Release Notes


16 November 2017

This release contains a large number of minor enhancements and bug fixes. Most significant are those related to alternative environments, such as Raspberry Pi systems.

Those running ddcutil on a 32 bit system, either i386 or Raspberry Pi, should upgrade to address a segfault in the environment command. Otherwise, there is no need to upgrade to this release unless you are experiencing problems.

Changes to facilitate scripting

  • Do not output a warning message regarding undetected VCP version if message level is --terse.
  • "Display not found" message now directed to stderr instead of stdout.


  • Changes to meet Debian distribution standards. Clean up debian/rules and other files in the debian directory.
  • Add a changelog file, normally installed in file /usr/share/doc/ddcutil/Changelog.
  • Rename NEWS file to
  • Add configure option --enable-x11/--disable-x11. The default is --enable-x11. If --disable-x11 is specified, ddcutil is built without X11 related code, which is used only for diagnostics within the environment command.
  • Directory package has been removed from the ddcutil git tree and is no longer included in the tarball. This more cleanly separates "downstream" from "upstream" activities. Packagers for Linux distributions should contact if this creates problems.

Command environment enhancements

The environment command is extensively revised. The increasingly large collection of tests has been consolidated. In particular:

  • Tests are now sensitive to machine architecture. Performs alternative tests on arm systems, inlcuding the Raspberry Pi.
  • Added a consolidated collection of suggestions at the end that suggest ways for the user to address configuration issues.
  • Fixed a bug that caused a segfault in 32 bit versions of ddcutil.
  • Use of shell commands has largely been replaced with API calls, addressing some obscure shell errors.


The required glib-2.0 version has been increased from 2.16 to 2.32 reflecting the use of glib thread functions. Consequently, ddcutil no longer builds on some older Linux distributions.

Bug fixes

  • Fix a logic error in the VCP version comparison function vcp_version_le(). This bug affected only audio related VCP feature codes x8f, x91, and x93. It also affected experimental command watch which listens for monitor changes not made by ddcutil, i.e. by pressing buttons on the monitor.
  • Properly handle the failure case where a display is detected on an I2C bus (EDID is read) but DDC communication fails, and the user specifies the display on the command line using the I2C bus number (option --bus).
  • Fix the cause of a segfault when the loadvcp command reads a user modified VCP file. The command now fails gracefully if none of MFG_ID, MODEL, and SN are present.
  • Fix the cause of a segfault when displaying I2C functionality flags in the --environment command. This segfault was seen only in 32 bit versions of ddcutil.
  • The --stats option showed incorrect time statistics on 32 bit versions of ddcutil.
  • Add "-lddcutil" to output of "pkgconfig ddcutil --libs". Thank you Federico.


  • Improve reporting of the individual errors that result in command failure due to maximum I2C retries exceeded.
  • interrogate, environment and usbenvironment now redirect all stderr output to stdout for easier capture
  • Added command line options -trcfile and -trcfunc to enable tracing by file name and function name.
  • Added configuration option --enable-x11 controlling whether X11 related diagnostics are included in the environment and interrogate commands, allowing ddcutil to build in embedded environments that lack X11. The default is yes.
  • Verbose EDID decription, e.g. on command detect --verbose, now reports EDID byte 24 (x14), the supported features bitmap. This is purely informational.
  • Ongoing work to make functions thread safe.
  • Code cleanup.


22 July 2017

This release primarily contains packaging changes to meet Fedora distribution standards. It also includes minor improvements and bug fixes. There is no need to install it unless you are experiencing problems.

Command ddcutil interrogate

  • Do not include usbenvironment diagnostics as part of the interrogate command. USB connected monitors are rare and USB related diagnostics clutter the report.


  • Do not package static libraries (.a and .la files) in libddcutil-devel.

Bug fixes

  • Prevent a segfault in monitor detection that occurrs when slave address x50 is detected on an I2C bus, but reading the EDID at that address fails.

  • Address an assert failure in the ddcutil usbenvironment command due to an unexpected library return code.

  • In the device identification cross reference report of ddcutil environment, properly handle I2C bus number -1, used internally to indicate unknown.


17 May 2017

This release introduces minor improvements to the diagnostic output of the environment, detect, and interrogate commands. There is no need to install it unless you are experiencing problems.

Command ddcutil detect

  • If a display appears to support DDC (I2C slave address x37 is active) but communication fails, perform a heuristic test as to whether it is a laptop display, issue message appropriately.
  • Changes to reduce the number of feature reads performed when detecting monitors, slightly improving performance.

Option --stats errors

  • Maintain and report a secondary count of the individual errors that are rolled up into "retry count exceeded" errors.

ddcutil environment --verbose changes

  • Eliminate checks for package i2c-tools
  • Report additional information about system hardware
  • Add a cross reference report for the multiple ways that different subsystems identify a monitor and its I2C bus
  • Consolidate some tests to reduce redundancy
  • Check for DisplayPort Multi-Stream Technology (DPMST) devices
  • Eliminate the cause of a bogus error message when scanning devices


  • Changes to the requirements of package i2c-tools. Generally speaking, it is no longer needed for building ddcutil, and is optionally used as part of the diagnostic output of the environment command.
    • Changed from Requires: to Suggests: for dpkg
    • Requires: and BuildRequires: eliminated for rpm packaging in general
    • BuildRequires: kept for SuSE rpm packaging. ddcutil needs file linux/i2c-dev.h, which is contained in package i2c-tools in SuSE releases through 13.2
  • Changes to packaging to conform to Debian standards
  • Changes to packaging to conform to Fedora standards


06 May 2017

This is a bugfix release. There is no need to install it unless you are experiencing the segfault described below.

  • Fixes a segfault that can occur after open() fails for a /dev/usb/hiddev device.


01 May 2017

Release 0.8.0 contains new features intended to address issues with particular monitors and user environments, to changes to improve performance, and code restructuring for future enhancements

General Changes

  • Monitor detection and cataloging has been extensively reworked. To support the library version of ddcutil used by the C (and future Python) APIs, the list of displays is determined at program/library initialization and saved.

  • The processing of the DDC Null Response has changed. Normally a Null Response indicates a processing error in the monitor. Some monitors also use the Null Response to indicate that a VCP feature is unsupported. ddcutil now performs an extended sleep when receiving a Null Message response to allow the monitor to recover. It also performs an initial check of whether a monitor uses DDC Null Response to indicate unsupported features. See DDC Null Response.

  • Linux error numbers no longer appear "modulated" in messages. That is, an EACCES error is reported as -13, not -1013. Note that the C API has always returned Linux error numbers in unmodulated form.

General Options

Option --async parallelizes the monitor checks executed during display detection. If there are multiple monitors, initial monitor checks are performed in multiple threads, improving performance. This may become the default in future releases.

Option --nodetect is valid if the display is specified by its I2C bus number, using option --bus. In that case the initial detection of all displays is skipped, improving performance. This may become the default in future releases.

Option --hiddev specifies a USB monitor connection by its hiddev device number. For example:

--hiddev 3

refers to device /dev/usb/hiddev3. There is no single letter specifier for this option.

Option --brief is now a synonym for --terse

Added --stats class elapsed. --stats time is a synonum for --stats elapsed. Shows program elapsed time.

Changes to Specific Commands:

environment and interrogate commands:

  • Report ddcutil version

probe, interrogate, and getvcp scan commands:

  • Do not include table features unless output level is VERBOSE. This speeds up execution as table reads for unsupported features execute the maximum number of retries. It also clarifies performance statistics. No monitors implementing table type features have yet been observed.

interrogate command:

  • Report performance statistics for individual parts of interrogate.

getvcp command:

  • If --terse output is specified, then the VCP data is output in a form that is easily machine readable. See Command Specific Options

setvcp and loadvcp commands:

By default, setvcp and loadvcp verify that a value has been successfully set by reading the value from the monitor after writing it. Option --noverify disables this check.

Bug Fixes

  • Fixed a segfault that occurred when a monitor returned a 0 length capabilities string.
  • Variables holding nanoseconds have been changed from type long to uint64_t to address overflow problems on 32 bit systems such as Raspberry Pi.


The C API has undergone extensive changes.

Name Changes

Names have been changed for consistency and clarity. Names of certain functions used in message generation have been shortened to simplify printf() type functions.

Old typedef name New typedef name
VCP_Feature_Code DDCA_Vcp_Feature_Code
Version_Feature_Info DDCA_Version_Feature_Info
Vcp_Value_Type DDCA_Vcp_Value_Type
Single_Vcp_Value DDCA_Single_Vcp_Value
Old function name New function name
ddca_status_code_name() ddca_rc_name()
ddca_status_code_desc() ddca_rc_desc()
ddca_repr_display_identifier() ddca_did_repr()
ddca_create_display_ref() ddca_get_display_ref()
ddca_repr_display_ref() ddca_dref_repr()
ddca_repr_display_handle() ddca_dh_repr()
ddca_get_displays() ddca_get_display_info_list()
ddca_repr_mccs_version() ddca_mccs_version_id_name()
ddca_mccs_version_id_string() ddca_mccs_version_id_desc()

- Function ddca_create_display_ref() was renamed to ddca_get_display_ref() to reflect the fact that it returns a DDCA_Display_Reference from the internal data structures representing detected displays.

Struct Old field name New field name
DDCA_Ddcutil_Version_Spec build micro
Enum Old value name New value name

Structs and Functions Removed

Structs removed Comment
DDCA_IO_Location Use DCCA_IO_Path
Functions removed Comment
ddca_built_with_adl() Use ddca_get_build_options()
ddca_built_with_usb() Use ddca_get_built_options()

Enums, Structs, and Functions Added or Changed

New enum Purpose
DDCA_Stats_Type Used as both values and bitflags for statistics types
New Structs Purpose
DDCA_Adlno Represents a ADL adapter number/display number pair
DDCA_IO_PATH Physical path for MCCS communication


  • Struct DDCA_Display_Location is replaced by DDCA_IO_Path, which contains only the io mode and device identifiers for that mode. USB bus and device numbers were removed because they are attributes by which the physical path (hiddev device) can be found, not the path itself.
  • Struct DDCA_Display_Info has been adjusted to use DDCA_IO_Path.
Function added Purpose
ddca_ddcutil_version() gets version as a struct of numbers
ddca_reset_stats() reset performance statistics
ddca_show_stats() report performance statistics
ddca_enable_verify() controls whether VCP values are read after being set
ddca_is_verify_enabled() queries whether VCP values are verified
ddca_free_display_info_list() free function for struct
ddca_create_usb_hiddev_display_identifier() create identifier for hiddev number
ddca_get_simple_nc_feature_value_name() lookup name of simple NC feature value

Other Changes

Functions now return Linux error number -EINVAL rather than DDC specific error number DDCL_ARG in case of invalid argument

The display selection lifecycle has been changed. The DDCA_Display_Identifier is not the only way to obtain a DDCA_Display_Reference. It can be found in one of 2 ways:

  • Function ddca_get_display_info_list() returns a collection DDCA_Display_Info, containing display identification information (e.g. ddcutil assigned display number, I2c Bus number, manufacturer id, etc.) and a display reference. The client program can search through the returned collection to find the desired monitor.
  • A DDCA_Display_Identifier can be created and passed to ddca_get_display_ref(), which returns a DDCA_Display_Reference.

See sample program demo_display_selection.c for examples.

Sample code files:

The following example files for using the C API have been added:

File name Purpose
demo_global_settings.c Report and modify global settings
demo_display_selection.c Display selection
demo_vcpinfo.c Query VCP feature code descriptons
demo_get_set_vcp.c Get and set VCP values

File clmain.c is now a miscellaneous catchall, mainly for testing.

Build system changes

  • src/ change dist-hook to use -prune option per Knut Omang; avoids eror in case there is nothing to erase (i.e. build hasn't happened)
  • Added cmake file FindDDCUtil.cmake from Dorian Vogel


05 March 2017

Maintenance release.

  • Add additional diagnostic tests to command ddcutil environment --verbose*.
  • New configure option --enable-drm.
    • If enabled (the default), command ddcutil environment --verbose includes additional tests that use DRM services to better analyze disrepancies observed in the field between the EDID read directly from the monitor and that reported by X11.
    • Option --enable-drm requires that a recent version (>= 2.4.67) of the development package for libdrm2 (libdrm-dev or libdrm-devel depending on Linux distribution) be installed when building ddcutil. If a sufficently recent version is not detected, configure issues a warning and reverts to --disable-drm.
    • If minimizing executable size is important, set --disable-drm, as DRM services are used only for diagnostics.
  • Additional information on ddcutil detect --verbose.
    • Report and interpret USB vendor id/product id when detect cannot open a hiddev device. This is usually benign, and identifying the device should make this evident.
    • Suggest that the user check that DDC/CI is enabled in the monitor's on screen display if I2C bus x37 (DDC) is responsive, but DDC communication fails.


01 February 2017

Maintenance release.

  • Fixes a critical bug in release 0.7.1 where insufficient privileges on a /dev/i2c-n device causes program termination.
  • ddcutil interrogate always executes with --set-slave-address in effect
  • Minor improvements to ddcutil environment and ddcutil interrogate output
  • Minor improvements to ddcutil detect --verbose
  • Add explanations for additional errno values


27 January 2017

Mantenance release.

  • Add option --force-slave-address which causes ddcutil to take control of slave addresses on the I2C bus even if they are in use. Internally, ioctl(I2C_FORCE_SLAVE) is called instead of ioctl(I2C_SLAVE). This is intended as a possible workaround for situations where monitors on an I2C bus are not detected. See Option --force-slave-address
  • Option --force disables various checking. It's use is likely to result in command failure, but may provide useful information as to the source of problems. See Option --force
  • Fix the cause of a segfault when displaying statistics for option --stats errors or simply --stats, or on the interrogate command.
  • make install will not install C header files unless configuration option --enable-lib is set.
  • Fix the cause of a Makefile error when trying to strip dependency information from during make install.
  • Extensive rework of Autotools files for future support of Python 3 as well as Python 2. This work is incomplete.
  • Emit diagnostic messages to aid in debugging certain situations seen only in the field.


03 January 2017

  • C API
    • Should be fairly stable, though not all features of ddcutil are exposed.
    • Reflecting the presence of APIs, the default for the --enable-lib configuration option is now "yes". The following shared object library is created:
    • Additional prebuilt packages: libddcutil0, libddcutil-dev (Debian, Ubuntu) or ddcutil-devel (Fedora, SUSE)
  • Python API
    • The Python API, implemented using SWIG, is a rough proof of concept and subject to major change.
    • SWIG support is only available by building from source, not from pre-built packages. Requires --enable-swig configuration option.
  • A new command line option --mfg, which allows for monitor selection using the 3 character manufacturer id found in the EDID. --model and -sn no longer need to specified together. If any of --mfg, -model, and -sn are specified, the first monitor to match all of the manufacturer/model/serial-numbers that are specified is chosen.
  • The monitor feature and capabilities portion of interrogate has been enhanced. This functionality is also exposed using the new probe command which operates on a single monitor.
  • Parsing of the capabilities string is enhanced to accomodate monitors that do not separate feature codes by blanks.


21 November 2016

Maintenance release.

  • Improve recovery and diagnotistic messages for certain exceptional conditions.
  • Command ddcutil interrogate now reports the differences between VCP codes declared in the capabilities string and those observed by scanning.
  • Extensive internal changes in preparation for future C and Python APIs.


01 October 2016

Rename project from ddctool to ddcutil.

  • Primary executable is now named ddcutil
  • Shared library is now named
  • Tarball is now named ddcutil.n.n.n.tar.gz


24 September 2016

Fix overzealous code cleanup in release 0.5.1.

  • Undefined reference to function is_module_loaded_using_sysfs() when building without ADL support.
  • Error in command parsing. Command arguments were ignored.


23 September 2016

Minor improvements to diagnostics of the environment and interrogate commands:

  • Check if i2c_dev is built into the kernel as an alternative to it being a loadable kernel module
  • Recognize amdgpu video driver


09 September 2016

This is the first formal release of ddcutil

  • Rework USB monitor support based on testing with Eizo Coloredge and NEC PA series
    -- Probing of USB connected monitors is greatly extended and refactored from ddcutil environment into a separate command ddcutil usbenv. Libraries hidraw and libusb are used as well as hiddev. HID Report Descriptors are parsed. This probing is included in ddcutil interrogate.
    -- configure option --enable-usb/--disable-usb controls whether ddcutil is built with USB monitor support. The default is --enable-usb. When building with USB support, packages for udev, hidraw, hiddev, and libusb are required.
  • Simplify Refer to for most documentation
  • The assumption that I2C buses are numbered consecutively is removed. Required for Raspberry Pi.
  • Allow a display to be specified on the command ddcutil loadvcp. This addresses the situation where a monitor presents both an I2C and a USB interface. Normally, the display is determined using the data stored in the VCP file, and the I2C display interface is chosen over the USB interface. If a display is specified, then the model and sn from its EDID must match those in the VCP data being loaded.
  • Add ddcutil command option --timestamp/--ts. If specified, trace messages are prefaced with an elapsed timestamp.
  • Because of changing package requirements, ddcutil no longer builds in the openSUSE Build Service (OBS) for older OS versions (openSUSE 13.1, Ubuntu 12.04, 14.04).
  • ddcutil currently does not build for any openSUSE version in the openSUSE Build Service due to violation of policy guidelines. This problem will be addressed in a subsequent release. openSUSE users can build ddcutil from its tarball.