XSETTINGS - cross toolkit configuration proposal

Owen Taylor


Version 0.5

Table of Contents

Existing Systems
Client behavior
Settings Manager behavior
A. Rational and Discussion


The intent of this specification is to specify a mechanism to allow the configuration of settings such as double click timeout, drag-and-drop threshold, and default foreground and background colors for all applications running within a desktop. The mechanism should:

allow for instant updates to be propagated across all applications at runtime
perform well, even for remote applications.

It is not intended:

for the storage of application-specific data
to be able to store large amounts of data
to store complex data types (other than as strings)

Existing Systems

The existing systems in this area are the Xrm database, and various other configuration database systems, mostly specific to particular desktops. (The kde configuration system, gnome-config, GConf, libproplist, etc.)

The Xrm database is tempting to use for this purpose since it is very well established and has a universally deployed existing implementation. However, it has various defects, that make it not suitable for this purpose.

  • The Xrm database merges information from various sources - the ~/.Xdefaults file on the root window, and a property on the root window. This means that to programatically configure the Xrm database in response to a GUI config tool is difficult and unreliable.

  • The Xrm database was not designed for on-the-fly changing of settings.

  • The Xrm database stores all information in a single text property on the root window. This makes it difficult to determine what settings have changed; it is necessary to parse the property and do string comparisons.

    Additionally, most systems have a large amount of application specific information in the Xrm database, which further slows down notification of changes.

Other configuration databases are more designed for this task. However, they are sophisticated systems that are not easily reimplementable. Also, picking one would mean difficulties integrating with other desktops that use different systems.

It is our contention that a system designed specifically for configuration of a small number of settings being changed at runtime can, compared to a more general existing system:

Be easier to bridge onto each systems native configuration mechanism.
Be easier to implement in whatever language/library combination people want to use.
Be more efficient.

Client behavior

On startup, each client that should identify the settings window by calling XGetSelectionOwner() for the _XSETTINGS_S[N] selection and select for notification on the settings window by calling XSelectInput() with a mask of StructureNotifyMask|PropertyChangeMask.

To prevent race conditions a client MUST grab the server while performing these operations using XGrabServer().

If there is no owner of the _XSETTINGS_S[N] selection, the client can determine when an owner is established by listening for client messages sent to root window of the screen with type MANAGER. (See section 2.8, Manager Selections of the ICCCM.) The format of this message is:

 event-mask:   StructureNotify
 event:        ClientMessage
 type:         MANAGER
 format:       32
 data[0]:      timestamp
 data[1]:      _XSETTINGS_S[N] (atom)
 data[2]:      New owner of the selection
 data[3]:      0 (reserved)

The client can then proceed to read contents of the _XSETTINGS_SETTINGS property from the settings window and interpret according to the information in the "_XSETTINGS_SETTINGS Format" section of this document.

Clients must trap X errors when reading the _XSETTING_SETTINGS property because it is possible that the selection window may be destroyed at any time.

When the client is notified that the settings window has been destroyed or discovers that the selection window has been destroyed, it should reset all settings to their default values and then proceed as on initial startup. [ See rational section ]

When a client receives a PropertyChangeNotify event for the window it should reread the _XSETTING_SETTINGS property. It can use the 'serial' field to tell what fields have been changed. The client must parse the entire property and read in all new values before taking action on changed settings such as notifying listeners for those settings to avoid using a mix of old and new data.


The _XSETTINGS_SETTINGS property is of form 8 and type _XSETTINGS_SETTINGS. The contents are a

 1	CARD8    byte-order
 3               unused
 4      CARD32   SERIAL
 4      CARD32   N_SETTINGS

Followed by N_SETTINGS settings records, which have a header:

 1      SETTING_TYPE  type
 1                    unused
 2      n             name-len
 n      STRING8       name
 P                    unused, p=pad(n)
 4      CARD32        last-change-serial


 0  XSettingsTypeInteger
 1  XSettingsTypeString
 2  XSettingsTypeColor

followed by the body. If TYPE = XSettingsTypeString the body is:

 4      n             value-len
 n      STRING8       value
 P                    unused, p=pad(n)

If TYPE == XSettingsTypeInteger, then the body is:

 4      INT32         value

If TYPE == XSettingsTypeColor, then the body is:

 2      CARD16        red
 2      CARD16        green
 2      CARD16        blue
 2      CARD16        alpha

If the setting does not need the alpha field, it should be set to 65535.

Setting names must be confined to the ascii characters:

 'A'-'Z' 'a'-'z' '0'-'9' '_' and '/'

With the additional restrictions that '/' cannot appear in the leading or trailing position, that two occurences of '/' cannot be consecutive, and that the first character of the name, and and the first character after a slash cannot be one of '0'-'9'. Names may not be empty.



are legitimate names, while


Are not legitimate names.

The names, types, contents, and default values of standard settings will be separately agreed upon.

Names beginning with 'Net/' and case variants of that string are reserved and must not be used without prior agreement.

The 'serial' field and and the 'last-change-serial' field of the each settings record can be used to tell which settings have changed since the last time a client retrieved the _XSETTINGS_SETTINGS property. Each time the client retrieves the contents of the _XSETTINGS_SETTINGS property it should store the contents of the 'serial' field. When it next retrieves the property, any settings whose 'last-change-serial' is greater than the stored value.

(Careful clients will make provisions for wrap-around of the serial field. This is, however, not expected to happen in practice.)

Settings Manager behavior

The _XSETTING_S[N] selection is managed as a manager selection according to section 2.8 of the ICCCM and the handling of the selections window, the _XSETTING_S[N] window and MANAGER client messages must conform to that specification.

The settings manager changes the contents of the _XSETTINGS_SETTINGS property of the root window whenever the source it derives them from changes, taking care to increment the 'serial' field at each increment and set the 'last-change-serial' fields appropriately.

A. Rational and Discussion

The reasons why an existing configuration mechanism, and in particular, the Xrm database, was not used is discussed above. Various other design decisions are discussed below:

Why aren't the contents of the property stored in XML?  The data format is designed to be space efficient and to be easily and efficiently parsed with minimal code. These are not things XML does well. Flexibility of structure, things that XML does well are not needed here. If needed, XML can be used for the contents of individual settings.

Why is the settings property screen specific?  While most settings are expected to be shared across all screens of a display, some settings, such as font sizes will be screen specific, and it is considered easier to let the settings manager propagate shared resources across screens then to have both screen-specific and screen-independent resources.

Why does there need to be a "settings manager" process running?  Having a process always present as the owner of the _XSETTING selection ensures that there are no race conditions and is simpler than trying to create a locking mechanism that can work without a persistant process. It is also expected that to properly handle notification of changes to the stored properties most desktops will want a process running to watch for changes in any case. In cases of tight resource usage, the settings manager can be combined with some other function, such as the window manager or session manager.

Why use a single property for all settings?  Using a single property has several advantages. First, retrieving all settings takes only a single round-trip to the server instead of a round-trip for each settings. Second, it means that when multiple settings can be changed at once, only a single notification is received by clients, and clients will see interrelated properties changed in an atomic fashion.

Why is the _XSETTINGS_SETTINGS property stored in the endianess of the manager instead of a neutral endianness?  This conforms to the way many other X mechanisms work. The main reason for doing it this way is to save conversions for the common case when the client and manager are on machines of the same endianness.

When the settings manager exits, why should clients reset the values to the default settings instead of keeping the current settings?  Resetting the settings to the default values is preferred to maintaining the current values because it makes sure that all programs on the desktop have consistent values for settings whether they were started before or after the settings manager exited. It is not expected that changes in the current settings manager will occur very often.