the Squawk (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_SQUAWK_VALID) {p += sprintf(p, ",%x", mm->modeA);}
    else                                         {p += sprintf(p, ",");}

    // Field 19 is the Squawk Changing Alert flag (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_FS_VALID) {
        if ((mm->fs >= 2) && (mm->fs <= 4)) {
            p += sprintf(p, ",-1");
        } else {
            p += sprintf(p, ",0");
        }
    } else {
        p += sprintf(p, ",");
    }

    // Field 20 is the Squawk Emergency flag (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_SQUAWK_VALID) {
        if ((mm->modeA == 0x7500) || (mm->modeA == 0x7600) || (mm->modeA == 0x7700)) {
            p += sprintf(p, ",-1");
        } else {
            p += sprintf(p, ",0");
        }
    } else {
        p += sprintf(p, ",");
    }

    // Field 21 is the Squawk Ident flag (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_FS_VALID) {
        if ((mm->fs >= 4) && (mm->fs <= 5)) {
            p += sprintf(p, ",-1");
        } else {
            p += sprintf(p, ",0");
        }
    } else {
        p += sprintf(p, ",");
    }

    // Field 22 is the OnTheGround flag (if we have it)
    if (mm->bFlags & MODES_ACFLAGS_AOG_VALID) {
        if (mm->bFlags & MODES_ACFLAGS_AOG) {
            p += sprintf(p, ",-1");
        } else {
            p += sprintf(p, ",0");
        }
    } else {
        p += sprintf(p, ",");
    }

    p += sprintf(p, "\r\n");

    completeWrite(&Modes.sbs_out, p);
}
//
//=========================================================================
//
void modesQueueOutput(struct modesMessage *mm) {
    modesSendSBSOutput(mm);
    modesSendBeastOutput(mm);
    modesSendRawOutput(mm);
}
//
//=========================================================================
//
// This function decodes a Beast binary format message
//
// The message is passed to the higher level layers, so it feeds
// the selected screen output, the network output and so forth.
//
// If the message looks invalid it is silently discarded.
//
// The function always returns 0 (success) to the caller as there is no
// case where we want broken messages here to close the client connection.
//
int decodeBinMessage(struct client *c, char *p) {
    int msgLen = 0;
    int  j;
    char ch;
    char * ptr;
    unsigned char msg[MODES_LONG_MSG_BYTES];
    struct modesMessage mm;
    MODES_NOTUSED(c);
    memset(&mm, 0, sizeof(mm));

    ch = *p++; /// Get the message type
    if (0x1A == ch) {p++;} 

    if       ((ch == '1') && (Modes.mode_ac)) { // skip ModeA/C unless user enables --modes-ac
        msgLen = MODEAC_MSG_BYTES;
    } else if (ch == '2') {
        msgLen = MODES_SHORT_MSG_BYTES;
    } else if (ch == '3') {
        msgLen = MODES_LONG_MSG_BYTES;
    }

    if (msgLen) {
        // Mark messages received over the internet as remote so that we don't try to
        // pass them off as being received by this instance when forwarding them
        mm.remote      =    1;

        ptr = (char*) &mm.timestampMsg;
        for (j = 0; j < 6; j++) { // Grab the timestamp (big endian format)
            ptr[5-j] = ch = *p++; 
            if (0x1A == ch) {p++;}
        }

        // record reception time as the time we read it.
        clock_gettime(CLOCK_REALTIME, &mm.sysTimestampMsg);

        ch = *p++;  // Grab the signal level
        mm.signalLevel = ((unsigned char)ch / 256.0);
        mm.signalLevel = mm.signalLevel * mm.signalLevel + 1e-5;
        if (0x1A == ch) {p++;}

        for (j = 0; j < msgLen; j++) { // and the data
            msg[j] = ch = *p++;
            if (0x1A == ch) {p++;}
        }

        if (msgLen == MODEAC_MSG_BYTES) { // ModeA or ModeC
            Modes.stats_current.remote_received_modeac++;
            decodeModeAMessage(&mm, ((msg[0] << 8) | msg[1]));
        } else {
            int result;

            Modes.stats_current.remote_received_modes++;
            result = decodeModesMessage(&mm, msg);
            if (result < 0) {
                if (result == -1)
                    Modes.stats_current.remote_rejected_unknown_icao++;
                else
                    Modes.stats_current.remote_rejected_bad++;
                return 0;
            } else {
                Modes.stats_current.remote_accepted[mm.correctedbits]++;
            }
        }

        useModesMessage(&mm);
    }
    return (0);
}
//
//=========================================================================
//
// Turn an hex digit into its 4 bit decimal value.
// Returns -1 if the digit is not in the 0-F range.
//
int hexDigitVal(int c) {
    c = tolower(c);
    if (c >= '0' && c <= '9') return c-'0';
    else if (c >= 'a' && c <= 'f') return c-'a'+10;
    else return -1;
}
//
//=========================================================================
//
// This function decodes a string representing message in raw hex format
// like: *8D4B969699155600E87406F5B69F; The string is null-terminated.
// 
// The message is passed to the higher level layers, so it feeds
// the selected screen output, the network output and so forth.
// 
// If the message looks invalid it is silently discarded.
//
// The function always returns 0 (success) to the caller as there is no 
// case where we want broken messages here to close the client connection.
//
int decodeHexMessage(struct client *c, char *hex) {
    int l = strlen(hex), j;
    unsigned char msg[MODES_LONG_MSG_BYTES];
    struct modesMessage mm;
    MODES_NOTUSED(c);
    memset(&mm, 0, sizeof(mm));

    // Mark messages received over the internet as remote so that we don't try to
    // pass them off as being received by this instance when forwarding them
    mm.remote      =    1;
    mm.signalLevel =    1e-5;

    // Remove spaces on the left and on the right
    while(l && isspace(hex[l-1])) {
        hex[l-1] = '\0'; l--;
    }
    while(isspace(*hex)) {
        hex++; l--;
    }

    // Turn the message into binary.
    // Accept *-AVR raw @-AVR/BEAST timeS+raw %-AVR timeS+raw (CRC good) <-BEAST timeS+sigL+raw
    // and some AVR records that we can understand
    if (hex[l-1] != ';') {return (0);} // not complete - abort

    switch(hex[0]) {
        case '<': {
            mm.signalLevel = ((hexDigitVal(hex[13])<<4) | hexDigitVal(hex[14])) / 256.0;
            mm.signalLevel = mm.signalLevel * mm.signalLevel + 1e-5;
            hex += 15; l -= 16; // Skip <, timestamp and siglevel, and ;
            break;}

        case '@':     // No CRC check
        case '%': {   // CRC is OK
            hex += 13; l -= 14; // Skip @,%, and timestamp, and ;
            break;}

        case '*':
        case ':': {
            hex++; l-=2; // Skip * and ;
            break;}

        default: {
            return (0); // We don't know what this is, so abort
            break;}
    }

    if ( (l != (MODEAC_MSG_BYTES      * 2)) 
      && (l != (MODES_SHORT_MSG_BYTES * 2)) 
      && (l != (MODES_LONG_MSG_BYTES  * 2)) )
        {return (0);} // Too short or long message... broken

    if ( (0 == Modes.mode_ac) 
      && (l == (MODEAC_MSG_BYTES * 2)) ) 
        {return (0);} // Right length for ModeA/C, but not enabled

    for (j = 0; j < l; j += 2) {
        int high = hexDigitVal(hex[j]);
        int low  = hexDigitVal(hex[j+1]);

        if (high == -1 || low == -1) return 0;
        msg[j/2] = (high << 4) | low;
    }

    // record reception time as the time we read it.
    clock_gettime(CLOCK_REALTIME, &mm.sysTimestampMsg);

    if (l == (MODEAC_MSG_BYTES * 2)) {  // ModeA or ModeC
        Modes.stats_current.remote_received_modeac++;
        decodeModeAMessage(&mm, ((msg[0] << 8) | msg[1]));
    } else {       // Assume ModeS
        int result;

        Modes.stats_current.remote_received_modes++;
        result = decodeModesMessage(&mm, msg);
        if (result < 0) {
            if (result == -1)
                Modes.stats_current.remote_rejected_unknown_icao++;
            else
                Modes.stats_current.remote_rejected_bad++;
            return 0;
        } else {
            Modes.stats_current.remote_accepted[mm.correctedbits]++;
        }
    }

    useModesMessage(&mm);
    return (0);
}
//
//=========================================================================
//
// Return a description of planes in json. No metric conversion
//

// usual caveats about function-returning-pointer-to-static-buffer apply
static const char *jsonEscapeString(const char *str) {
    static char buf[1024];
    const char *in = str;
    char *out = buf, *end = buf + sizeof(buf) - 10;

    for (; *in && out < end; ++in) {
        unsigned char ch = *in;
        if (ch == '"' || ch == '\\') {
            *out++ = '\\';
            *out++ = ch;
        } else if (ch < 32 || ch > 127) {
            out += snprintf(out, end - out, "\\u%04x", ch);
        } else {
            *out++ = ch;
        }
    }

    *out++ = 0;
    return buf;
}

char *generateAircraftJson(const char *url_path, int *len) {
    uint64_t now = mstime();
    struct aircraft *a;
    int buflen = 1024; // The initial buffer is incremented as needed
    char *buf = (char *) malloc(buflen), *p = buf, *end = buf+buflen;
    int first = 1;

    MODES_NOTUSED(url_path);

    p += snprintf(p, end-p,
                  "{ \"now\" : %.1f,\n"
                  "  \"messages\" : %u,\n"
                  "  \"aircraft\" : [",
                  now / 1000.0,
                  Modes.stats_current.messages_total + Modes.stats_alltime.messages_total);

    for (a = Modes.aircrafts; a; a = a->next) {
        if (a->modeACflags & MODEAC_MSG_FLAG) { // skip any fudged ICAO records Mode A/C
            continue;
        }

        if (a->messages < 2) { // basic filter for bad decodes
            continue;
        }

        if (first)            
            first = 0;
        else
            *p++ = ',';
            
        p += snprintf(p, end-p, "\n    {\"hex\":\"%s%06x\"", (a->addr & MODES_NON_ICAO_ADDRESS) ? "~" : "", a->addr & 0xFFFFFF);
        if (a->bFlags & MODES_ACFLAGS_SQUAWK_VALID)
            p += snprintf(p, end-p, ",\"squawk\":\"%04x\"", a->modeA);
        if (a->bFlags & MODES_ACFLAGS_CALLSIGN_VALID)
            p += snprintf(p, end-p, ",\"flight\":\"%s\"", jsonEscapeString(a->flight));
        if (a->bFlags & MODES_ACFLAGS_LATLON_VALID)
            p += snprintf(p, end-p, ",\"lat\":%f,\"lon\":%f,\"nucp\":%u,\"seen_pos\":%.1f", a->lat, a->lon, a->pos_nuc, (now - a->seenLatLon)/1000.0);
        if ((a->bFlags & MODES_ACFLAGS_AOG_VALID) && (a->bFlags & MODES_ACFLAGS_AOG))
            p += snprintf(p, end-p, ",\"altitude\":\"ground\"");
        else if (a->bFlags & MODES_ACFLAGS_ALTITUDE_VALID)
            p += snprintf(p, end-p, ",\"altitude\":%d", a->altitude);
        if (a->bFlags & MODES_ACFLAGS_VERTRATE_VALID)
            p += snprintf(p, end-p, ",\"vert_rate\":%d", a->vert_rate);
        if (a->bFlags & MODES_ACFLAGS_HEADING_VALID)
            p += snprintf(p, end-p, ",\"track\":%d", a->track);
        if (a->bFlags & MODES_ACFLAGS_SPEED_VALID)
            p += snprintf(p, end-p, ",\"speed\":%d", a->speed);

        p += snprintf(p, end-p, ",\"messages\":%ld,\"seen\":%.1f,\"rssi\":%.1f}",
                      a->messages, (now - a->seen)/1000.0,
                      10 * log10((a->signalLevel[0] + a->signalLevel[1] + a->signalLevel[2] + a->signalLevel[3] +
                                  a->signalLevel[4] + a->signalLevel[5] + a->signalLevel[6] + a->signalLevel[7] + 1e-5) / 8));
        
        // If we're getting near the end of the buffer, expand it.
        if ((end - p) < 256) {
            int used = p - buf;
            buflen *= 2;
            buf = (char *) realloc(buf, buflen);
            p = buf+used;
            end = buf + buflen;
        }
    }

    p += snprintf(p, end-p, "\n  ]\n}\n");
    *len = p-buf;
    return buf;
}

static char * appendStatsJson(char *p,
                              char *end,
                              struct stats *st,
                              const char *key)
{
    int i;

    p += snprintf(p, end-p,
                  "\"%s\":{\"start\":%.1f,\"end\":%.1f",
                  key,
                  st->start / 1000.0,
                  st->end / 1000.0);

    if (!Modes.net_only) {
        p += snprintf(p, end-p,
                      ",\"local\":{\"blocks_processed\":%u"
                      ",\"blocks_dropped\":%u"
                      ",\"modeac\":%u"
                      ",\"modes\":%u"
                      ",\"bad\":%u"
                      ",\"unknown_icao\":%u",
                      st->blocks_processed,
                      st->blocks_dropped,
                      st->demod_modeac,
                      st->demod_preambles,
                      st->demod_rejected_bad,
                      st->demod_rejected_unknown_icao);

        for (i=0; i <= Modes.nfix_crc; ++i) {
            if (i == 0) p += snprintf(p, end-p, ",\"accepted\":[%u", st->demod_accepted[i]);
            else p += snprintf(p, end-p, ",%u", st->demod_accepted[i]);
        }

        p += snprintf(p, end-p, "]");

        if (st->signal_power_count > 0)
            p += snprintf(p, end-p,",\"signal\":%.1f", 10 * log10(st->signal_power_sum / st->signal_power_count));
        if (st->noise_power_count > 0)
            p += snprintf(p, end-p,",\"noise\":%.1f", 10 * log10(st->noise_power_sum / st->noise_power_count));
        if (st->peak_signal_power > 0)
            p += snprintf(p, end-p,",\"peak_signal\":%.1f", 10 * log10(st->peak_signal_power));

        p += snprintf(p, end-p,",\"strong_signals\":%d}", st->strong_signal_count);
    }

    if (Modes.net) {
        p += snprintf(p, end-p,
                      ",\"remote\":{\"modeac\":%u"
                      ",\"modes\":%u"
                      ",\"bad\":%u"
                      ",\"unknown_icao\":%u",
                      st->remote_received_modeac,
                      st->remote_received_modes,
                      st->remote_rejected_bad,
                      st->remote_rejected_unknown_icao);

        for (i=0; i <= Modes.nfix_crc; ++i) {
            if (i == 0) p += snprintf(p, end-p, ",\"accepted\":[%u", st->remote_accepted[i]);
            else p += snprintf(p, end-p, ",%u", st->remote_accepted[i]);
        }

        p += snprintf(p, end-p, "]");

        p += snprintf(p, end-p, "},\"http_requests\":%u", st->http_requests);
    }

    {
        uint64_t demod_cpu_millis = (uint64_t)st->demod_cpu.tv_sec*1000UL + st->demod_cpu.tv_nsec/1000000UL;
        uint64_t reader_cpu_millis = (uint64_t)st->reader_cpu.tv_sec*1000UL + st->reader_cpu.tv_nsec/1000000UL;
        uint64_t background_cpu_millis = (uint64_t)st->background_cpu.tv_sec*1000UL + st->background_cpu.tv_nsec/1000000UL;

        p += snprintf(p, end-p,
                      ",\"cpr\":{\"global_ok\":%u"
                      ",\"global_bad\":%u"
                      ",\"global_range\":%u"
                      ",\"global_speed\":%u"
                      ",\"global_skipped\":%u"
                      ",\"local_ok\":%u"
                      ",\"local_skipped\":%u"
                      ",\"local_range\":%u"
                      ",\"local_speed\":%u"
                      ",\"filtered\":%u}"
                      ",\"cpu\":{\"demod\":%llu,\"reader\":%llu,\"background\":%llu}"
                      ",\"tracks\":{\"all\":%u"
                      ",\"single_message\":%u}"
                      ",\"messages\":%u}",
                      st->cpr_global_ok,
                      st->cpr_global_bad,
                      st->cpr_global_range_checks,
                      st->cpr_global_speed_checks,
                      st->cpr_global_skipped,
                      st->cpr_local_ok,
                      st->cpr_local_skipped,
                      st->cpr_local_range_checks,
                      st->cpr_local_speed_checks,
                      st->cpr_filtered,
                      (unsigned long long)demod_cpu_millis,
                      (unsigned long long)reader_cpu_millis,
                      (unsigned long long)background_cpu_millis,
                      st->unique_aircraft,
                      st->single_message_aircraft,
                      st->messages_total);
    }

    return p;
}
    
char *generateStatsJson(const char *url_path, int *len) {
    struct stats add;
    char *buf = (char *) malloc(4096), *p = buf, *end = buf + 4096;

    MODES_NOTUSED(url_path);

    p += snprintf(p, end-p, "{\n");
    p = appendStatsJson(p, end, &Modes.stats_current, "latest");
    p += snprintf(p, end-p, ",\n");

    p = appendStatsJson(p, end, &Modes.stats_1min[Modes.stats_latest_1min], "last1min");
    p += snprintf(p, end-p, ",\n");

    p = appendStatsJson(p, end, &Modes.stats_5min, "last5min");
    p += snprintf(p, end-p, ",\n");

    p = appendStatsJson(p, end, &Modes.stats_15min, "last15min");
    p += snprintf(p, end-p, ",\n");

    add_stats(&Modes.stats_alltime, &Modes.stats_current, &add);
    p = appendStatsJson(p, end, &add, "total");
    p += snprintf(p, end-p, "\n}\n");    

    assert(p <= end);

    *len = p-buf;
    return buf;
}

//
// Return a description of the receiver in json.
//
char *generateReceiverJson(const char *url_path, int *len)
{
    char *buf = (char *) malloc(1024), *p = buf;
    int history_size;

    MODES_NOTUSED(url_path);

    // work out number of valid history entries
    if (Modes.json_aircraft_history[HISTORY_SIZE-1].content == NULL)
        history_size = Modes.json_aircraft_history_next;
    else
        history_size = HISTORY_SIZE;

    p += sprintf(p, "{ " \
                 "\"version\" : \"%s\", "
                 "\"refresh\" : %.0f, "
                 "\"history\" : %d",
                 MODES_DUMP1090_VERSION, 1.0*Modes.json_interval, history_size);

    if (Modes.json_location_accuracy && (Modes.fUserLat != 0.0 || Modes.fUserLon != 0.0)) {
        if (Modes.json_location_accuracy == 1) {
            p += sprintf(p, ", "                \
                         "\"lat\" : %.2f, "
                         "\"lon\" : %.2f",
                         Modes.fUserLat, Modes.fUserLon);  // round to 2dp - about 0.5-1km accuracy - for privacy reasons
        } else {
            p += sprintf(p, ", "                \
                         "\"lat\" : %.6f, "
                         "\"lon\" : %.6f",
                         Modes.fUserLat, Modes.fUserLon);  // exact location
        }
    }

    p += sprintf(p, " }\n");

    *len = (p - buf);
    return buf;
}

char *generateHistoryJson(const char *url_path, int *len)
{
    int history_index = -1;

    if (sscanf(url_path, "/data/history_%d.json", &history_index) != 1)
        return NULL;

    if (history_index < 0 || history_index >= HISTORY_SIZE)
        return NULL;

    if (!Modes.json_aircraft_history[history_index].content)
        return NULL;

    *len = Modes.json_aircraft_history[history_index].clen;
    return strdup(Modes.json_aircraft_history[history_index].content);
}

// Write JSON to file
void writeJsonToFile(const char *file, char * (*generator) (const char *,int*))
{
#ifndef _WIN32
    char pathbuf[PATH_MAX];
    char tmppath[PATH_MAX];
    int fd;
    int len = 0;
    mode_t mask;
    char *content;

    if (!Modes.json_dir)
        return;

    snprintf(tmppath, PATH_MAX, "%s/%s.XXXXXX", Modes.json_dir, file);
    tmppath[PATH_MAX-1] = 0;
    fd = mkstemp(tmppath);
    if (fd < 0)
        return;
    
    mask = umask(0);
    umask(mask);
    fchmod(fd, 0644 & ~mask);

    snprintf(pathbuf, PATH_MAX, "/data/%s", file);
    pathbuf[PATH_MAX-1] = 0;
    content = generator(pathbuf, &len);

    if (write(fd, content, len) != len)
        goto error_1;

    if (close(fd) < 0)
        goto error_2;

    snprintf(pathbuf, PATH_MAX, "%s/%s", Modes.json_dir, file);
    pathbuf[PATH_MAX-1] = 0;
    rename(tmppath, pathbuf);
    free(content);
    return;

 error_1:
    close(fd);
 error_2:
    unlink(tmppath);
    free(content);
    return;
#endif
}



//
//=========================================================================
//
#define MODES_CONTENT_TYPE_HTML "text/html;charset=utf-8"
#define MODES_CONTENT_TYPE_CSS  "text/css;charset=utf-8"
#define MODES_CONTENT_TYPE_JSON "application/json;charset=utf-8"
#define MODES_CONTENT_TYPE_JS   "application/javascript;charset=utf-8"
#define MODES_CONTENT_TYPE_GIF  "image/gif"

static struct {
    char *path;
    char * (*handler)(const char*,int*);
    char *content_type;
    int prefix;
} url_handlers[] = {
    { "/data/aircraft.json", generateAircraftJson, MODES_CONTENT_TYPE_JSON, 0 },
    { "/data/receiver.json", generateReceiverJson, MODES_CONTENT_TYPE_JSON, 0 },
    { "/data/stats.json", generateStatsJson, MODES_CONTENT_TYPE_JSON, 0 },
    { "/data/history_", generateHistoryJson, MODES_CONTENT_TYPE_JSON, 1 },
    { NULL, NULL, NULL, 0 }
};

//
// Get an HTTP request header and write the response to the client.
// gain here we assume that the socket buffer is enough without doing
// any kind of userspace buffering.
//
// Returns 1 on error to signal the caller the client connection should
// be closed.
//
int handleHTTPRequest(struct client *c, char *p) {
    char hdr[512];
    int clen, hdrlen;
    int httpver, keepalive;
    int statuscode = 500;
    const char *statusmsg = "Internal Server Error";
    char *url, *content = NULL;
    char *ext;
    char *content_type;
    int i;

    if (Modes.debug & MODES_DEBUG_NET)
        printf("\nHTTP request: %s\n", c->buf);

    // Minimally parse the request.
    httpver = (strstr(p, "HTTP/1.1") != NULL) ? 11 : 10;
    if (httpver == 10) {
        // HTTP 1.0 defaults to close, unless otherwise specified.
        //keepalive = strstr(p, "Connection: keep-alive") != NULL;
    } else if (httpver == 11) {
        // HTTP 1.1 defaults to keep-alive, unless close is specified.
        //keepalive = strstr(p, "Connection: close") == NULL;
    }
    keepalive = 0;

    // Identify he URL.
    p = strchr(p,' ');
    if (!p) return 1; // There should be the method and a space
    url = ++p;        // Now this should point to the requested URL
    p = strchr(p, ' ');
    if (!p) return 1; // There should be a space before HTTP/
    *p = '\0';

    if (Modes.debug & MODES_DEBUG_NET) {
        printf("\nHTTP keep alive: %d\n", keepalive);
        printf("HTTP requested URL: %s\n\n", url);
    }
    
    // Ditch any trailing query part (AJAX might add one to avoid caching)
    p = strchr(url, '?');
    if (p) *p = 0;

    statuscode = 404;
    statusmsg = "Not Found";
    for (i = 0; url_handlers[i].path; ++i) {
        if ((url_handlers[i].prefix && !strncmp(url, url_handlers[i].path, strlen(url_handlers[i].path))) ||
            (!url_handlers[i].prefix && !strcmp(url, url_handlers[i].path))) {
            content_type = url_handlers[i].content_type;
            content = url_handlers[i].handler(url, &clen);
            if (!content)
                continue;

            statuscode = 200;
            statusmsg = "OK";
            if (Modes.debug & MODES_DEBUG_NET) {
                printf("HTTP: 200: %s -> internal (%d bytes, %s)\n", url, clen, content_type);
            }
            break;
        }
    }
            
    if (!content) {
        struct stat sbuf;
        int fd = -1;
        char rp[PATH_MAX], hrp[PATH_MAX];
        char getFile[1024];

        if (strlen(url) < 2) {
            snprintf(getFile, sizeof getFile, "%s/gmap.html", HTMLPATH); // Default file
        } else {
            snprintf(getFile, sizeof getFile, "%s/%s", HTMLPATH, url);
        }

        if (!realpath(getFile, rp))
            rp[0] = 0;
        if (!realpath(HTMLPATH, hrp))
            strcpy(hrp, HTMLPATH);

        clen = -1;
        content = strdup("Server error occured");
        if (!strncmp(hrp, rp, strlen(hrp))) {
            if (stat(getFile, &sbuf) != -1 && (fd = open(getFile, O_RDONLY)) != -1) {
                content = (char *) realloc(content, sbuf.st_size);
                if (read(fd, content, sbuf.st_size) != -1) {
                    clen = sbuf.st_size;
                    statuscode = 200;
                    statusmsg = "OK";
                }
            }
        } else {
            errno = ENOENT;
        }

        if (clen < 0) {
            content = realloc(content, 128);
            clen = snprintf(content, 128,"Error opening HTML file: %s", strerror(errno));
            statuscode = 404;
            statusmsg = "Not Found";
        }
        
        if (fd != -1) {
            close(fd);
        }

        // Get file extension and content type
        content_type = MODES_CONTENT_TYPE_HTML; // Default content type
        ext = strrchr(getFile, '.');
        
        if (strlen(ext) > 0) {
            if (strstr(ext, ".json")) {
                content_type = MODES_CONTENT_TYPE_JSON;
            } else if (strstr(ext, ".css")) {
                content_type = MODES_CONTENT_TYPE_CSS;
            } else if (strstr(ext, ".js")) {
                content_type = MODES_CONTENT_TYPE_JS;
            } else if (strstr(ext, ".gif")) {
                content_type = MODES_CONTENT_TYPE_GIF;
            }
        }

        if (Modes.debug & MODES_DEBUG_NET) {
            printf("HTTP: %d %s: %s -> %s (%d bytes, %s)\n", statuscode, statusmsg, url, rp, clen, content_type);
        }
    }


    // Create the header and send the reply
    hdrlen = snprintf(hdr, sizeof(hdr),
        "HTTP/1.1 %d %s\r\n"
        "Server: Dump1090\r\n"
        "Content-Type: %s\r\n"
        "Connection: %s\r\n"
        "Content-Length: %d\r\n"
        "Cache-Control: no-cache, must-revalidate\r\n"
        "Expires: Sat, 26 Jul 1997 05:00:00 GMT\r\n"
        "\r\n",
        statuscode, statusmsg,
        content_type,
        keepalive ? "keep-alive" : "close",
        clen);

    if (Modes.debug & MODES_DEBUG_NET) {
        printf("HTTP Reply header:\n%s", hdr);
    }

    // Send header and content.
#ifndef _WIN32
    if ( (write(c->fd, hdr, hdrlen) != hdrlen) 
      || (write(c->fd, content, clen) != clen) ) {
#else
    if ( (send(c->fd, hdr, hdrlen, 0) != hdrlen) 
      || (send(c->fd, content, clen, 0) != clen) ) {
#endif
        free(content);
        return 1;
    }
    free(content);
    Modes.stats_current.http_requests++;
    return !keepalive;
}
//
//=========================================================================
//
// This function polls the clients using read() in order to receive new
// messages from the net.
//
// The message is supposed to be separated from the next message by the
// separator 'sep', which is a null-terminated C string.
//
// Every full message received is decoded and passed to the higher layers
// calling the function's 'handler'.
//
// The handler returns 0 on success, or 1 to signal this function we should
// close the connection with the client in case of non-recoverable errors.
//
void modesReadFromClient(struct client *c, char *sep,
                         int(*handler)(struct client *, char *)) {
    int left;
    int nread;
    int fullmsg;
    int bContinue = 1;
    char *s, *e, *p;

    while(bContinue) {

        fullmsg = 0;
        left = MODES_CLIENT_BUF_SIZE - c->buflen;
        // If our buffer is full discard it, this is some badly formatted shit
        if (left <= 0) {
            c->buflen = 0;
            left = MODES_CLIENT_BUF_SIZE;
            // If there is garbage, read more to discard it ASAP
        }
#ifndef _WIN32
        nread = read(c->fd, c->buf+c->buflen, left);
#else
        nread = recv(c->fd, c->buf+c->buflen, left, 0);
        if (nread < 0) {errno = WSAGetLastError();}
#endif

        // If we didn't get all the data we asked for, then return once we've processed what we did get.
        if (nread != left) {
            bContinue = 0;
        }

        if (nread == 0) { // End of file
            modesCloseClient(c);
            return;
        }

#ifndef _WIN32
        if (nread < 0 && (errno == EAGAIN || errno == EWOULDBLOCK)) { // No data available (not really an error)
#else
        if (nread < 0 && errno == EWOULDBLOCK) { // No data available (not really an error)
#endif
            return;
        }

        if (nread < 0) { // Other errors
            modesCloseClient(c);
            return;
        }

        c->buflen += nread;

        // Always null-term so we are free to use strstr() (it won't affect binary case)
        c->buf[c->buflen] = '\0';

        e = s = c->buf;                                // Start with the start of buffer, first message

        if (c->service == Modes.bis) {
            // This is the Beast Binary scanning case.
            // If there is a complete message still in the buffer, there must be the separator 'sep'
            // in the buffer, note that we full-scan the buffer at every read for simplicity.

            left = c->buflen;                                  // Length of valid search for memchr()
            while (left && ((s = memchr(e, (char) 0x1a, left)) != NULL)) { // The first byte of buffer 'should' be 0x1a
                s++;                                           // skip the 0x1a
                if        (*s == '1') {
                    e = s + MODEAC_MSG_BYTES      + 8;         // point past remainder of message
                } else if (*s == '2') {
                    e = s + MODES_SHORT_MSG_BYTES + 8;
                } else if (*s == '3') {
                    e = s + MODES_LONG_MSG_BYTES  + 8;
                } else {
                    e = s;                                     // Not a valid beast message, skip
                    left = &(c->buf[c->buflen]) - e;
                    continue;
                }
                // we need to be careful of double escape characters in the message body
                for (p = s; p < e; p++) {
                    if (0x1A == *p) {
                        p++; e++;
                        if (e > &(c->buf[c->buflen])) {
                            break;
                        }
                    }
                }
                left = &(c->buf[c->buflen]) - e;
                if (left < 0) {                                // Incomplete message in buffer
                    e = s - 1;                                 // point back at last found 0x1a.
                    break;
                }
                // Have a 0x1a followed by 1, 2 or 3 - pass message less 0x1a to handler.
                if (handler(c, s)) {
                    modesCloseClient(c);
                    return;
                }
                fullmsg = 1;
            }
            s = e;     // For the buffer remainder below

        } else {
            //
            // This is the ASCII scanning case, AVR RAW or HTTP at present
            // If there is a complete message still in the buffer, there must be the separator 'sep'
            // in the buffer, note that we full-scan the buffer at every read for simplicity.
            //
            while ((e = strstr(s, sep)) != NULL) { // end of first message if found
                *e = '\0';                         // The handler expects null terminated strings
                if (handler(c, s)) {               // Pass message to handler.
                    modesCloseClient(c);           // Handler returns 1 on error to signal we .
                    return;                        // should close the client connection
                }
                s = e + strlen(sep);               // Move to start of next message
                fullmsg = 1;
            }
        }

        if (fullmsg) {                             // We processed something - so
            c->buflen = &(c->buf[c->buflen]) - s;  //     Update the unprocessed buffer length
            memmove(c->buf, s, c->buflen);         //     Move what's remaining to the start of the buffer
        } else {                                   // If no message was decoded process the next client
            return;
        }
    }
}

#define TSV_MAX_PACKET_SIZE 160

static void writeFATSV() {
    struct aircraft *a;
    uint64_t now;
    static uint64_t next_update;

    if (!Modes.fatsv_out.connections) {
        return; // no active connections
    }

    now = mstime();
    if (now < next_update) {
        return;
    }

    // scan once a second at most
    next_update = now + 1000;

    for (a = Modes.aircrafts; a; a = a->next) {
        int altValid = 0;
        int alt = 0;
        int groundValid = 0;
        int ground = 0;
        int latlonValid = 0;
        int useful = 0;
        uint64_t emittedMillisAgo;
        char *p, *end;

        // skip non-ICAO
        if (a->addr & MODES_NON_ICAO_ADDRESS)
            continue;

        if (a->messages < 2)  // basic filter for bad decodes
            continue;

        // don't emit if it hasn't updated since last time
        if (a->seen < a->fatsv_last_emitted) {
            continue;
        }

        emittedMillisAgo = (now - a->fatsv_last_emitted);

        // don't emit more than once every five seconds
        if (emittedMillisAgo < 5000) {
            continue;
        }

        if (a->bFlags & MODES_ACFLAGS_ALTITUDE_VALID) {
            altValid = 1;            
            alt = a->altitude;
        }
        
        if (a->bFlags & MODES_ACFLAGS_AOG_VALID) {
            groundValid = 1;

            if (a->bFlags & MODES_ACFLAGS_AOG) {
                alt = 0;
                ground = 1;
            }
        }

        if (a->bFlags & MODES_ACFLAGS_LATLON_VALID) {
            latlonValid = 1;
        }

        // if it's over 10,000 feet, don't emit more than once every 10 seconds
        if (alt > 10000 && emittedMillisAgo < 10000) {
            continue;
        }

        // disable if you want only ads-b
        // also don't send mode S very often
        if (!latlonValid) {
            if (emittedMillisAgo < 30000) {
                continue;
            }
        } else {
            // if it hasn't changed altitude very much and it hasn't changed 
            // heading very much, don't update real often
            if (abs(a->track - a->fatsv_emitted_track) < 2 && abs(alt - a->fatsv_emitted_altitude) < 50) {
                if (alt < 10000) {
                    // it hasn't changed much but we're below 10,000 feet 
                    // so update more frequently
                    if (emittedMillisAgo < 10000) {
                        continue;
                    }
                } else {
                    // above 10,000 feet, don't update so often when it 
                    // hasn't changed much
                    if (emittedMillisAgo < 30000) {
                        continue;
                    }
                }
            }
        }

        p = prepareWrite(&Modes.fatsv_out, TSV_MAX_PACKET_SIZE);
        if (!p)
            return;

        end = p + TSV_MAX_PACKET_SIZE;
#       define bufsize(_p,_e) ((_p) >= (_e) ? (size_t)0 : (size_t)((_e) - (_p)))
        p += snprintf(p, bufsize(p,end), "clock\t%ld\thexid\t%06X", (long)(a->seen / 1000), a->addr);

        if (*a->flight != '\0') {
            p += snprintf(p, bufsize(p,end), "\tident\t%s", a->flight);
        }

        if (a->bFlags & MODES_ACFLAGS_SQUAWK_VALID) {
            p += snprintf(p, bufsize(p,end), "\tsquawk\t%04x", a->modeA);
        }

        if (altValid) {
            p += snprintf(p, bufsize(p,end), "\talt\t%d", alt);
            useful = 1;
        }

        if (a->bFlags & MODES_ACFLAGS_SPEED_VALID) {
            p += snprintf(p, bufsize(p,end), "\tspeed\t%d", a->speed);
            useful = 1;
        }

        if (groundValid) {
            if (ground) {
                p += snprintf(p, bufsize(p,end), "\tairGround\tG");
            } else {
                p += snprintf(p, bufsize(p,end), "\tairGround\tA");
            }
        }

        if (latlonValid) {
            p += snprintf(p, bufsize(p,end), "\tlat\t%.5f\tlon\t%.5f", a->lat, a->lon);
            useful = 1;
        }

        if (a->bFlags & MODES_ACFLAGS_HEADING_VALID) {
            p += snprintf(p, bufsize(p,end), "\theading\t%d", a->track);
            useful = 1;
        }

        // if we didn't get at least an alt or a speed or a latlon or
        // a heading, bail out. We don't need to do anything special
        // to unwind prepareWrite().
        if (!useful) {
            continue;
        }

        p += snprintf(p, bufsize(p,end), "\n");

        if (p <= end)
            completeWrite(&Modes.fatsv_out, p);
        else
            fprintf(stderr, "fatsv: output too large (max %d, overran by %d)\n", TSV_MAX_PACKET_SIZE, (int) (p - end));
#       undef bufsize

        a->fatsv_last_emitted = now;
        a->fatsv_emitted_altitude = alt;
        a->fatsv_emitted_track = a->track;
    }
}

//
// Perform periodic network work
//
void modesNetPeriodicWork(void) {
	struct client *c, **prev;
        uint64_t now = mstime();
	int j;
	int need_heartbeat = 0, need_flush = 0;

	// Accept new connetions
	modesAcceptClients();

	// Read from clients
	for (c = Modes.clients; c; c = c->next) {
		if (c->service == Modes.ris) {
			modesReadFromClient(c,"\n",decodeHexMessage);
		} else if (c->service == Modes.bis) {
			modesReadFromClient(c,"",decodeBinMessage);
		} else if (c->service == Modes.https) {
			modesReadFromClient(c,"\r\n\r\n",handleHTTPRequest);
		}
	}

        // Generate FATSV output
        writeFATSV();

	// If we have generated no messages for a while, generate
	// a dummy heartbeat message.
	if (Modes.net_heartbeat_interval) {
		for (j = 0; j < MODES_NET_SERVICES_NUM; j++) {
			if (services[j].writer &&
			    services[j].writer->connections &&
			    (services[j].writer->lastWrite + Modes.net_heartbeat_interval) <= now) {
				need_flush = 1;
				if (services[j].writer->dataUsed == 0) {
					need_heartbeat = 1;
					break;
				}
			}
		}
        }

        if (need_heartbeat) {
		//
		// We haven't sent any traffic for some time. To try and keep any TCP
		// links alive, send a null frame. This will help stop any routers discarding our TCP
		// link which will cause an un-recoverable link error if/when a real frame arrives.
		//
		// Fudge up a null message
		struct modesMessage mm;

		memset(&mm, 0, sizeof(mm));
		mm.msgbits      = MODES_SHORT_MSG_BITS;
		mm.timestampMsg = 0;
		mm.msgtype      = -1;

		// Feed output clients
		modesQueueOutput(&mm);
        }

	// If we have data that has been waiting to be written for a while,
	// write it now.
	for (j = 0; j < MODES_NET_SERVICES_NUM; j++) {
		if (services[j].writer &&
		    services[j].writer->dataUsed &&
		    (need_flush || (services[j].writer->lastWrite + Modes.net_output_flush_interval) <= now)) {
			flushWrites(services[j].writer);
		}
	}

	// Unlink and free closed clients
	for (prev = &Modes.clients, c = *prev; c; c = *prev) {
		if (c->fd == -1) {
			// Recently closed, prune from list
			*prev = c->next;
			free(c);
		} else {
			prev = &c->next;
		}
	}
}

//
// =============================== Network IO ===========================
//