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umockdev mocks Linux devices for creating integration tests for hardware related libraries and programs. It also provides tools to record the properties and behaviour of particular devices, and to run a program or test suite under a test bed with the previously recorded devices loaded. This allows developers of software like gphoto or libmtp to receive these records in bug reports and recreate the problem on their system without having access to the affected hardware.

The UMockdevTestbed class builds a temporary sandbox for mock devices. You can add a number of devices including arbitrary sysfs attributes and udev properties, and then run your software in that test bed that is independent of the actual hardware it is running on. With this you can simulate particular hardware in virtual environments up to some degree, without needing any particular privileges or disturbing the whole system.

You can use this from the command line, and a wide range of programming languages (C, Vala, and everything which supports gobject-introspection, such as JavaScript or Python).

Right now umockdev supports the following features:

  • Emulation of arbitrary sysfs devices, attributes, and udev properties.

  • Synthesis of arbitrary uevents.

  • Emulation of /dev device nodes; they look just like the original real device (i. e. stat() delivers a block/char device with appropriate major/minor), but are backed by a PTY (for terminal devices) or a plain file (for everything else) by default. You can manually create other kinds of fake devices in your tests, too.

  • Recording and replay of read()s/recv()s and write()s/send()s from/to a character device (e. g. for emulating modems) or an Unix socket (e. g. for Android's /dev/socket/rild). These records are called "scripts". Replay can optionally use a configurable fuzz factor in case the expected (previously recorded) script data doesn't perfectly match what is actually being sent from the tested application.

  • Replay of usbdevfs (e. g. for PtP/MTP devices). Two methods are available for flexible and pure in-order replay. The --ioctl based replay may allow interactive emulation while the pcap/usbmon based replay is purely in-order and supports control transfer replay.

  • Recording and replay of evdev (touch pads, Wacom tablets, etc.) ioctls.

  • Recording and replay of spidev ioctls and read/write commands using --ioctl for both umockdev-record and umockdev-run. This is an in-order record/replay of all SPI transfers. Similar to scripts with the difference that full duplex transfers via ioctl are supported. Timinges/errors are currently not recorded.

  • Recording and replay of evdev input events using the evemu events format. Unlike recorded evdev scripts these are architecture independent and human readable.

  • Mocking of files and directories in /proc

Other aspects and functionality will be added in the future as use cases arise.

Component overview

umockdev consists of the following parts:

  • The umockdev-record program generates text dumps (conventionally called *.umockdev) of some specified, or all of the system's devices and their sysfs attributes and udev properties. It can also record ioctls and reads/writes that a particular program sends and receives to/from a device, and store them into a text file (conventionally called *.ioctl for ioctl records, and *.script for read/write records).

  • The libumockdev library provides the UMockdevTestbed GObject class which builds sysfs and /dev testbeds, provides API to generate devices, attributes, properties, and uevents on the fly, and can load *.umockdev and *.ioctl records into them. It provides VAPI and GI bindings, so you can use it from C, Vala, and any programming language that supports introspection. This is the API that you should use for writing regression tests. You can find the API documentation in docs/reference/.

  • The libumockdev-preload library intercepts access to /sys, /dev/, /proc/, the kernel's netlink socket (for uevents) and ioctl() and re-routes them into the sandbox built by libumockdev. You don't interface with this library directly, instead you need to run your test suite or other program that uses libumockdev through the umockdev-wrapper program.

  • The umockdev-run program builds a sandbox using libumockdev, can load *.umockdev, *.ioctl, and *.script files into it, and run a program in that sandbox. I. e. it is a CLI interface to libumockdev, which is useful in the "debug a failure with a particular device" use case if you get the text dumps from a bug report. This automatically takes care of using the preload library, i. e. you don't need umockdev-wrapper with this. You cannot use this program if you need to simulate uevents or change attributes/properties on the fly; for those you need to use libumockdev directly.

Mocking /proc and /dev

When enabled, the preload library diverts access to /proc and /dev to the corresponding directories in $UMOCKDEV_DIR, aka. umockdev_testbed_get_root(). However, if a path does not exist there, it falls through the real /proc and /dev. Thus you can easily replace files like /proc/cpuinfo or add new ones without losing standard files such as /dev/null or /proc/pid/*. Currently there is no way to "remove" files from the real directories or fully control them. You can get the effect of removing a file by creating a broken symlink in the umockdev directory though.

In contrast, an UMockdevTestbed fully controls the visible /sys directory; for a program there is no (regular) way to see the real /sys, unless it circumvents the libc API.


API: Create a fake battery

Batteries, and power supplies in general, are simple devices in the sense that userspace programs such as upower only communicate with them through sysfs and uevents. No /dev nor ioctls are necessary. docs/examples/ has two example programs how to use libumockdev to create a fake battery device, change it to low charge, sending an uevent, and running upower on a local test system D-BUS in the testbed, with watching what happens with upower --monitor-detail. battery.c shows how to do that with plain GObject in C, is the equivalent program in Python that uses the GI binding.

Command line: Record and replay PtP/MTP USB devices (unordered)

With this method of record and replay a tree of dependent USB URBs is generated and replayed. The advantage is that discontinuities may occur during replay, as the replayer will always try to find the appropriate response, possibly changing the order of replay.

If you need completely in-order replay or USB control commands, then the pcap based replayer will be more appropriate.

  • Connect your digital camera, mobile phone, or other device which supports PtP or MTP, and locate it in lsusb. For example

    Bus 001 Device 012: ID 0fce:0166 Sony Ericsson Xperia Mini Pro
  • Dump the sysfs device and udev properties:

    umockdev-record /dev/bus/usb/001/012 > mobile.umockdev
  • Now record the dynamic behaviour (i. e. usbfs ioctls) of various operations. You can store multiple different operations in the same file, which will share the common communication between them. For example:

    umockdev-record --ioctl /dev/bus/usb/001/012=mobile.ioctl mtp-detect
    umockdev-record --ioctl /dev/bus/usb/001/012=mobile.ioctl mtp-emptyfolders
  • Now you can disconnect your device, and run the same operations in a mocked testbed. Please note that /dev/bus/usb/001/012 merely echoes what is in mobile.umockdev and it is independent of what is actually in the real /dev directory. You can rename that device in the generated *.umockdev files and on the command line.

    umockdev-run --device mobile.umockdev --ioctl /dev/bus/usb/001/012=mobile.ioctl mtp-detect
    umockdev-run --device mobile.umockdev --ioctl /dev/bus/usb/001/012=mobile.ioctl mtp-emptyfolders

Note that if your *.ioctl files get too large for some purpose, you can xz-compress them.

Command line: Record and replay USB devices using usbmon pcap captures

This method of USB replay is a pure in-order replay. This has the advantage that timeouts will be correctly emulated rather than causing discontinuities in the replayer and possibly incorrect device state emulation. pcap currently also has the advantage of correctly replaying USB control transfers.

  • Connect your device and locate it in lsusb. For example

    Bus 001 Device 004: ID 06cb:00bd Synaptics, Inc. Prometheus MIS Touch Fingerprint Reader
  • Dump the sysfs device and udev properties:

    umockdev-record /dev/bus/usb/001/004 > fingerprint.umockdev
  • Use wireshark to record the bus in question (bus 001, usbmon1). By default this will record all devices. To minimize the size of the capture, you may use a filter to only record/save communication to the device in question.

    After starting the capture, run the command to capture the required interactions. For example the synaptics/ test script from libfprint.

  • Now you can disconnect your device, and run the same operations in a mocked testbed. To do so, load the sysfs device and udev properties. Then specify the --pcap option with the corresponding sysfs path of the device. Doing so will create the appropriate usbdevfs device node.

    Note that you need to specify the sysfs path from the device description.

    umockdev-run --device fingerprint.umockdev --pcap /sys/devices/pci0000:00/0000:00:14.0/usb1/1-9=fingerprint.pcapng synaptics/

Command line: Record and replay tty devices

This example records the behaviour of an USB 3G stick with ModemManager.

  • Dump the sysfs device and udev properties of the relevant tty devices (a Huawei stick creates ttyUSB{0,1,2}):

    umockdev-record /dev/ttyUSB* > huawei.umockdev
  • Record the communication that goes on between ModemManager and the 3G stick into a file ("script"):

    umockdev-record -s /dev/ttyUSB0=0.script -s /dev/ttyUSB1=1.script \
        -s /dev/ttyUSB2=2.script -- modem-manager --debug

    (The --debug option for ModemManager is not necessary, but it's nice to see what's going on). Note that you should shut down the running system instance for that, or run this on a private D-BUS.

  • Now you can disconnect the stick (not necessary, just to clearly prove that the following does not actually talk to the stick), and replay in a test bed:

    umockdev-run -d huawei.umockdev -s /dev/ttyUSB0=0.script -s /dev/ttyUSB1=1.script \
         -s /dev/ttyUSB2=2.script -- modem-manager --debug

Record and replay an Unix socket

This example records the behaviour of ofonod when talking to Android's rild through /dev/socket/rild.

  • Record the communication:

    sudo pkill ofonod
    sudo umockdev-record -s /dev/socket/rild=phonecall.script -- ofonod -n -d

    Now make a call, send a SMS, or anything else you want to replay later. Press Control-C when you are done.

  • ofonod's messages that get sent to rild are not 100% predictable, some bytes in some messages are always different. Edit the recorded rild.script to set a fuzz factor of 5, i. e. at most 5% of the bytes in a message are allowed to be different from the recorded ones. Insert a line

    f 5 -

    at the top of the file. See docs/script-format.txt for details.

  • Now you can run ofonod in a testbed with the mocked rild:

    sudo pkill ofonod
    sudo umockdev-run -u /dev/socket/rild=phonecall.script -- ofonod -n -d

    Note that you don't need to record device properties or specify -d/--device for unix sockets, since their path is all that is to be known about them.

    With the API, you would do this with a call like

    umockdev_testbed_load_socket_script(testbed, "/dev/socket/rild",
                                        SOCK_STREAM, "phonecall.script", &error);

    Note that for Unix sockets you cannot use umockdev_testbed_get_dev_fd(), you can only use scripts with them. If you need full control in your test suite, you can of course create the socket in <testbed root>/<socket path> and handle the bind/accept/communication yourself.

Record and replay input devices

For those the "evemu" format is preferable as it is platform independent (scripts depend on the architecture endianess and size of time_t) and human readable. ioctls need to be recorded as well, as they specify the input device's capability beyond what it is already exposed in sysfs, particularly for multi-touch devices.

This uses the evtest program, but you can use anything which listens to evdev devices.

  • Record the static device data, ioctls, and some events. This needs to run as root:

    sudo umockdev-record /dev/input/event3 > mouse.umockdev
    sudo umockdev-record -i /dev/input/event3=mouse.ioctl \
      -e /dev/input/ -- evtest /dev/input/event3

    Now cause some events on the devices (key presses, mouse clicks, touch clicks, etc.), and stop evtest with Control-C.

  • Replay is straightforward. It does not need root privileges:

    umockdev-run -d mouse.umockdev -i /dev/input/event3=mouse.ioctl \
        -e /dev/input/ -- evtest /dev/input/event3

    Press Control-C again to stop evtest.

Command line: Mock file in /proc

By default, /proc is the standard system directory:

$ umockdev-run -- head -n2 /proc/cpuinfo
processor	: 0
vendor_id	: GenuineIntel

But you can replace files (or directories) in it by the ones in the mock dir:

$ umockdev-run -- sh -c 'mkdir $UMOCKDEV_DIR/proc;
>   echo hello > $UMOCKDEV_DIR/proc/cpuinfo;
>   cat /proc/cpuinfo'

Build, Test, Run

If you want to build umockdev from a git checkout, install the necessary build dependencies first. On a Debian based system:

sudo apt install -y meson pkg-config valac libglib2.0-dev libudev-dev libgudev-1.0-dev libpcap-dev python3-gi gobject-introspection libgirepository1.0-dev gir1.2-glib-2.0 gir1.2-gudev-1.0 gtk-doc-tools

In order to run all integration tests, install the test dependencies:

sudo apt install -y udev xserver-xorg-video-dummy xserver-xorg-input-evdev xserver-xorg-input-synaptics xinput usbutils gphoto2

umockdev uses the meson build system. Configure a build tree with desired options with

meson setup build/
cd build/

You may want to supply --prefix=/usr or similar options, see meson setup --help.

  • Build the configured build directory with meson compile.
  • Run tests against the build tree with meson test.
  • Generate a code coverage report with configuring the build tree with -Db_coverage=true and running ninja coverage-text.
  • Install into the configured prefix with sudo meson install (/usr/local by default).

If you don't want to install umockdev but use it from the build tree, run the programs with these environment variables, assuming that your current directory is the build directory:

LD_LIBRARY_PATH=`pwd` GI_TYPELIB_PATH=`pwd` ./umockdev-run ...


To debug umockdev itself and what it's doing, you can set the $UMOCKDEV_DEBUG environment variable to a list (comma or space separated) of

  • path: Redirection of paths in /sys, /dev etc. to testbed
  • netlink: Redirection of netlink socket and uevent synthesis
  • script: Script (device reads/writes) recording and replay
  • ioctl: ioctl recording and replay
  • ioctl-tree: detailed parsing and traversal of recorded ioctl trees
  • all: All debug categories


umockdev is being developed and released on

umockdev is very much demand driven. If you want to work on a new feature (such as adding support for more ioctls) or contribute a bug fix, please check out the git repository, push your changes to github, and create a pull request. Contributions are appreciated, and I will do my best to provide timely reviews.

If you find a bug in umockdev or have an idea about a new feature but don't want to implement it yourself, please file a report in the github issue tracker. Please always include the version of umockdev that you are using, and a complete runnable reproducer of the problem (i. e. the code and recorded scripts/ioctls, etc.), unless it is a feature request.


  • Copyright (C) 2012 - 2014 Canonical Ltd.
  • Copyright (C) 2017 - 2021 Martin Pitt

umockdev is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.

umockdev 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 Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program; If not, see