Chapter 2. Peer-to-Peer Communication by Means of Selections

Table of Contents

Selections are the primary mechanism that X Version 11 defines for the exchange of information between clients, for example, by cutting and pasting between windows. Note that there can be an arbitrary number of selections (each named by an atom) and that they are global to the server. Use of Selection Atoms. discusses the choice of an atom. Each selection is owned by a client and is attached to a window.

Selections communicate between an owner and a requestor. The owner has the data representing the value of its selection, and the requestor receives it. A requestor wishing to obtain the value of a selection provides the following:

The name of the selection
The name of a property
A window
The atom representing the data type required
Optionally, some parameters for the request

If the selection is currently owned, the owner receives an event and is expected to do the following:

Convert the contents of the selection to the requested data type
Place this data in the named property on the named window
Send the requestor an event to let it know the property is available

Clients are strongly encouraged to use this mechanism. In particular, displaying text in a permanent window without providing the ability to select and convert it into a string is definitely considered antisocial.

Note that all data transferred between an owner and a requestor must usually go by means of the server in an X Version 11 environment. A client cannot assume that another client can open the same files or even communicate directly. The other client may be talking to the server by means of a completely different networking mechanism (for example, one client might be DECnet and the other TCP/IP). Thus, passing indirect references to data (such as, file names, host names, and port numbers) is permitted only if both clients specifically agree.

Acquiring Selection Ownership

A client wishing to acquire ownership of a particular selection should call SetSelectionOwner, which is defined as follows:

SetSelectionOwner

selection: ATOM

owner: WINDOW or None

time: TIMESTAMP or CurrentTime

The client should set the specified selection to the atom that represents the selection, set the specified owner to some window that the client created, and set the specified time to some time between the current last-change time of the selection concerned and the current server time. This time value usually will be obtained from the timestamp of the event that triggers the acquisition of the selection. Clients should not set the time value to CurrentTime, because if they do so, they have no way of finding when they gained ownership of the selection. Clients must use a window they created so that requestors can route events to the owner of the selection.^[2]

Convention
Clients attempting to acquire a selection must set the time value of the SetSelectionOwner request to the timestamp of the event triggering the acquisition attempt, not to CurrentTime. A zero-length append to a property is a way to obtain a timestamp for this purpose; the timestamp is in the corresponding PropertyNotify event.

If the time in the SetSelectionOwner request is in the future relative to the server's current time or is in the past relative to the last time the specified selection changed hands, the SetSelectionOwner request appears to the client to succeed, but ownership is not actually transferred.

Because clients cannot name other clients directly, the specified owner window is used to refer to the owning client in the replies to GetSelectionOwner, in SelectionRequest and SelectionClear events, and possibly as a place to put properties describing the selection in question. To discover the owner of a particular selection, a client should invoke GetSelectionOwner, which is defined as follows:

GetSelectionOwner

selection: ATOM

owner: WINDOW or None

Convention
Clients are expected to provide some visible confirmation of selection ownership. To make this feedback reliable, a client must perform a sequence like the following:

SetSelectionOwner(selection=PRIMARY, owner=Window, time=timestamp)
owner = GetSelectionOwner(selection=PRIMARY)
if (owner != Window) Failure

If the SetSelectionOwner request succeeds (not merely appears to succeed), the client that issues it is recorded by the server as being the owner of the selection for the time period starting at the specified time.

Responsibilities of the Selection Owner

When a requestor wants the value of a selection, the owner receives a SelectionRequest event, which is defined as follows:

SelectionRequest

owner: WINDOW

selection: ATOM

target: ATOM

property: ATOM or None

requestor: WINDOW

time: TIMESTAMP or CurrentTime

The specified owner and selection will be the values that were specified in the SetSelectionOwner request. The owner should compare the timestamp with the period it has owned the selection and, if the time is outside, refuse the SelectionRequest by sending the requestor window a SelectionNotify event with the property set to None (by means of a SendEvent request with an empty event mask).

More advanced selection owners are free to maintain a history of the value of the selection and to respond to requests for the value of the selection during periods they owned it even though they do not own it now.

If the specified property is None, the requestor is an obsolete client. Owners are encouraged to support these clients by using the specified target atom as the property name to be used for the reply.

Otherwise, the owner should use the target to decide the form into which the selection should be converted. Some targets may be defined such that requestors can pass parameters along with the request. The owner will find these parameters in the property named in the selection request. The type, format, and contents of this property are dependent upon the definition of the target. If the target is not defined to have parameters, the owner should ignore the property if it is present. If the selection cannot be converted into a form based on the target (and parameters, if any), the owner should refuse the SelectionRequest as previously described.

If the specified property is not None, the owner should place the data resulting from converting the selection into the specified property on the requestor window and should set the property's type to some appropriate value, which need not be the same as the specified target.

Convention
All properties used to reply to SelectionRequest events must be placed on the requestor window.

In either case, if the data comprising the selection cannot be stored on the requestor window (for example, because the server cannot provide sufficient memory), the owner must refuse the SelectionRequest, as previously described. See also Large Data Transfers.

If the property is successfully stored, the owner should acknowledge the successful conversion by sending the requestor window a SelectionNotify event (by means of a SendEvent request with an empty mask). SelectionNotify is defined as follows:

SelectionNotify

requestor: WINDOW

selection, target: ATOM

property: ATOM or None

time: TIMESTAMP or CurrentTime

The owner should set the specified selection, target, time, and property arguments to the values received in the SelectionRequest event. (Note that setting the property argument to None indicates that the conversion requested could not be made.)

Convention
The selection, target, time, and property arguments in the SelectionNotify event should be set to the values received in the SelectionRequest event.

If the owner receives more than one SelectionRequest event with the same requestor, selection, target, and timestamp it must respond to them in the same order in which they were received.

Rationale
It is possible for a requestor to have multiple outstanding requests that use the same requestor window, selection, target, and timestamp, and that differ only in the property. If this occurs, and one of the conversion requests fails, the resulting SelectionNotify event will have its property argument set to None. This may make it impossible for the requestor to determine which conversion request had failed, unless the requests are responded to in order.

The data stored in the property must eventually be deleted. A convention is needed to assign the responsibility for doing so.

Convention
Selection requestors are responsible for deleting properties whose names they receive in SelectionNotify events (See Requesting a Selection ) or in properties with type MULTIPLE.

A selection owner will often need confirmation that the data comprising the selection has actually been transferred. (For example, if the operation has side effects on the owner's internal data structures, these should not take place until the requestor has indicated that it has successfully received the data.) Owners should express interest in PropertyNotify events for the specified requestor window and wait until the property in the SelectionNotify event has been deleted before assuming that the selection data has been transferred. For the MULTIPLE request, if the different conversions require separate confirmation, the selection owner can also watch for the deletion of the individual properties named in the property in the SelectionNotify event.

When some other client acquires a selection, the previous owner receives a SelectionClear event, which is defined as follows:

SelectionClear

owner: WINDOW

selection: ATOM

time: TIMESTAMP

The timestamp argument is the time at which the ownership changed hands, and the owner argument is the window the previous owner specified in its SetSelectionOwner request.

If an owner loses ownership while it has a transfer in progress (that is, before it receives notification that the requestor has received all the data), it must continue to service the ongoing transfer until it is complete.

If the selection value completely changes, but the owner happens to be the same client (for example, selecting a totally different piece of text in the same xterm as before), then the client should reacquire the selection ownership as if it were not the owner, providing a new timestamp. If the selection value is modified, but can still reasonably be viewed as the same selected object, ^[3] the owner should take no action.

Giving Up Selection Ownership

Clients may either give up selection ownership voluntarily or lose it forcibly as the result of some other client's actions.

Voluntarily Giving Up Selection Ownership

To relinquish ownership of a selection voluntarily, a client should execute a SetSelectionOwner request for that selection atom, with owner specified as None and the time specified as the timestamp that was used to acquire the selection.

Alternatively, the client may destroy the window used as the owner value of the SetSelectionOwner request, or the client may terminate. In both cases, the ownership of the selection involved will revert to None.

Forcibly Giving Up Selection Ownership

If a client gives up ownership of a selection or if some other client executes a SetSelectionOwner for it and thus reassigns it forcibly, the previous owner will receive a SelectionClear event. For the definition of a SelectionClear event, see Responsibilities of the Selection Owner

The timestamp is the time the selection changed hands. The specified owner is the window that was specified by the current owner in its SetSelectionOwner request.

Requesting a Selection

A client that wishes to obtain the value of a selection in a particular form (the requestor) issues a ConvertSelection request, which is defined as follows:

ConvertSelection

selection, target: ATOM

property: ATOM or None

requestor: WINDOW

time: TIMESTAMP or CurrentTime

The selection argument specifies the particular selection involved, and the target argument specifies the required form of the information. For information about the choice of suitable atoms to use, see Use of Selection Atoms The requestor should set the requestor argument to a window that it created; the owner will place the reply property there. The requestor should set the time argument to the timestamp on the event that triggered the request for the selection value. Note that clients should not specify CurrentTime.

Convention
Clients should not use CurrentTime for the time argument of a ConvertSelection request. Instead, they should use the timestamp of the event that caused the request to be made.

The requestor should set the property argument to the name of a property that the owner can use to report the value of the selection. Requestors should ensure that the named property does not exist on the window before issuing the ConvertSelection request.^[4] The exception to this rule is when the requestor intends to pass parameters with the request (see below).

Rationale
It is necessary for requestors to delete the property before issuing the request so that the target can later be extended to take parameters without introducing an incompatibility. Also note that the requestor of a selection need not know the client that owns the selection nor the window on which the selection was acquired.

Some targets may be defined such that requestors can pass parameters along with the request. If the requestor wishes to provide parameters to a request, they should be placed in the specified property on the requestor window before the requestor issues the ConvertSelection request, and this property should be named in the request.

Some targets may be defined so that parameters are optional. If no parameters are to be supplied with the request of such a target, the requestor must ensure that the property does not exist before issuing the ConvertSelection request.

The protocol allows the property field to be set to None, in which case the owner is supposed to choose a property name. However, it is difficult for the owner to make this choice safely.

Conventions

Requestors should not use None for the property argument of a ConvertSelection request.
Owners receiving ConvertSelection requests with a property argument of None are talking to an obsolete client. They should choose the target atom as the property name to be used for the reply.

The result of the ConvertSelection request is that a SelectionNotify event will be received. For the definition of a SelectionNotify event, see Responsibilities of the Selection Owner.

The requestor, selection, time, and target arguments will be the same as those on the ConvertSelection request.

If the property argument is None, the conversion has been refused. This can mean either that there is no owner for the selection, that the owner does not support the conversion implied by the target, or that the server did not have sufficient space to accommodate the data.

If the property argument is not None, then that property will exist on the requestor window. The value of the selection can be retrieved from this property by using the GetProperty request, which is defined as follows:

GetProperty

window: WINDOW

property: ATOM

type: ATOM or AnyPropertyType

long-offset, long-length: CARD32

delete: BOOL

type: ATOM or None

format: {0, 8, 16, 32}

bytes-after: CARD32

value: LISTofINT8 or LISTofINT16 or LISTofINT32

GetProperty to retrieve the value of a selection, the property argument should be set to the corresponding value in the SelectionNotify event. Because the requestor has no way of knowing beforehand what type the selection owner will use, the type argument should be set to AnyPropertyType. Several GetProperty requests may be needed to retrieve all the data in the selection; each should set the long-offset argument to the amount of data received so far, and the size argument to some reasonable buffer size (see Large Data Transfers. ). If the returned value of bytes-after is zero, the whole property has been transferred.

Once all the data in the selection has been retrieved (which may require getting the values of several properties -- see Use of Selection Properties. ), the requestor should delete the property in the SelectionNotify request by using a GetProperty request with the delete argument set to True. As previously discussed, the owner has no way of knowing when the data has been transferred to the requestor unless the property is removed.

Convention
The requestor must delete the property named in the SelectionNotify once all the data has been retrieved. The requestor should invoke either DeleteProperty or GetProperty (delete==True) after it has successfully retrieved all the data in the selection. For further information, see Large Data Transfers.

Large Data Transfers

Selections can get large, which poses two problems:

Transferring large amounts of data to the server is expensive.
All servers will have limits on the amount of data that can be stored in properties. Exceeding this limit will result in an Alloc error on the ChangeProperty request that the selection owner uses to store the data.

The problem of limited server resources is addressed by the following conventions:

Conventions

Selection owners should transfer the data describing a large selection (relative to the maximum-request-size they received in the connection handshake) using the INCR property mechanism (see INCR Properties. ).
Any client using SetSelectionOwner to acquire selection ownership should arrange to process Alloc errors in property change requests. For clients using Xlib, this involves using the XSetErrorHandler function to override the default handler.
A selection owner must confirm that no Alloc error occurred while storing the properties for a selection before replying with a confirming SelectionNotify event.
When storing large amounts of data (relative to maximum-request-size), clients should use a sequence of ChangeProperty (mode==Append) requests for reasonable quantities of data. This avoids locking servers up and limits the waste of data an Alloc error would cause.
If an Alloc error occurs during the storing of the selection data, all properties stored for this selection should be deleted and the ConvertSelection request should be refused (see Responsibilities of the Selection Owner. ).
To avoid locking servers up for inordinate lengths of time, requestors retrieving large quantities of data from a property should perform a series of GetProperty requests, each asking for a reasonable amount of data.

Advice to Implementors
Single-threaded servers should take care to avoid locking up during large data transfers.

Use of Selection Atoms

Defining a new atom consumes resources in the server that are not released until the server reinitializes. Thus, reducing the need for newly minted atoms is an important goal for the use of the selection atoms.

Selection Atoms

There can be an arbitrary number of selections, each named by an atom. To conform with the inter-client conventions, however, clients need deal with only these three selections:

PRIMARY
SECONDARY
CLIPBOARD

Other selections may be used freely for private communication among related groups of clients.

The PRIMARY Selection

The selection named by the atom PRIMARY is used for all commands that take only a single argument and is the principal means of communication between clients that use the selection mechanism.

The SECONDARY Selection

The selection named by the atom SECONDARY is used:

As the second argument to commands taking two arguments (for example, "exchange primary and secondary selections")
As a means of obtaining data when there is a primary selection and the user does not want to disturb it

The CLIPBOARD Selection

The selection named by the atom CLIPBOARD is used to hold data that is being transferred between clients, that is, data that usually is being cut and then pasted or copied and then pasted. Whenever a client wants to transfer data to the clipboard:

It should assert ownership of the CLIPBOARD.
If it succeeds in acquiring ownership, it should be prepared to respond to a request for the contents of the CLIPBOARD in the usual way (retaining the data to be able to return it). The request may be generated by the clipboard client described below.
If it fails to acquire ownership, a cutting client should not actually perform the cut or provide feedback that would suggest that it has actually transferred data to the clipboard.

The owner should repeat this process whenever the data to be transferred would change.

Clients wanting to paste data from the clipboard should request the contents of the CLIPBOARD selection in the usual way.

Except while a client is actually deleting or copying data, the owner of the CLIPBOARD selection may be a single, special client implemented for the purpose. This client maintains the content of the clipboard up-to-date and responds to requests for data from the clipboard as follows:

It should assert ownership of the CLIPBOARD selection and reassert it any time the clipboard data changes.
If it loses the selection (because another client has some new data for the clipboard), it should:
- Obtain the contents of the selection from the new owner by using the timestamp in the SelectionClear event.
- Attempt to reassert ownership of the CLIPBOARD selection by using the same timestamp.
- Restart the process using a newly acquired timestamp if this attempt fails. This timestamp should be obtained by asking the current owner of the CLIPBOARD selection to convert it to a TIMESTAMP. If this conversion is refused or if the same timestamp is received twice, the clipboard client should acquire a fresh timestamp in the usual way (for example by a zero-length append to a property).
It should respond to requests for the CLIPBOARD contents in the usual way.

A special CLIPBOARD client is not necessary. The protocol used by the cutting client and the pasting client is the same whether the CLIPBOARD client is running or not. The reasons for running the special client include:

Stability - If the cutting client were to crash or terminate, the clipboard value would still be available.
Feedback - The clipboard client can display the contents of the clipboard.
Simplicity - A client deleting data does not have to retain it for so long, thus reducing the chance of race conditions causing problems.

The reasons not to run the clipboard client include:

Performance - Data is transferred only if it is actually required (that is, when some client actually wants the data).
Flexibility - The clipboard data may be available as more than one target.

Target Atoms

The atom that a requestor supplies as the target of a ConvertSelection request determines the form of the data supplied. The set of such atoms is extensible, but a generally accepted base set of target atoms is needed. As a starting point for this, the following table contains those that have been suggested so far.

Atom	Type	Data Received
ADOBE_PORTABLE_DOCUMENT_FORMAT	STRING	[1]
APPLE_PICT	APPLE_PICT	[2]
BACKGROUND	PIXEL	A list of pixel values
BITMAP	BITMAP	A list of bitmap IDs
CHARACTER_POSITION	SPAN	The start and end of the selection in bytes
CLASS	TEXT	(see WM_CLASS Property. )
CLIENT_WINDOW	WINDOW	Any top-level window owned by the selection owner
COLORMAP	COLORMAP	A list of colormap IDs
COLUMN_NUMBER	SPAN	The start and end column numbers
COMPOUND_TEXT	COMPOUND_TEXT	Compound Text
DELETE	NULL	(see DELETE. )
DRAWABLE	DRAWABLE	A list of drawable IDs
ENCAPSULATED_POSTSCRIPT	STRING	[3], Appendix H ^[a]
ENCAPSULATED_POSTSCRIPT_INTERCHANGE	STRING	[3], Appendix H
FILE_NAME	TEXT	The full path name of a file
FOREGROUND	PIXEL	A list of pixel values
HOST_NAME	TEXT	(see WM_CLIENT_MACHINE Property. )
INSERT_PROPERTY	NULL	(see INSERT_PROPERTY. )
INSERT_SELECTION	NULL	(see INSERT_SELECTION. )
LENGTH	INTEGER	The number of bytes in the selection ^[b]
LINE_NUMBER	SPAN	The start and end line numbers
LIST_LENGTH	INTEGER	The number of disjoint parts of the selection
MODULE	TEXT	The name of the selected procedure
MULTIPLE	ATOM_PAIR	(see the discussion that follows)
NAME	TEXT	(see WM_NAME Property. )
ODIF	TEXT	ISO Office Document Interchange Format
OWNER_OS	TEXT	The operating system of the owner client
PIXMAP	PIXMAP ^[c]	A list of pixmap IDs
POSTSCRIPT	STRING	[3]
PROCEDURE	TEXT	The name of the selected procedure
PROCESS	INTEGER, TEXT	The process ID of the owner
STRING	STRING	ISO Latin-1 (+TAB+NEWLINE) text
TARGETS	ATOM	A list of valid target atoms
TASK	INTEGER, TEXT	The task ID of the owner
TEXT	TEXT	The text in the owner's choice of encoding
TIMESTAMP	INTEGER	The timestamp used to acquire the selection
USER	TEXT	The name of the user running the owner
^[a] Earlier versions of this document erroneously specified that conversion of the PIXMAP target returns a property of type DRAWABLE instead of PIXMAP. Implementors should be aware of this and may want to support the DRAWABLE type as well to allow for compatibility with older clients. ^[b] The targets ENCAPSULATED_POSTSCRIPT and ENCAPSULATED_POSTSCRIPT_INTERCHANGE are equivalent to the targets _ADOBE_EPS and _ADOBE_EPSI (respectively) that appear in the selection targets registry. The _ADOBE_ targets are deprecated, but clients are encouraged to continue to support them for backward compatibility. ^[c] This definition is ambiguous, as the selection may be converted into any of several targets that may return differing amounts of data. The requestor has no way of knowing which, if any, of these targets corresponds to the result of LENGTH. Clients are advised that no guarantees can be made about the result of a conversion to LENGTH; its use is thus deprecated.

References:

Adobe Systems, Incorporated. Portable Document Format Reference Manual. Reading, MA, Addison-Wesley, ISBN 0-201-62628-4.
Apple Computer, Incorporated. Inside Macintosh, Volume V. Chapter 4, "Color QuickDraw," Color Picture Format. ISBN 0-201-17719-6.
Adobe Systems, Incorporated. PostScript Language Reference Manual. Reading, MA, Addison-Wesley, ISBN 0-201-18127-4.

It is expected that this table will grow over time.

Selection owners are required to support the following targets. All other targets are optional.

TARGETS - The owner should return a list of atoms that represent the targets for which an attempt to convert the current selection will succeed (barring unforseeable problems such as Alloc errors). This list should include all the required atoms.
MULTIPLE - The MULTIPLE target atom is valid only when a property is specified on the ConvertSelection request. If the property argument in the SelectionRequest event is None and the target is MULTIPLE, it should be refused.
When a selection owner receives a SelectionRequest (target==MULTIPLE) request, the contents of the property named in the request will be a list of atom pairs: the first atom naming a target and the second naming a property ( None is not valid here). The effect should be as if the owner had received a sequence of SelectionRequest events (one for each atom pair) except that:
- The owner should reply with a SelectionNotify only when all the requested conversions have been performed.
- If the owner fails to convert the target named by an atom in the MULTIPLE property, it should replace that atom in the property with None.
Convention
The entries in a MULTIPLE property must be processed in the order they appear in the property. For further information, see Selection Targets with Side Effects.
The requestor should delete each individual property when it has copied the data from that conversion, and the property specified in the MULTIPLE request when it has copied all the data.
The requests are otherwise to be processed independently, and they should succeed or fail independently. The MULTIPLE target is an optimization that reduces the amount of protocol traffic between the owner and the requestor; it is not a transaction mechanism. For example, a client may issue a MULTIPLE request with two targets: a data target and the DELETE target. The DELETE target will still be processed even if the conversion of the data target fails.
TIMESTAMP - To avoid some race conditions, it is important that requestors be able to discover the timestamp the owner used to acquire ownership. Until and unless the protocol is changed so that a GetSelectionOwner request returns the timestamp used to acquire ownership, selection owners must support conversion to TIMESTAMP, returning the timestamp they used to obtain the selection.

Selection Targets with Side Effects

Some targets (for example, DELETE) have side effects. To render these targets unambiguous, the entries in a MULTIPLE property must be processed in the order that they appear in the property.

In general, targets with side effects will return no information, that is, they will return a zero length property of type NULL. (Type NULL means the result of InternAtom on the string "NULL", not the value zero.) In all cases, the requested side effect must be performed before the conversion is accepted. If the requested side effect cannot be performed, the corresponding conversion request must be refused.

Conventions
Targets with side effects should return no information (that is, they should have a zero-length property of type NULL).
The side effect of a target must be performed before the conversion is accepted.
If the side effect of a target cannot be performed, the corresponding conversion request must be refused.

Problem
The need to delay responding to the ConvertSelection request until a further conversion has succeeded poses problems for the Intrinsics interface that need to be addressed.

These side-effect targets are used to implement operations such as "exchange PRIMARY and SECONDARY selections."

DELETE

When the owner of a selection receives a request to convert it to DELETE, it should delete the corresponding selection (whatever doing so means for its internal data structures) and return a zero-length property of type NULL if the deletion was successful.

INSERT_SELECTION

When the owner of a selection receives a request to convert it to INSERT_SELECTION, the property named will be of type ATOM_PAIR. The first atom will name a selection, and the second will name a target. The owner should use the selection mechanism to convert the named selection into the named target and should insert it at the location of the selection for which it got the INSERT_SELECTION request (whatever doing so means for its internal data structures).

INSERT_PROPERTY

When the owner of a selection receives a request to convert it to INSERT_PROPERTY, it should insert the property named in the request at the location of the selection for which it got the INSERT_SELECTION request (whatever doing so means for its internal data structures).

Use of Selection Properties

The names of the properties used in selection data transfer are chosen by the requestor. The use of None property fields in ConvertSelection requests (which request the selection owner to choose a name) is not permitted by these conventions.

The selection owner always chooses the type of the property in the selection data transfer. Some types have special semantics assigned by convention, and these are reviewed in the following sections.

In all cases, a request for conversion to a target should return either a property of one of the types listed in the previous table for that target or a property of type INCR and then a property of one of the listed types.

Certain selection properties may contain resource IDs. The selection owner should ensure that the resource is not destroyed and that its contents are not changed until after the selection transfer is complete. Requestors that rely on the existence or on the proper contents of a resource must operate on the resource (for example, by copying the contents of a pixmap) before deleting the selection property.

The selection owner will return a list of zero or more items of the type indicated by the property type. In general, the number of items in the list will correspond to the number of disjoint parts of the selection. Some targets (for example, side-effect targets) will be of length zero irrespective of the number of disjoint selection parts. In the case of fixed-size items, the requestor may determine the number of items by the property size. Selection property types are listed in the table below. For variable-length items such as text, the separators are also listed.

Type Atom	Format	Separator
APPLE_PICT	8	Self-sizing
ATOM	32	Fixed-size
ATOM_PAIR	32	Fixed-size
BITMAP	32	Fixed-size
C_STRING	8	Zero
COLORMAP	32	Fixed-size
COMPOUND_TEXT	8	Zero
DRAWABLE	32	Fixed-size
INCR	32	Fixed-size
INTEGER	32	Fixed-size
PIXEL	32	Fixed-size
PIXMAP	32	Fixed-size
SPAN	32	Fixed-size
STRING	8	Zero
WINDOW	32	Fixed-size

It is expected that this table will grow over time.

TEXT Properties

In general, the encoding for the characters in a text string property is specified by its type. It is highly desirable for there to be a simple, invertible mapping between string property types and any character set names embedded within font names in any font naming standard adopted by the Consortium.

The atom TEXT is a polymorphic target. Requesting conversion into TEXT will convert into whatever encoding is convenient for the owner. The encoding chosen will be indicated by the type of the property returned. TEXT is not defined as a type; it will never be the returned type from a selection conversion request.

If the requestor wants the owner to return the contents of the selection in a specific encoding, it should request conversion into the name of that encoding.

In the table in Target Atoms, the word TEXT (in the Type column) is used to indicate one of the registered encoding names. The type would not actually be TEXT; it would be STRING or some other ATOM naming the encoding chosen by the owner.

STRING as a type or a target specifies the ISO Latin-1 character set plus the control characters TAB (octal 11) and NEWLINE (octal 12). The spacing interpretation of TAB is context dependent. Other ASCII control characters are explicitly not included in STRING at the present time.

COMPOUND_TEXT as a type or a target specifies the Compound Text interchange format; see the Compound Text Encoding.

There are some text objects where the source or intended user, as the case may be, does not have a specific character set for the text, but instead merely requires a zero-terminated sequence of bytes with no other restriction; no element of the selection mechanism may assume that any byte value is forbidden or that any two differing sequences are equivalent. ^[5] For these objects, the type C_STRING should be used.

Rationale
An example of the need for C_STRING is to transmit the names of files; many operating systems do not interpret filenames as having a character set. For example, the same character string uses a different sequence of bytes in ASCII and EBCDIC, and so most operating systems see these as different filenames and offer no way to treat them as the same. Thus no character-set based property type is suitable.

Type STRING, COMPOUND_TEXT, and C_STRING properties will consist of a list of elements separated by null characters; other encodings will need to specify an appropriate list format.

INCR Properties

Requestors may receive a property of type INCR ^[6] in response to any target that results in selection data.

This indicates that the owner will send the actual data incrementally. The contents of the INCR property will be an integer, which represents a lower bound on the number of bytes of data in the selection. The requestor and the selection owner transfer the data in the selection in the following manner.

The selection requestor starts the transfer process by deleting the (type==INCR) property forming the reply to the selection.

The selection owner then:

Appends the data in suitable-size chunks to the same property on the same window as the selection reply with a type corresponding to the actual type of the converted selection. The size should be less than the maximum-request-size in the connection handshake.
Waits between each append for a PropertyNotify (state==Deleted) event that shows that the requestor has read the data. The reason for doing this is to limit the consumption of space in the server.
Waits (after the entire data has been transferred to the server) until a PropertyNotify (state==Deleted) event that shows that the data has been read by the requestor and then writes zero-length data to the property.

The selection requestor:

Waits for the SelectionNotify event.
Loops:
- Retrieving data using GetProperty with the delete argument True.
- Waiting for a PropertyNotify with the state argument NewValue.
Waits until the property named by the PropertyNotify event is zero-length.
Deletes the zero-length property.

The type of the converted selection is the type of the first partial property. The remaining partial properties must have the same type.

DRAWABLE Properties

Requestors may receive properties of type PIXMAP, BITMAP, DRAWABLE, or WINDOW, which contain an appropriate ID. While information about these drawables is available from the server by means of the GetGeometry request, the following items are not:

Foreground pixel
Background pixel
Colormap ID

In general, requestors converting into targets whose returned type in the table in Target Atoms is one of the DRAWABLE types should expect to convert also into the following targets (using the MULTIPLE mechanism):

FOREGROUND returns a PIXEL value.
BACKGROUND returns a PIXEL value.
COLORMAP returns a colormap ID.

SPAN Properties

Properties with type SPAN contain a list of cardinal-pairs with the length of the cardinals determined by the format. The first specifies the starting position, and the second specifies the ending position plus one. The base is zero. If they are the same, the span is zero-length and is before the specified position. The units are implied by the target atom, such as LINE_NUMBER or CHARACTER_POSITION.

Manager Selections

Certain clients, often called managers, take on responsibility for managing shared resources. A client that manages a shared resource should take ownership of an appropriate selection, named using the conventions described in Naming Conventions and Discriminated Names. A client that manages multiple shared resources (or groups of resources) should take ownership of a selection for each one.

The manager may support conversion of various targets for that selection. Managers are encouraged to use this technique as the primary means by which clients interact with the managed resource. Note that the conventions for interacting with the window manager predate this section; as a result many interactions with the window manager use other techniques.

Before a manager takes ownership of a manager selection, it should use the GetSelectionOwner request to check whether the selection is already owned by another client, and, where appropriate, it should ask the user if the new manager should replace the old one. If so, it may then take ownership of the selection. Managers should acquire the selection using a window created expressly for this purpose. Managers must conform to the rules for selection owners described in Acquiring Selection Ownership and Responsibilities of the Selection Owner , and they must also support the required targets listed in Use of Selection Atoms.

If a manager loses ownership of a manager selection, this means that a new manager is taking over its responsibilities. The old manager must release all resources it has managed and must then destroy the window that owned the selection. For example, a window manager losing ownership of WM_S2 must deselect from SubstructureRedirect on the root window of screen 2 before destroying the window that owned WM_S2.

When the new manager notices that the window owning the selection has been destroyed, it knows that it can successfully proceed to control the resource it is planning to manage. If the old manager does not destroy the window within a reasonable time, the new manager should check with the user before destroying the window itself or killing the old manager.

If a manager wants to give up, on its own, management of a shared resource controlled by a selection, it must do so by releasing the resources it is managing and then by destroying the window that owns the selection. It should not first disown the selection, since this introduces a race condition.

Clients who are interested in knowing when the owner of a manager selection is no longer managing the corresponding shared resource should select for StructureNotify on the window owning the selection so they can be notified when the window is destroyed. Clients are warned that after doing a GetSelectionOwner and selecting for StructureNotify, they should do a GetSelectionOwner again to ensure that the owner did not change after initially getting the selection owner and before selecting for StructureNotify.

Immediately after a manager successfully acquires ownership of a manager selection, it should announce its arrival by sending a ClientMessage event. This event should be sent using the SendEvent protocol request with the following arguments:

Argument	Value
destination:	the root window of screen 0, or the root window of the appropriate screen if the manager is managing a screen-specific resource
propogate:	False
event-mask:	`StructureNotify`
event:	`ClientMessage`
type:	MANAGER
format:	32
data[0] ^[a]	timestamp
data[1]:	manager selection atom
data[2]:	the window owning the selection
data[3]:	manager-selection-specific data
data[4]:	manager-selection-specific data
^[a] We use the notation data[n] to indicate the n ^th element of the LISTofINT8, LISTofINT16, or LISTofINT32 in the data field of the `ClientMessage`, according to the format field. The list is indexed from zero.

Clients that wish to know when a specific manager has started should select for StructureNotify on the appropriate root window and should watch for the appropriate MANAGER ClientMessage.

^[2] At present, no part of the protocol requires requestors to send events to the owner of a selection. This restriction is imposed to prepare for possible future extensions.

^[3] The division between these two cases is a matter of judgment on the part of the software developer.

^[4] This requirement is new in version 2.0, and, in general, existing clients do not conform to this requirement. To prevent these clients from breaking, no existing targets should be extended to take parameters until sufficient time has passed for clients to be updated. Note that the MULTIPLE target was defined to take parameters in version 1.0 and its definition is not changing. There is thus no conformance problem with MULTIPLE.

^[5] Note that this is different from STRING, where many byte values are forbidden, and from COMPOUND_TEXT, where, for example, inserting the sequence 27,\ 40,\ 66 (designate ASCII into GL) at the start does not alter the meaning.

^[6] These properties were called INCREMENTAL in an earlier draft. The protocol for using them has changed, and so the name has changed to avoid confusion.