These notes are meant as an aid to reading and understanding some of the main code files forming the gpsd daemon and its support libraries. They are not a line-by-line explanation but only pick out the important (or unusual) aspects of the code.

They do not address the many programs that are used for test purposes, the individual device driver files or any of the supplied applications that use gpsd.

This guide was written in 2007 before the move to a JSON-based protocol and before AIS support was added. The machinery relating to JSON and AIS is not yet documented here. While the information on the rest of the code has been updated as of 2011.


This file only contains macros that convert data variables from the native format of the device output (MSB-first or LSB-first) into a standard form for gpsd use. This makes data parsing consistent regardless of the underlying format. If the device driver file makes no deliberate selection, the default is to assume the device emits data in big-endian (network byte) order.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: Handles initialization of and communication with the DBUS.

int initialize_dbus_connection(void)

Initializes the DBUS connection.

void send_dbus_fix(struct gps_device_t* channel)

Sends the current fix data for this channel via dbus

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: Parser for NMEA strings, generic and proprietary.

static void do_lat_lon(char *field[], struct gps_fix_t *out)

Reads a four element array containing the value and hemisphere of the latitude and longitude of a location as text. It converts them into signed (-ve for S and W) values. The gps_data_t structure is updated with the new value(s).

static void merge_ddmmyy(char *ddmmyy, struct gps_device_t *session)

If the century has not yet been stored in the nmea driver private data, take the supplied ddmmyy date and generate and store a ddmmyyyy date, using the century value compiled in from gpsd.h.

static void merge_hhmmss(char *hhmmss, struct gps_device_t *session)

Stash the present hour value before updating it from the incoming data. If the new hour is less than the stashed value, we have passed midnight, so update the day value. Finally update the minutes, seconds and fractions of a second from the incoming data.

static gps_mask_t processGPRMC(int count, char *field[], struct gps_device_t *session)

Handle a $GPRMC sentence stored in an array of strings, one member per field.

Check if the message is stamped valid or not.

If it is invalid, set the status and fix mode to NO_FIX and save the corresponding flags locally; also save the online flag to indicate we have handled a known sentence.

If the fix is autonomous and valid, start to decode the fields.

First, test if there are enough fields available; then handle the date and time via merge_ddmmyy() and merge_hhmmss(), storing the TIME_SET flag and storing the fix time as a UNIX-epoch relative value.

If the sentence time and this fix time are different, we have started a new cycle of observation, so update the sentence time and the store the CYCLE_START_SET flag.

Whatever the number of fields, store the fix coordinates via do_lat_lon(), store the speed and the track and save the corresponding flags.

Return the local aggregated flags to allow the main copy in the session data to be updated.

static gps_mask_t processGPGLL(int count, char *field[], struct gps_device_t *session)

Preload the local flag with the ERROR_SET flag.

Check that the sentence is usable, exiting with the preset error flag if it is not.

If it is usable, clear the local flags and start processing the fields, updating any local flag fields on the way.

If the year is already known, update the time and check for the start of cycle (see processGPRMC() above).

Handle the fix location and, if the number of received fixes is more than 8 and set fix status.

If the present mode is less than 2D_FIX, update it to 2D_FIX.

Write the stashed value of newstatus into the session status and return all the locally aggregated flags.

static gps_mask_t processGPGGA(int c UNUSED, char *field[], struct gps_device_t *session)

Stash the last fix time. Set the status to the value in the message and update the local flag variable.

If the status is STATUS_UNK, exit immediately, returning the locally aggregated flags. If there is a fix, process it.

Handle the time as in processGPGLL() above. Handle the latitiude and longitude with a call to do_lat_lon() above and set the local flag.

Update the satellites_used field and stash the altitude.

If the altitude is empty, force the fix mode and status to 2D if it was 3D previously.

If it is not empty, stash the old value of altitude and replace it with the new value stashed earlier and set the local flag variable. If the mode is presently less than 3D, update it to 3D and set the local flag.

If the stashed old altitude is NaN or the stashed fix time and current fix time are equal, set the climb rate to 0 otherwise calculate it by dividing the altitude difference by the time difference and set the local flag.

If the geoid separation is available, store it, otherwise store the value from wgs84_separation() that depends on current location.

Finally, return all the locally aggregated flags.

static gps_mask_t processGPGSA(int count, char *field[], struct gps_device_t *session)

Start with a simple validity check on the number of fields (for i.Trek M3) and bail out with a simple indication of on-line status if it fails.

Set the fix mode from the sentence and either clear the local flag variable (if an Antaris chipset says we are in dead-reckoning mode) or set the MODE_SET flag.

Update all the DOP fields from the sentence, clear the count of used satellites, then scan all the satellite data.

If any satellite is good (prn != 0), store the prn and increment the count of used satellites.

Finally, set the local flags to indicate that DOPs are available and return all the locally aggregated flags.

static gps_mask_t processGPGSV(int count, char *field[], struct gps_device_t *session)

Check if the sentence has too few fields or the wrong number of fields. In this case, clear the data for all satellites and return with an error indication.

Start to parse the sentence. First, note how many sentences are to be expected to complete the data transfer.

If the sentence number is invalid, clear the data for all satellites and return with an error indication.

If this is the first sentence of the sequence, clear the data for all satellites.

Loop through the sentence fields, updating the session’s satellite data.

If any satellite number is higher than the number of channels, clear all satellite data and break out of the loop.

Assuming this is not a buggy chipset (e.g. Motorola Oncore GT+), update the satellite count and loop again.

If this was the last sentence of the block and the number of satellites seen is not the same as the number reported, generate an error log.

If this is not the last sentence of the block, exit early and return an error flag as a guard.

Finally, on the last sentence, carry out a sanity check and either return an error flag or a SATELLITE_SET flag.

static gps_mask_t processPGRME(int c UNUSED, char *field[], struct gps_device_t *session)

Check that the error estimate data is good. If not, set all error estimate fields to 100m and return an error flag.

If they are good, calculate the error value and store it. Return the appropriate flag values.

static gps_mask_t processGPZDA(int c UNUSED, char *field[], struct gps_device_t *session)

Set the local flag variable to indicate that the time is available.

Store the actual time by a call to merge_hhmmss() and fill in the other fields from the sentence data.

If the sentence is not timestamped the same as the fixtime, set the CYCLE_START_SET flag.

Update the fixtime to the sentence timestamp.

Finally, return all the locally aggregated flags.

static gps_mask_t processTNTHTM(int c UNUSED, char *field[], struct gps_device_t *session)

Set the local variable to indicate the unit is on-line.

Fill all appropriate fields from the sentence and set the associated flags in the local flag variable.

Set the fix status and return all the locally aggregated flags.

static short nmea_checksum(char *sentence, unsigned char *correct_sum)

Calculate and return the checksum of an NMEA sentence.

gps_mask_t nmea_parse(char *sentence, struct gps_device_t *session)

Test that the length of the NMEA sentence is acceptable, simply returning an on-line indication if it is too long to handle.

If it is within limits, make a local copy and split it on the commas into an array, one field per element.

Use the first element to match the command to the table of decodable commands.

Check if it is supported and the number of fields is reasonable, invoke the correct decoder and return the value from that call.

If it fails the check, simply return an on-line status.

This function is also responsible for performing adaptive end-of-cycle detection.

void nmea_add_checksum(char *sentence)

Calculate the checksum then append '' + the checksum + CR/LF to the end of an NMEA sentence, skipping any existing ''.

int nmea_write(struct gps_device_t *session, const char *fmt, ... )

Ship a string to an NMEA device, adding a checksum and CR/LF if needed. A checksum is added only if the sentence begins with '$'. Bytes written are returned.

int nmea_send(struct gps_device_t *session, , const char *fmt, ... )

A wrapper around nmea_write() to give it sprintf-like varargs behavior.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


This is a dummy file to use as a skeleton for writing a new driver. The driver is heavily commented, so refer to the source code.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This is a decoder for the RTCM-104 serial protocol.

void rtcm2_unpack(struct rtcm2_t *tp, char *buf)

Splits RTCM2 raw data into fields.

bool rtcm2_repack(struct rtcm2_t *tp, isgps30bits_t *buf)

Packs RTCM2 fields into a raw data stream.

static bool preamble_match(isgps30bits_t *w)

Tells if the preamble field in a message is valid or not.

static bool length_check(struct gps_packet_t *lexer)

Tells if the message length is valid or not.

enum isgpsstat_t rtcm2_decode(struct gps_packet_t *lexer, unsigned int c)

A simple call to isgps_decode().

void rtcm2_dump(struct rtcm2_t *rtcm2, char buf[], size_t buflen)

Dump the contents of a parsed RTCM104v2 message.

void rtcm2_output_magnavox(isgps30bits_t *ip, FILE *fp)

Ship an RTCM2 message in the format emitted by Magnavox DGPS receivers.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This is the main driver for NMEA, both plain vanilla and any manufacturer specific variety of NMEA output. This is NOT where manufacturer specific binary matters are handled.

ssize_t pass_rtcm(struct gps_device_t *session, char *buf, size_t rtcmbytes)

This writes the rtcm data to the device.

gps_mask_t nmea_parse_input(struct gps_device_t *session)

If the packet is a comment packet, ignore it and return 0.

If it is one of several binary packets, issue a warning to that effect.

If the relevant driver exists, switch to that driver and get it to parse the packet. The return value is that of the binary parser.

If the driver does not exist, ignore the packet and return 0.

If after all this, it is likely to be an NMEA packet, so try to parse it with the NMEA parser. If that returns a 0 (unsuccessful), then check if it is a trigger packet for one of the triggerable drivers.

If this succeeds, return 1, otherwise issue an unrecognised packet error.

Finally checks are made against openBSD’s NMEA Line Discipline to make use of timestamping and if ntpd synchronising is active, the time is passed to the ntp daemon.

static void nmea_probe_subtype(struct gps_device_t *session, unsigned int seq)

This tries to probe for the NMEA mode of several popular chipsets. Most are detected by a trigger string.

Several functions are omitted which are device specific. These are not covered here since they ARE device specific.

static gps_mask_t rtcm104_analyze(struct gps_device_t *session)

This function calls rtcm_unpack() to unpack rtcm packets into the session buffer.

The last important thing in this code is an array of addresses, one for each compiled-in driver. This address gets us to the “capabilities” table in the bottom of each driver where we find what functions a driver supports and the entry address for each function.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: Provides conversion between the ECEF and WGS84 coordinate reference systems.

static double bilinear(double x1, double y1, double x2, double y2, double x, double y, double z11, double z12, double z21, double z22)

This is a piece of mathematical manipulation only used internally in this code, so wrestle with it if you will! It’s used to perform bilinear interpolation on the grid points in the WGS4 geoidal-separation array.

double wgs84_separation(double lat, double lon)

This is used by drivers to get the separation of MSL from the WGS84 datum at a pair of Latitude/Longitude coordinates. The data is in a pre-defined table.

void ecef_to_wgs84fix(struct gps_data_t *gpsdata, double x, double y, double z, double vx, double vy, double vz)

This does exactly what it says! Coordinate system conversion from ecef to the wgs84 equivalent.

static double fix_minuz(double d)

This forces a value of -0.0 to be the same as 0.0 so that certain trig functions act in a predictable manner.

int main(int argc, char **argv)

This is a local test routine which exercises the other code for debugging purposes. It is normally shut off by an #ifdef.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Structures: The interface of the libgps library, provides data structures the library needs and that the daemon fills.

struct gps_fix_t {}

This is the fix related data gathered from the gps device. Each client session has 2, the current fix and the previous fix.

struct rtcm2_t {}

This holds the data being processed by the rtcm functions.

struct gps_data_t {}

This contains ALL the data available from the daemon; the actual fix, status flags, headings etc. Basically any good stuff that we might want goes here.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This is the main body of the daemon.

static void onsig(int sig)

This is a simple catchall routine to trap wanted signal. Simply store the signal number in a variable to advise the main loop which signal to handle.

static int daemonize(void)

Try to fork() a child process. The parent will get a return value of either -1 (on a failure to fork()) or non-zero (the child’s PID). The parent routine will use this information to either return -1 or exit with an exit code of 0 (i.e. the parent terminates, leaving the child running).

The child instance gets a return value of 0 (on an unsuccessful fork() there is no child), so this value is used as the trigger to do the following useful stuff.

First, the child tries to create a new session, returning -1 if unable to do so. If successful, it will have inherited the exiting parent’s session.

Next switch to the root directory and try to open “/dev/null”. If that succeeds, force stdin, stdout and stderr to duplicate the fd of “/dev/null”. If the fd being used for the operation is >2, release it.

Finally, set the flag which indicates the process is in the background and return a value of 0.

static void usage(void)

Simply print a big list of the invocation parameters to the default gpsd port (2947, allocated by IANA).

static int passivesock_af(char *service, char *tcp_or_udp, int qlen)

Initialise an Internet socket address structure and preload the family and address fields to accept Internet Protocol and any address.

Test to see if the incoming service and protocol exist in /etc/services. If they do, store the port number in the structure (massaging byte order as needed); if they don’t, exit with a -1.

Test to see if the protocol is listed in /etc/services, exiting with -1 if it is not.

Test if the protocol is udp or not, setting the type accordingly.

Try to open a socket with the accumulated settings, exiting with -1 if it fails.

Try to set the socket options correctly, again exiting with -1 if it fails.

Try to bind to the open socket; if it fails exit with -1 as earlier, but give a special warning if the error indicates that gpsd may already be active on the socket.

If we are using a stream type socket and we are unable to listen to the port we exit with -1.

The last possibility is a successful set of operations which is signalled by returning the socket fd number.

static int passivesocks(char *service, char *tcp_or_udp, int qlen, int socks[])

Open a passive socket for each supported address family; presently the supported families are IPV4 and IPv6. This socket will be used to listen for client command connections. Sockets are left in the final array argument, and the number successfully opened is returned.

static int filesock(char *filename)

Try and open a socket for Local (UNIX) communications in streaming mode. If the open fails, return with a -1.

If it opens, copy the incoming filename into the socket control structure, bind to the socket and try to listen on it.

Signal a failure by returning -1 and success by returning the socket fd number.

static void adjust_max_fd(int fd, bool on)

If the incoming boolean flag is active, check if the fd number passed is greater than the highest seen so far. If so, save it as the new highest value.

If the boolean is passive we can take some further action, depending if we are interested in limiting the maximum number of devices and client fds (set by compile time options).

If we are not limiting ourselves, then we check for the case when we are actually at the highest fd seen so far. In that case, scan through all fds available to the system and store the highest active fd number in our allocation set as the new highest value.

static struct subscriber_t* allocate_client(void)

Scan through all the client file descriptors, looking for one which does not have a device allocated to it.

On a match, exit early, returning this fd.

If none are available, return a NULL.

static void detach_client(struct subscriber_t *sub)

Close the given fd and remove it from our allocation set.

Make a call to adjust_max_fd() to housekeep the highest fd marker if needed.

Set important fields in the client’s datablock to safe values for the next re-use, then return.

static ssize_t throttled_write(struct subscriber_t *sub, char *buf, ssize_t len)

Check if we have a high enough debug level active to warrant printing out the information we are about to send to the client.

Make the actual write() call and if that was successful, return the counter value from that operation.

If we have suffered some kind of failure, try to analyse it.

On a short write, detach the client and return a 0.

Trap EAGAIN or EINTR and return a 0.

Trap EBADF or a EWOULDBLOCK where the client has not read data for more than a reasonable amount of time and generate a suitable report.

For all other errors, generate a general error. In these last several cases, call detach_cient().

Finally, return the status (-1 in this case).

static void notify_watchers(struct gps_device_t *device, const char *sentence, ...)

For every possible subscriber, check if the subscriber is in watcher mode and is interested in the gps device indicated in the calling parameter gps_device_t.

If so, send the data via a call to throttled_write().

static struct gps_device_t *find_device(const char *device_name)

For every possible channel, check if the channel is allocated and if the device on the channel is the one passed to us.

If it is so, exit early and return the channel number.

If there is no match, return a NULL.

static void deactivate_device(struct gps_device_t *device)

Deactivate device, but leave it in the device pool; do not free it. This means it will be available to be watched on subsequent client opens.

bool open_device(struct gps_device_t *devp)

Try to activate the device via a call to gpsd_activate().

If this fails return “false”.

If it succeeds, add the fd to our list of active fds, housekeep the highest fd number and return “true”.

static bool add_device(const char *device_name)

Add a device to the pool of those available. If in nowait mode, open it immediately; otherwise initialize it and make it available for future watches, but don’t open it yet.

static bool awaken(struct subscriber_t *user, struct gps_device_t *device)

If the device is not initialized, attempt to open the specified device on behalf of the specified user. If you succeed and the device has an active fd, you’re done. If it does not, make a call to gpsd_activate().

If this fails, return “false”, if not, add the fd to our list of active fds and housekeep the highest fd.

Check if the user is in watcher mode but not tied to a specific device.

static char *snarfline(char *p, char **out)

Copy the input line into a new buffer stopping at the first non-printable or whitespace character.

static bool privileged_user(struct gps_device_t *device)

Scan all subscribers and count all who are connected to the device. If only the one user is connected, return “true”, otherwise return “false”.

static void handle_request(struct subscriber_t* sub, char *buf, const char **after, char *reply, size_t replylen)

Perform a single GPSD JSON command. Accept the command response into a reply buffer, and update the after pointer to point just after the parsed JSON object.

+static int handle_gpsd_request(struct subscriber_t *sub, const char *buf) +

Parse multiple GPSD JSON commands out of a buffer and perform each. Ship all responses back to the user via throttled_write().

static void handle_control(int sfd, char *buf)

This code is similar in function to handle_gpsd_request() in that it parses user input. It expects the commands to be one per line and despatches them according to the leading character, which is limited to one of '-', '+' or '!'.

In the first case, the body of the command is assumed to be a device to remove from the search list. If found, it is removed, any clients are advised and “OK” is written to the calling socket fd. If the device is not found “ERROR” is written to the calling socket fd.

In the second case, the body of the command is assumed to be a device to be used by the daemon. If the device is already known, or does not respond to open_device(), “ERROR” is written to the calling socket fd, otherwise “OK” is written.

In the third case, the command is assumed to be a device-specific control string in the form “!device_name=control_string”. If the string is ill-formed or the device is not found “ERROR” is written to the calling socket fd. If all is well, the control string is written to the device and “OK” is written to the calling socket fd.

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

If the 1PPS function is compiled in, initialise the local mutex structure for use by the program.

A while() loop reads in any command line arguments which are options and handles the options. Most set an internal variable to control action when running, either to a fixed value or to the associated option’s parameter.

Carry out a series of calls to routines to set things up ready for the main task (e.g. opening a control socket if one is needed). We also take care of tasks such as daemonizing when appropriate. The last piece of preparation is to set the permissions of the default devices correctly if we are daemonizing and are presently running as root.

Switch to the compiled in user name (typically “nobody”) and the group used by the tty devices.

Now we clear important data for all the records in the subscriber list.

Use setjmp() to prepare things for when the daemon terminates.

Clear the semaphore variable which will contain the signal number if one arrives and set some important signals so they are trapped by the stub handler in onsig().

Add the command and RTCM sockets (if active) to the list of active fds, housekeeping the highest fd number and pre-clear the list of control fds.

Process the remaining parameter on the command line which should be the device name and try to open the specified device.

Enter the main execution loop, a while() loop which terminates if a signal sets the semaphore variable. What follows will repeat over and over until an external termination happens.

First we make a working copy of the active fds and then we make a time-limited (1 second time limit) call to select() using the working copy of the fds. This means that when the select() returns, we will either have returned on timeout or because some fd became ready to read.

First we check if any new clients have come active and (if we have resources) allocate a subscriber slot to it, doing housekeeping such as adding it to the main list of active fds and removing it from the local copy of the list. If RTCM support is compiled in, the last operation is repeated for any new RTCM client. The operation is then repeated for any new control socket clients.

If we are expecting DGPS reports, make a call to netgnss_poll() and if there are no ready reports, clear the fd from the main and local active fd lists.

Check if any of the active control sockets has sent one or more commands.

For every one which has sent commands, make calls to handle_control() to process them and remove the associated fd from the main and control lists of active fds.

Poll every active gps device and send RTCM data to it (if needed), followed by reading its output (if any). If the device returns an error, disable the device. If it has gone off-line, disable the device.

If we get here, we have something to handle, so we take care of a device which we know about, but do not have a subtype for.

We send the available data to all subscribers who are connected to this device. If the data is RTCM information, pass it to all GPS devices that can accept the data.

Handle any subscribers who are in watcher mode building up an appropriate set of requests, depending on the available data and passing the requests to handle_gpsd_request().

If we care about DBUS, send the fix to the DBUS.

Note that this small section of code is presently disabled pending development of the DGNSS function. If DGNSS is available and we have a fix, we poll a DGNSS report via dgnss_autoconnect().

Loop round all clients and process active ones. We check for input from them and if the read fails, the client is released with detach_client(). If it succeeds, any data is handled via handle_rtc_request() or handle_gpsd_request().

If the transaction fails, the client is released with detach_client().

If the client has timed out with no device assigned, it is released with detach_client().

If the client has a device, but has timed out on no response (when not in raw or watcher modes) it is released with detach_client().

If we are not running in “nowait” mode, we are supposed to go idle after a timeout when there are no clients.

If a device (with a known type) has no active clients, then we can actually make it idle via gpsd_deactivate().

If we reach here, we are out of the endless while loop. We check if the signal was SIGHUP and restart the program if it was. If it is any other signal, we deallocate all channels and wrap up any devices. Finally we check for the existence of a control socket or a pid file and delete them.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Structures: Provides fundamental types and structures for the GPS daemon. Only key structures are analysed here. Simple definitions and prototype definitions are ignored.

This file is created at configure time by combining gpsd.h-head, certain configuration options from the auto-generated file gpsd_configure.h and gpsd.h-tail. This file (gpsd.h) should not be edited directly, neither should gpsd.h-head. You should only edit gpsd.h-tail as needed.

struct gps_packet_t {}

Provides the data structure used by packet_getter(). 1 per gpsd session.

struct gps_context_t {}

Provides context data like number of good fixes seen. 1 per gpsd session.

struct gps_type_t {}

This is the method table that every driver uses to communicate with the daemon. It is a table of entry points or externally visible data. 1 per driver type.

struct gps_device_t {}

This is the data belonging to a session (global state) and also private storage for the session’s associated device driver (if needed). 1 per gpsd session.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: Direct access to GPSes on serial or USB devices.

void gpsd_log(int errlevel, const struct errout_t, const char *fmt, ... )

This code is used for error reporting, but is dependant on SQUELCH_DISABLE so that embedded systems (for example) are not burdened with unnecessary noise. The first thing to check is if the error level offered is high enough to be of interest (controlled by the debug level we are running at).

If we are interested, the first step is to protect the code with a mutex if we are using the 1PPS input.

Now we build a message buffer which has a fixed header ("`gpsd: `") and the incoming data. The output buffer is prepared (load the start with a NULL) and then the input buffer is scanned, byte-by-byte, up to its terminating NULL. The scanned data is transferred on the fly to the output buffer subject to the following tests:

If the character is printable, it passes through unchanged.

If it is a space and either of the next two bytes is NULL it will also pass through unchanged.

In any other case, it is copied across as a hexadecimal string like “x09”.

The completed output buffer is then either sent to the system logger if we are in background mode (daemon) or to the stderr file if we are in foreground mode.

int gpsd_switch_driver(struct gps_device_t *session, char* typename)

Test if the function is called with the same name as the active driver.

If it is, test if the driver has a configurator function and is able to be reconfigured. A good result here will call the driver event hook with type 'driver_switch', and return a 0.

For an entry with a different driver name, scan all available drivers to see if the wanted one is there. An unmatched name exits, returning 0.

If we got a match, get the baudrate for the device with gpsd_assert_sync(), probe for the subtype if we have one.

If the device has a configurator and is reconfigurable, trigger the configurator.

Return a 1 to indicate a device switch.

void gpsd_init(struct gps_device_t *session, struct gps_context_t *context, const char *device)

Copy the device name to the session data structure, initialise important data fields and call gpsd_tty_init(), gpsd_zero_satellites() and packet_reset().

void gpsd_deactivate(struct gps_device_t *session)

All actions below, except the last one are conditional on the ntpd interface being compiled in.

Release the ntpd resources, including the 1PPS resources if they are active.

If the device has a revert function, trigger it.

If it has an NMEA mode switcher, invoke it.

If it has a wrapup routine, invoke it.

Finally, close the device.

static void *gpsd_ppsmonitor(void *arg)

An ioctl() call is made which returns either 0 if the status of the port changed, or an error.

If we got a change, we read the modem control bits and extract the 1PPS information.

We check the returned value and see if it has changed recently. A counter of 10 unchanged events will disable further testing.

If we are still hanging in there, we now see if we already have had more than 4 good fixes, otherwise we can’t trust the 1PPS.

We then finally test the pulse duration. If it is either a genuine 1PPS or a 2Hz square wave, we call ntpshm_pps().

Short or long PPS pulses are dropped with an error report.

int gpsd_activate(struct gps_device_t *session, bool reconfigurable)

If the devicename matches an NTRIP or DGNSS URI, hand off to special code for opening a socket to that source over the network.

Try and open the device, returning -1 if we fail.

Probe all possible drivers to see if one recognises the device.

Set some fundamental data to a clean value.

Handle the initialisation of NTP and 1PPS functions if needed.

If we did succeed in probing some device type, try and get the subtype.

If we need to do so, we now configure the device.

Finally, signal success by returning the file descriptor given by the device open call.

void gpsd_error_model(struct gps_device_t *session, struct gps_fix_t *fix, struct gps_fix_t *oldfix)

Check we have a 2D fix (or better) and if the gps didn’t provide an eph value, use the HDOP to calculate one or fail to NAN.

Do the same with epv/VDOP if we have a 3D or better fix.

Do the same with epe/PDOP.

Consider speed error; check if we have two fixes with differing timestamps and use their times and eph values to calculate the speed.

If we have two valid 3D fixes, we can calculate the climb/sink rate.

Finally, just before exiting, save this fix as the old fix for the next comparison round.

gps_mask_t gpsd_poll(struct gps_device_t *session)

Make a call to gps_clear_fix() to prepare the newdata structure to receive data from an incoming packet.

Check if we know the device type. If we do, stash the count of of characters we are able to get from it.

If the read has given a full packet, we can call the subtype probing method, if the device supports it.

If we don’t know the device type, try and figure out what it is, exiting if we can’t.

Make some checks if the device is offline or the packet is incomplete, using the stashed count of characters and the full packet indicator.

If a full packet is available, we try to get the fix data and update the main data structure. We also compute the DOPs so we can fill them in if they are not included in the gps device output.

Mopst of the possible driver events are called from somewhere in here.

void gpsd_wrap(struct gps_device_t *session)

Simple call to gpsd_deactivate(session).

void gpsd_zero_satellites(struct gps_data_t *out)

Zero the status data for all satellites.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: Code shared between low-level and high-level interfaces. Most of them do exactly what they say on the tin.

void gps_clear_fix(struct gps_fix_t *fixp)

Stuff a fix structure with recognizable out-of-band values.

unsigned int gps_valid_fields(struct gps_fix_t *fixp)

Clears certain status flags, depending if the associated fields are invalid.

time_t mkgmtime(register struct tm *t)

Convert a struct tm {} (see time.h) to seconds since UNIX Epoch.

timespec_t iso8601_to_timespec(char *isotime)

Convert an ISO8601 UTC string (like 2006-01-25T13:15:22.9Z) to a timespec_t (time_t,long).

char *timespec_to_iso8601(timespec_t ts, char isotime[], int len)

Convert a struct timepsec into an ISO8601 UTC string (like 2006-01-25T13:15:22.9Z).

static double CalcRad(double lat)

Return the earth’s radius of curvature in meters at specified latitude.

double earth_distance(double lat1, double lon1, double lat2, double lon2)

Return the distance in meters between two points specified in degrees.

static int invert(double mat[4][4], double inverse[4][4])

Try and invert an array. If it is possible, fill the inverted array and return a 1. Return a 0 if it is not possible.

gps_mask_t dop(struct gps_data_t *gpsdata)

Take a gps data structure and try to calculate the DOP values from the other data in it. Return 0 if it is not possible or update the appropriate DOP fields in the structure and set a mask which tells which fields were updated.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This contains low level handling for the data transmission format of the satellite downlink and the RTCM2 radio data stream. ESR comments “You are not expected to understand any of this.”. Let’s try anyhow.

static unsigned char parity_array[]

An array of parity values to allow fast lookup instead of calculation.

static unsigned int reverse_bits[]

An array of conversion values to allow fast lookup instead of calculation.

unsigned int isgps_parity(isgps30bits_t th)

Calculate the ISGPS parity for the incoming 30 bit word. This involves lots of masking (with preset values or values searched through parity_array[]) and shifting

void isgps_init(struct gps_packet_t *session)

Initialise the ISGPS data for the current session.

enum isgpsstat_t isgps_decode(struct gps_packet_t *session, bool (*preamble_match)(isgps30bits_t *), bool (*length_check)(struct gps_packet_t *), size_t maxlen, unsigned int c)

Check the validity of the ISGPS word tag (incoming data unsigned int c) and exit with an error if it is not valid.

If the session has not already locked with the datastream, loop through the data and try and sync up. If sync is achieved, exit early with an indication of sync, or fall through still marked unsynced.

If the session is already locked to the datastream, read the data into the session buffer. All the while, take care that the datastream is not corrupted. Any error will result in an early return with an appropriate error code. At the end of this section, return the status of the operations.

Finally, if the above two sections failed, exit with an error indicating that lock was not achieved.

void isgps_output_magnavox(isgps30bits_t *ip, unsigned int len, FILE *fp)

Ship an IS-GPS-200 message to standard output in Magnavox format.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: The client interface library for the gpsd daemon.

struct gps_data_t *gps_open(const char *host, const char *port)

Open a connection to a gps daemon.

Try to get a gps_data_t structure. If it fails, return a NULL.

Test for a specified host and/or port number, using defaults if nothing is specified in the command line invocation.

Try to connect to the now defined socket; on error, release the resources and return NULL. On success, initialise an internal struct gps_data_t and return the pointer to it.

struct gps_data_t *gps_open_r(const char *host, const char *port, struct gps_data_t *gpsdata)

Variant of gps_open() for when re-entrancy is required. Initializes storage passed by the caller.

int gps_close(struct gps_data_t *gpsdata)

Close the fd associated with the gps_data_t structure and stash the result.

If there is a device id stored, NULL it and the associated path.

If the device list has any data in it, free this and mark the number of available devices as invalid.

Free the gps_data_t structure itself and return the stashed close() result.

static void gps_unpack(char *buf, struct gps_data_t *gpsdata)

Keep looping through the data in buf for JSON objects. Each time one is found, analyze it and unpack it into the struct pointed at by gpsdata, setting the gpsdata→set mask appropriately to indicate which data is valid

This function is intended for GPSD internal use only and should not be considered a stable part oof the API.

int gps_poll(struct gps_data_t *gpsdata)

Poll the daemon and if there is no data or an error, return -1.

If there is something to read, clear the buffer, note the time as the received data time and the online time, then unpack the data.

If profiling is active, use the received data time, the fix time and the present time to calculate the decode time and the client receipt time.

int gps_send(struct gps_data_t *gpsdata, const char *fmt, ... )

Gather up the instructions to a gpsd instance for information to return and write them to the device.

If the write fails, return -1. This entry point is deprecated in favor of gps_stream()

bool gps_waiting(struct gps_data_t *gpsdata)

Return a boolean indicating whether input data is waiting on the daemon socket. Does not block.

int gps_stream(struct gps_data_t *gpsdata, unsigned int, void *d)

Set watch and policy flags. This evaluates the flag mask passed as the second argument, composes a corresponding command in the GPSD write protocol, and ships it to the daemon.

The following three functions are guarded by an #ifdef so they only compile if it is required to be able to test the library and set up a command line exerciser interface.

static void data_dump(struct gps_data_t *collect, time_t now)

A data dumper used when debugging. It outputs data according to the command line input data.

static void dumpline(struct gps_data_t *ud UNUSED, char *buf, size_t ulen UNUSED, int level UNUSED)

A simple call to UNIX puts().

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

A simple command line parser and endless loop to exercise the daemon when debugging.

Notes based mostly on code as of Mon Apr 5 21:38:06 2010 -040.


Functions: The client interface library for the gpsd daemon.

char *deg_to_str(enum deg_str_type type, double f)

Convert the absolute value of double degrees to a static string and return a pointer to it.

Makes a simple check on invalid degree values (not more than 360) and returns "nan" on error.

For valid values, it generates the appropriate string according to the string type enumeration: dd, ddmm or ddmmss.

Warning: not thread safe.

+char *deg_to_str2(enum deg_str_type type, double f, char *buf, unsigned int buf_size, const char *suffix_pos, const char *suffix_neg) +

Convert the absolute value of double degrees to a string and place in the buffer "buf". Return a pointer to the buffer. "buf_size" is the size of the buffer.

Makes a simple check on invalid degree values (not more than 360) and returns "nan" on error.

For valid values, it generates the appropriate string according to the string type enumeration: dd, ddmm or ddmmss. If the degrees are positive, append "suffix_pos", else append "suffix_neg".

enum unit gpsd_units(void)

Simple check of the environment to determine what units are required. If all else fails, use compiled in units.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: General string output routine for any module to use.

void gpsd_log(int errlevel, struct gpsd_errout_t, const char *fmt, ... )

Provides the outputting of strings to stderr or through a specified hook function in the errout structure for anyone who needs it. Used extensively by many functions to log errors or progress. Responds according to the errlevel argument, which is filled by the DEBUG command line argument, so that the verbosity of output is conditional on the user’s wishes at runtime.

Notes based on code as of Sat Mar 7 10:11:55 EST 2015


C++ class wrapper for the libgps C binding.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Headers for libgpsmm.cpp.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This file provides the interface to Differential GNSS (Global Navigation Satellite Systems) services.

bool netgnss_url(char *name)

Check if a URL is valid for GNSS/DGPS service.

int netgnss_open(struct gps_context_t *context, char *dgnss_service)

Try to open a connection to the nominated service. If the service cannot be opened, the return is -1. The supported services are dgpsip (differential corrections via IP) and ntrip (differential corrections in http form).

int netgnss_poll(struct gps_context_t *context)

Try to poll the GNSS service for a correction report. If no socket is active, simply return 0. If a socket is active, on successful read it stores the current timestamp and the report in the context buffer then returns 0. If no data is ready or an error (except EAGAIN) occurs, drop the connection and return -1.

void netgnss_report(struct gps_device_t *session)

Call the dgpsip_report() or ntrip_report() function if either is active.

void netgnss_autoconnect(struct gps_context_t *context, double lat, double lon)

Call the autoconnect() function in dgpsip.c

void rtcm_relay(struct gps_device_t *session)

If there is a DGNSS connection report in the context buffer, pass it to the caller. If the transaction fails, generate an error log, otherwise, update the session timestamp.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This file provides the actual services for dgpsip corrections to dgnss.c.

socket_t dgpsip_open(struct gps_context_t *context, const char *dgpsserver)

Tries to open a connection to a dgpsip server. Returns either -1 on failure or the socket number on success. On success, it initializes the connection, sends a “HELO” to the server and stores the type of dgnss service in the context data.

void dgpsip_report(struct gps_device_t *session)

If we have not contacted the server and we have at received least 10 fixes, send a report to the server and set the flag, so we don’t do it again.

static int srvcmp(const void *s, const void *t)

Compare the distance of two servers from our location. Return -ve, 0 or ve depending if server `+s` is nearer, the same distance away or further away than server t.

void dgpsip_autoconnect(struct gps_context_t *context, double lat, double lon, const char *serverlist)

This function takes a list of servers and tries to get a connection to the closest one relative to our location. If no servers are within 1600 km or none are in the list, the dsock value is set to -2 to lock out the function.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: Gather and dispatch DGNSS data from Ntrip broadcasters.

static char *ntrip_field_iterate(char *start, char *prev, const char *eol)

Extract individual fields from the STR data block using ';' as the separator but handling string-embedded ';' characters correctly.

static void ntrip_str_parse(char *str, size_t len, struct ntrip_stream_t *hold)

Break an STR data block down using ntrip_field_iterate() and store the relevant details in an accumulation buffer.

static int ntrip_sourcetable_parse(int fd, char *buf, ssize_t blen, const char *stream, struct ntrip_stream_t *keep)

Parse a block of ntrip data by passing the STR data to ntrip_str_parse().

Return a 0 on success or -1 on any of the many possible failures.

static int ntrip_stream_probe(const char *caster, const char *port, const char *stream, struct ntrip_stream_t *keep)

Send a request to the broadcaster for a block of correction data.

If successful, return the result of parsing the table. On error return -1.

static int ntrip_auth_encode(const struct ntrip_stream_t *stream, const char *auth, char buf[], size_t size)

Check the authorisation of the stream and return 0 if there is none or it is basic. Return -1 in all other cases.

static int ntrip_stream_open(const char *caster, const char *port, const char *auth, struct gps_context_t *context, struct ntrip_stream_t *stream)

Check the authorisation of the stream and if successful, try to connect.

On connection, try to read data and test for various errors.

On success, return the socket number, on error return -1.

int ntrip_open(struct gps_context_t *context, char *caster)

Try to establish a connection to the ntrip server. On success, return the socket number. Signal error with a return of -1.

void ntrip_report(struct gps_device_t *session)

If we have not contacted the server already and we have at least 10 fixes, send a report to the server and set the flag, so we don’t do it again.

Notes based on codec as of Tue Apr 6 10:17:55 2010 -0400.


Functions: This provides socket connectivity.

int netlib_connectsock(const char *host, const char *service, const char *protocol)

This attempts to connect the to nominated service on the nominated host using the nominated protocol. On success, the return value is the socket number. On error, an appropriate system defined error code is returned.

char *sock2ip(int fd)

This makes a call to getpeername using the supplied fd. On success, the returned string is the ip address in dotted notation. On error, "<unknown>" is returned.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This puts time information into a shared memory segment for the ntp daemon to use.

static struct shmTime *getShmTime(int unit)

This requests a block of shared memory to communicate to the ntp daemon. It returns a pointer to the memory on success or NULL on failure.

void ntpshm_init(struct gps_context_t *context, bool enablepps)

Attaches all ntp shared memory segments, flagging the availability of the NMEA and 1pps capabilities as appropriate.

int ntpshm_alloc(struct gps_context_t *context)

This tries to allocate a free ntp shared memory segment.

If one is available, initialise it for use and flag it as in use.

Returns the segment number on success or -1 on failure.

bool ntpshm_free(struct gps_context_t *context, int segment)

This tries to release a previously allocated ntp shared memory segment. Indicates the outcome by returning true (success) or false.

int ntpshm_put(struct gps_device_t *session, double fixtime)

If the shared memory index allocated to the session is valid and the shmTime structure of the session is valid, store the time of the fix.

Success is indicated by a return value of 1 and errors are indicated by a return value of 0.

int ntpshm_pps(struct gps_device_t *session, struct timeval *tv)

This code is only available if the 1PPS function is compiled in.

The shared memory structures are checked for validity and if not valid, a 0 is returned.

The time received is then checked to be within 100 milliseconds of the PC’s internal time and if not, a -1 is returned.

The time received is then checked to be within 500 milliseconds of the second boundary and if not, the shared memory structure is advised that lock is lost and a -1 is returned.

If good, the time is stored in the shared memory and 1 is returned.

Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.


Functions: The packet-sniffing engine for reading data packets from GPS devices.

static void nextstate(struct gps_packet_t *lexer, unsigned char c)

This is the main packet-sniffer loop. It scans the character against the definitions of all the packet structures known to gpsd and, if possible, sets a new packet state.

If the state is “xxx_RECOGNIZED”, the packet_parse routine will dispatch the packet to the appropriate driver.

static void packet_accept(struct gps_packet_t *lexer, int packet_type)

This shifts a packet that has been recognized into the output buffer, provided it is not bigger than the buffer.

static void packet_discard(struct gps_packet_t *lexer)

This clears a handled packet out of the input buffer.

static void character_discard(struct gps_packet_t *lexer)

This is called if the nextstate() function returns “GROUND_STATE”.

In this case the character does not match any pattern, so to discard it, the input buffer is shifted back by one character to overwrite the “bad” character.

ssize_t packet_parse(struct gps_packet_t *lexer, size_t fix)

Call the nextstate() function to process the available data and set the recognition state correctly.

When a packet is matched to a driver, call packet_accept() and packet_discard() to handle the packet. If it is not matched, call packet_discard() and set the state to “GROUND_STATE”

Return the number of characters handled.

ssize_t packet_get(int fd, struct gps_packet_t *lexer)

Reads raw data from the input port.

Returns the number of characters read (0 or more) or BAD_PACKET if there was an error in reading.

Errors EAGAIN and EINTR are not classed as failures and cause a return of 0.

In case of a good read of more than 0 characters, the return value is the output from a call to packet_parse().

void packet_reset(struct gps_packet_t *lexer)

This simply resets the entire packet state machine to the ground state.

void packet_nit(struct gps_packet_t *lexer)

Zeros some counters, then resets the entire packet state machine to the ground state.

void packet_pushback(struct gps_packet_t *lexer)

This pushes back the last packet from the output buffer to the input buffer, provided doing so would not overflow the input buffer.

Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.


This is a file generated by the “make” process from packet_states.h. It takes all the state names in that file and surrounds them with speech marks. These values are read into a table of packet names, state_table[], in packet.c.

Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.


This is a list of every possible state generated by every driver. The inclusion of each driver’s states is controlled by #ifdef blocks, so we only include the states for drivers selected for compilation. These form an enumeration list in packet.c which indexes the state_table[] array mentioned in packet_names.h.

Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.


Functions: Generate pseudo-NMEA corresponding to binary packet reports

static double degtodm(double a)

Converts a fractional degree value (like 125.3567) into an NMEA field as dddmm.xxx (like 12521.402).

int gpsd_position_fix_dump(struct gps_device_t *session, /char bufp[], size_t len)

If possible, create a $GPGGA message (full time, position and fix data) from the fix data, taking care about the validity of subsidiary fields like HDOP and altitude.

static void gpsd_transit_fix_dump(struct gps_device_t *session, char bufp[], size_t len)

Create a $GPRMC message (minimum navigation data) from the fix data.

static void gpsd_binary_fix_dump(struct gps_device_t *session, char bufp[], size_t len)

Make a call to gpsd_position_fix_dump() and gpsd_transit_fix_dump().

static void gpsd_binary_satellite_dump(struct gps_device_t *session, char bufp[], size_t len)

Create one or more $GPGSV messages from the status data of all satellites we know about.

static void gpsd_binary_quality_dump(struct gps_device_t *session, char bufp[], size_t len)

Create a $GPGSA message taking care that if any xDOP is not valid, we substitute a “0.0” value.

If any of the error estimates is available, generate a $PGRME message again taking care about possibly invalid values.

static void gpsd_binary_dump(struct gps_device_t *session, char bufp[], size_t len)

Dump such binary data as we have available; a fix, a quality value and a satellite status block. Each is enabled by the appropriate status bits in the session mask variable.

Notes based on code as of Mon Apr 5 21:38:06 2010 -0400.


Functions: This provides serial port handling services to the daemon. It handles the tricky task of scanning for and changing port parameters, especially baudrate.

void gpsd_tty_init(struct gps_device_t *session)

To be called on allocating a device. Mark GPS fd closed and its baud rate unknown.

If we are supporting ntpd shared memory segments, ensure they are initially unused.

void cfmakeraw(struct termios *termios_p)

Workaround for systems (Cygwin, Solaris), which are missing cfmakeraw(). It is pasted from man page and added in serial.c arbitrarily.

speed_t gpsd_get_speed(struct termios* ttyctl)

Calls cfgetospeed() and returns the baud rate, if known. Default otherwise is 115200.

bool gpsd_set_raw(struct gps_device_t *session)

Tries to set port to raw mode and returns success or not.

void gpsd_set_speed(struct gps_device_t *session, speed_t speed, unsigned char parity, unsigned int stopbits)

Sets the speed, parity and stopbits.

Lots of black magic fiddling goes on to ensure the port is flushed on the baud rate change and wakeup strings are fired off just in case the device needs prodding into life.


Prior to exit, a call is made to packet_reset() to ensure the packet state machine is initialised.

int gpsd_open(struct gps_device_t *session)

Test the device and flag it as R/W if it is a character device, or R/O if it isn’t.

Try to open it in non-blocking and no-control mode.

If that fails, try again, adding read-only mode. If that also fails, exit with an error.

On no error, force the saved baudrate if we have a fixed port speed (typically embedded devices). Check if we have a saved baudrate and if so, activate it.

Preset the packet type to BAD_PACKET.

Check if the device we have opened is a tty. If it is a tty, read the original terminal parameters.

Exit with an error code -1 on failure to do so.

Save the old parameters, set important control flags, then set the speed.

Finally, return the allocated fd.

bool gpsd_write(struct gps_device_t *session, void const *buf, size_t len)

If the device is read-only, simply return 0.

If not, try to write len characters to the device. Waiting until all data has been sent.

Return the number of bytes written.

bool gpsd_next_hunt_setting(struct gps_device_t *session)

Check if we have had SNIFF_RETRIES attempts at current baudrate. If not, return “true”.

If we have exceeded the limit, reset the counter and see if there are any more rates to try at.

If no (fixed baudrate or all attempts exhausted), return “false”, otherwise, set the next speed and return “true”.

void gpsd_assert_sync(struct gps_device_t *session)

To be called when we want to register that we’ve synced with a device. We’ve achieved first sync with the device. Remember the baudrate so we can try it first next time this device is opened.

void gpsd_close(struct gps_device_t *session)

If there is an active fd, check if it is a tty device. If it is, force the baudrate to 0 (should terminate the connection and de-assert control lines).

Set the HUPCL flag in the original data, write the old data to the port, close the fd and clear that fd number from the session data.

Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.


Functions: This code interprets satellite subframe data.

void gpsd_interpret_subframe(struct gps_device_t *session,
unsigned int gnssID,
unsigned int prn,
unsigned int words[]
unsigned int numwords)

Extracts leap-second from subframe data.

Notes based on code as of Tue Apr 6 10:17:55 2010 -0400. With a few updates.



November 2007

Revision History






25 January 2020


Converted to AsciiDoc


25 Jan 2011


The raw_hook member is gone.


6 April 2010


Updated to match current reality, as far as it goes. Some newer parts of the codebase, notably the JSON parsing machinery and AIS support, aren’t documented.


14 November 2007


Updated to version svn revision 4420


This file is Copyright 2007 by the GPSD project
SPDX-License-Identifier: BSD-2-clause