gpsfake - test harness for gpsd, simulating a GNSS receiver


gpsfake [OPTIONS] infile

gpsfake -h

gpsfake -V


gpsfake is a test harness for gpsd and its clients. It opens a pty (pseudo-TTY), launches a gpsd instance that thinks the slave side of the pty is its GNSS device, and repeatedly feeds the contents of one or more test logfiles through the master side to the GNSS receiver. If there are multiple logfiles, sentences from them are interleaved in the order the files are specified.

gpsfake does not require root privileges, but will run fine as root. It can be run concurrently with a production gpsd instance without causing problems, as long as you use the -P option. Running under sudo will cause minor loss of functionality.

The logfiles may contain packets in any supported format, including in particular NMEA, SiRF, TSIP, or Zodiac. Leading lines beginning with # will be treated as comments and ignored, except in the following special cases.

Thse are interpreted directly by gpsfake:

• a comment of the form #Serial: [0-9] [78][NOE][12] may be used to set serial parameters for the log - baud rate, word length, stop bits.

• a comment of the form #Transport: UDP may be used to fake a UDP source rather than the normal pty.

• a comment of the form #Transport: TCP may be used to fake a TCP source rather than the normal pty.

Thse are interpreted directly by gpsd:

• a comment of the form # Date: yyyy-mm-dd (ISO8601 date format) may be used to set the initial date for the log.

The gpsd instance is run in foreground. The thread sending fake GNSS data to the daemon is run in background.


-?, -h, --help

Print a usage message and exit.

-1, --singleshot

The logfile is interpreted once only rather than repeatedly. This option is intended to facilitate regression testing.

-b, --baton

Enable a twirling-baton progress indicator on standard error. At termination, it reports elapsed time.

-c COUNT, --cycle COUNT

Sets the delay between sentences in seconds. Fractional values of seconds are legal. The default is zero (no delay).

-d LVL, --debug LVL

Pass a -D option to the daemon: thus -D 4 is shorthand for -o="-D 4".

-g, -G, --gdb, --lldb

Use the monitor facility to run the gpsd instance within gpsfake under control of gdb or lldb, respectively. They also disable the timeout on daemon inactivity, to allow for breakpointing. If necessary, the timeout can be reenabled by a subsequent -W or --wait . If xterm and $DISPLAY are available, these options launch the debugger in a separate xterm window, to separate the debugger dialog from the program output, but otherwise run it directly. In the gdb case, -tui is used with xterm but not otherwise, since curses and program output don’t play nicely together. Although lldb lacks an equivalent option, some versions have a 'gui' command.

-i, --promptme

Single-step through logfiles. It dumps the line or packet number (and the sentence if the protocol is textual) followed by "? ". Only when the user keys Enter is the line actually fed to gpsd.

-l, --linedump

Print a line or packet number just before each sentence is fed to the daemon. If the sentence is textual (e.g. NMEA), the text is printed as well. If not, the packet will be printed in hexadecimal (except for RTCM packets, which aren’t dumped at all). This option is useful for checking that gpsfake is getting packet boundaries right.

-m PROG, --monitor PROG

Specify a monitor program (PROG) inside which the daemon should be run. This option is intended to be used with valgrind(1) , gdb(1) and similar programs.

-n, --nowait

Pass -n to the daemon to start the daemon reading the GNSS receiver without waiting for a client (equivalent to -o="-n").

-o="OPTS", --option="OPTS"

Specify options to pass to the daemon. The equal sign (=) and quotes are required so that gpsd options are not confused with gpsfake options. To start the daemon reading the GNSS receiver without waiting for a client use -o="-n" (equivalent to the -n) which passes -n to the gpsd daemon. The option -o="-D 4" passes a -D 4 to the daemon, equivalent to the using -D 4.

-p, --pipe

Sets watcher mode and dump the NMEA and GPSD notifications generated by the log to standard output. This is useful for regression testing.

-p PORT, --port PORT

Sets the daemon’s listening port to PORT.

-q, --quiet

Tell gpsfake to suppress normal progress output and thus act in a quiet manner.

-r STR, --clientinit STR

Specify an initialization command to use in pipe mode. The default is ?WATCH={"enable":true,"json":true}.

-s SPEED, --speed SPEED

Sets the baud rate for the slave tty. The default is 4800.

-S, --slow

Tells gpsfake to insert realistic delays in the test input rather than trying to stuff it through the daemon as fast as possible. This will make the test(s) run much slower, but avoids flaky failures due to machine load and possible race conditions in the pty layer.

-t, --tcp

Forces the test framework to use TCP rather than pty devices. Besides being a test of TCP source handling, this may be useful for testing from within chroot jails where access to pty devices is locked out.

-T, --sysinfo

Makes gpsfake print some system information and then exit.

-u, --udp

Forces the test framework to use UDP rather than pty devices. Besides being a test of UDP source handling, this may be useful for testing from within chroot jails where access to pty devices is locked out.

-v, --verbose

Enable verbose progress reports to stderr. Use multiple times to increase verbosity. It is mainly useful for debugging gpsfake itself.

-w SEC, --wait SEC

Set the timeout on daemon inactivity, in seconds. The default timeout is 60 seconds, and a value of 0 suppresses the timeout altogether. Note that the actual timeout is longer due to internal delays, typically by about 20 seconds.

-x, --predump

Dump packets as gpsfake gathers them. It is mainly useful for debugging gpsfake itself.

The last argument(s) must be the name of a file or files containing the data to be cycled at the device. gpsfake will print a notification each time it cycles.

Normally, gpsfake creates a pty for each logfile and passes the slave side of the device to the daemon. If the header comment in the logfile contains the string "UDP", packets are instead shipped via UDP port 5000 to the address You can monitor the packet with tcpdump this way:

tcpdump -s0 -n -A -i lo udp and port 5000


Certain magic comments in test load headers can change the conditions of the test. These are:


May contain a serial-port setting such as 4800 7N2 - baud rate followed by 7 or 8 for byte length, N or O or E for parity and 1 or 2 for stop bits. The test is run with those settings on the slave port that the daemon sees.


Values 'TCP' and 'UDP' force the use of TCP and UDP feeds respectively (the default is a pty).


Must be followed by two whitespace-separated fields, a delimiter character and a numeric delay in seconds. Instead of being broken up by packet boundaries, the test load is split on the delimiters. The delay is performed after each feed. Can be useful for imposing write boundaries in the middle of packets.


gpsfake is a trivial wrapper around a Python module, also named gpsfake, that can be used to fully script sessions involving a gpsd instance, any number of client sessions, and any number of fake GPSes feeding the daemon instance with data from specified sentence logs.

Source and embedded documentation for this module is shipped with the gpsd development tools. You can use it to torture test either gpsd itself or any gpsd-aware client application.

Logfiles for the use with gpsfake can be retrieved using gpspipe, gpscat, or cgps from the gpsd distribution, or any other application which is able to create a compatible output.


For unknown reasons gpsfake may sometimes time out and fail. Set the WRITE_PAD environment value to a larger value to avoid this issue. A starting point might be "WRITE_PAD = 0.005". Values as large os 0.200 may be required.

If gpsfake exits with "Cannot execute gpsd: executable not found." the environment variable GPSD_HOME can be set to the path where gpsd can be found. (instead of adding that folder to the PATH environment variable



on success.


on failure


gpsd(8), gps(1), gpspipe(1), gpscat(1), cgps(1), tcpdump(1), gdb(1), lldb(1), valgrind(1)


Project web site: <>;


This file is Copyright 2013 by the GPSD project
SPDX-License-Identifier: BSD-2-clause


Eric S. Raymond