14 Aug 2004 3 3 The GPSD Project GPSD Documentation libgps C service library for communicating with the GPS daemon C: #include <gps.h> int gps_open char *server char * port struct gps_data_t *gpsdata int gps_send struct gps_data_t *gpsdata char *fmt... int gps_read struct gps_data_t *gpsdata bool gps_waiting struct gps_data_t *gpsdata void gps_close struct gps_data_t *gpsdata int gps_stream struct gps_data_t *gpsdata unsigned intflags void *data char *gps_errstr int err Python: import gps session = gps.gps(host="localhost", port="2947") session.stream(flags=WATCH_JSON) for report in session: process(report) del session libgps is a service library which supports communicating with an instance of the gpsd8; link it with the linker option -lgps. Take care to conditionalize your code on the major and minor API version symbols in gps.h; ideally, force a compilation failure if GPSD_API_MAJOR_VERSION is not a version you recognize. See the GPSD project website for more information on the protocol and API changes. Calling gps_open() initializes a GPS-data structure to hold the data collected by the GPS, and returns a socket attached to gpsd1. gps_open() returns NULL on errors and is re-entrant. errno is set depending on the error returned from the the socket layer; see gps.h for values and explanations; also see gps_errstr(). The host address may be a DNS name, an IPv4 dotted quad, or an IPV6 address; the library will do the right thing for any of these. gps_close() ends the session. gps_send() writes a command to the daemon. The second argument must be a format string containing elements from the command set documented at gpsd1. It may have % elements as for sprintf3, which will be filled in from any following arguments. This function returns a -1 if there was a Unix-level write error, otherwise 0. Please read the LIMITATIONS section for additional information and cautions. See gps_stream() as a possible alternative. gps_read() accepts a response, or sequence of responses, from the daemon and interprets it as though it were a query response (the return value is as for a query). This function does a blocking read waiting for data from the daemon (but see POLL_NONBLOCK below); it returns a count of bytes read for success, -1 with errno set on a Unix-level read error, -1 with errno not set if the socket to the daemon has closed, and 0 if POLL_NONBLOCK was passed to gps_stream() and no data is yet available. gps_waiting() can be used to check whether there is data from the daemon. It returns true if there is, false on no data waiting or error condition. It does not block waiting for input. gps_stream() asks gpsd to stream the reports it has at you, to be made available when you poll. It is preferable to the older-style (pre-2.90) way of doing this, gps_query() with a "w+" argument, because it insulates your code from whether your client library and your gpsd are using old or new protocol. The second argument is a flag mask that sets various policy bits; see the list below. Calling gps_stream() more than once with different flag masks is allowed. WATCH_DISABLE Disable the reporting modes specified by the other WATCH_ flags. Cannot be used to disable POLL_NONBLOCK. WATCH_ENABLE Disable the reporting modes specified by the other WATCH_ flags. This is the default. WATCH_JSON Enable JSON reporting of data. If WATCH_ENABLE is set, and no other WATCH flags are set, this is the default. WATCH_NMEA Enable generated pseudo-NMEA reporting on binary devices. WATCH_RARE Enable reporting of binary packets in encoded hex. WATCH_RAW Enable literal passthrough of binary packets. WATCH_SCALED When reporting AIS data, scale integer quantities to floats if they have a divisor or rendering formula associated with them. WATCH_NEWSTYLE Force issuing a JSON initialization and getting new-style responses. This is the default. WATCH_OLDSTYLE Force issuing a W or R command and getting old-style responses. Warning: this flag (and the capability) will be removed in a future release. WATCH_DEVICE Restrict watching to a specified device, patch given as second argument. POLL_NONBLOCK Normally gps_read() blocks until either there is a read error or some data is received from the daemon. In this mode, gps_read() returns immediately with a value of 0 if there is no input waiting. gps_errstr() returns an ASCII string (in English) describing the error indicated by a nonzero return value from gps_open(). Consult gps.h to learn more about the data members and associated timestamps. Note that information will accumulate in the session structure over time, and the 'valid' field is not automatically zeroed by each gps_read(). It is up to the client to zero that field when appropriate and to keep an eye on the fix and sentence timestamps. The Python implementation supports the same facilities as the C library. gps_open() is replaced by the initialization of a gps session object; the other calls are methods of that object, and have the same names as the corresponding C functions. However, it is simpler just to use the sessiion object as an iterator, as in the example given below. Resources within the session object will be properly released when it is garbage-collected. Note one limitation: POLL_NONBLOCK is not yet supported in Python; use the waiting() method instead. The following is an excerpted and simplified version of the libgps interface code from cgps1. struct gps_data_t gps_data; ret = gps_open(hostName, hostPort, &gps_data); (void) gps_stream(&gps_data, WATCH_ENABLE | WATCH_JSON, NULL); /* Put this in a loop with a call to a high resolution sleep () in it. */ if (gps_waiting (&gps_data)) { errno = 0; if (gps_read (&gps_data) == -1) { ... } else { /* Display data from the GPS receiver. */ if (gps_data.set & ... } } /* When you are done... */ (void) gps_stream(&gps_data, WATCH_DISABLE, NULL); gps_close (&gps_data); In the C API, incautious use of gps_send() may lead to subtle bugs. In order to not bloat struct gps_data_t with space used by responses that are not expected to be shipped in close sequence with each other, the storage for fields associated with certain responses are combined in a union. The risky set of responses includes VERSION, DEVICELIST, RTCM2, RTCM3, and AIS; it may not be limited to that set. The logic of the daemon's watcher mode is careful to avoid dangerous sequences, but you should read and understand the layout of struct gps_data_t before using gps_send() to request any of these responses. The gps_query() supported in major versions 1 and 2 of this library has been removed. With the new streaming-oriented wire protocol behind this library, it is extremely unwise to assume that the first transmission from the daemon after a command is shipped to it will be the response to command. If you must send commands to the daemon explicitly, use gps_send() but beware that this ties your code to the GPSD wire protocol. It is not recommended. See the comment above the symbol GPSD_API_MAJOR_VERSION in gps.h for recent changes. gpsd8, gps1, libgpsd3. libgpsmm3. Eric S. Raymond <esr@thyrsus.com>, C sample code Charles Curley <charlescurley@charlescurley.com>