# The XIM Transport Specification

Revision 0.1
X Version 11, Release 7
Takashi
Fujiwara
FUJITSU LIMITED
1994
FUJITSU LIMITED
1994
X Consortium
Revision 0.1

This specification describes the transport layer interfaces between Xlib and IM Server, which makes various channels usable such as X protocol or TCP/IP, DECnet and etc.

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## X Transport Specification

### Introduction

The Xlib XIM implementation is layered into three functions, a protocol layer, an interface layer and a transport layer. The purpose of this layering is to make the protocol independent of transport implementation. Each function of these layers are:

The protocol layer

  implements overall function of XIM and calls the interface layer functions when it needs to communicate to IM Server.

The interface layer

  separates the implementation of the transport layer from the protocol layer, in other words, it provides implementation independent hook for the transport layer functions.

The transport layer

  handles actual data communication with IM Server. It is done by a set of several functions named transporters.

This specification describes the interface layer and the transport layer, which makes various communication channels usable such as X protocol or, TCP/IP, DECnet, STREAM, etc., and provides the information needed for adding another new transport layer. In addition, sample implementations for the transporter using the X connection is described in section 4.

### Initialization

#### Registering structure to initialize

The structure typed as TransportSW contains the list of the transport layer the specific implementations supports.

    typedef struct {
          char *transport_name;
          Bool (*config);
    } TransportSW;

transport_name | name of transport Refer to "The Input Method Protocol: Appendix B
config | initial configuration function

A sample entry for the Xlib supporting transporters is shown below:

    TransportSW _XimTransportRec[] = {
    /*     char *:
     *     transport_name,     Bool (*config)()
     */
          "X",             _XimXConf,
          "tcp",           _XimTransConf,
          "local",         _XimTransConf,
          "decnet",        _XimTransConf,
          "streams",       _XimTransConf,
          (char *)NULL,    (Bool (*)())NULL,
    };

#### Initialization function

The following function will be called once when Xlib configures the transporter functions.

      Bool (*config)
      XIM im
      char *transport_data

im

  Specifies XIM structure address.

transport_data

  Specifies the data specific to the transporter, in IM Server address. Refer to "The Input Method Protocol: Appendix B

This function must setup the transporter function pointers.

The actual config function will be chosen by IM Server at the pre-connection time, matching by the transport_name specified in the `_XimTransportRec` array; The specific members of XimProto structure listed below must be initialized so that point they appropriate transporter functions.

If the specified transporter has been configured successfully, this function returns True. There is no Alternative Entry for config function itself.

The structure XimProto contains the following function pointers:

    Bool (*connect)();               /* Open connection */
    Bool (*shutdown)();              /* Close connection */
    Bool (*write)();                 /* Write data */
    Bool (*read)();                  /* Read data */
    Bool (*flush)();                 /* Flush data buffer */
    Bool (*register_dispatcher)();   /* Register asynchronous data handler */
    Bool (*call_dispatcher)();       /* Call dispatcher */

These functions are called when Xlib needs to communicate the IM Server. These functions must process the appropriate procedure described below.

### The interface/transport layer functions

Following functions are used for the transport interface.

#### The Transport Layer Functions

Alternate Entry (Interface Layer) | XimProto member (Transport Layer) | Section
--- | --- | ---
_XimConnect | connect | 3.1
_XimShutdown | shutdown | 3.2
_XimWrite | write | 3.3
_XimRead | read | 3.4
_XimFlush | flush | 3.5
_XimRegisterDispatcher | register_dispatcher | 3.6
_XimCallDispatcher | call_dispatcher | 3.7

The Protocol layer calls the above functions using the Alternative Entry in the left column. The transport implementation defines XimProto member function in the right column. The Alternative Entry is provided so as to make easier to implement the Protocol Layer.

#### Opening connection

When `XOpenIM` is called, the following function is called to connect with the IM Server.

      Bool (*connect)
      XIM im

im

  Specifies XIM structure address.

This function must establishes the connection to the IM Server. If the connection is established successfully, this function returns True. The Alternative Entry for this function is:

      Bool  _XimConnect
      XIM im

im

  Specifies XIM structure address.

#### Closing connection

When `XCloseIM` is called, the following function is called to disconnect the connection with the IM Server. The Alternative Entry for this function is:

      Bool  (*shutdown)
      XIM im

im

  Specifies XIM structure address.

This function must close connection with the IM Server. If the connection is closed successfully, this function returns True. The Alternative Entry for this function is:

      Bool _XimShutdown
      XIM im

im

  Specifies XIM structure address.

#### Writing data

The following function is called, when Xlib needs to write data to the IM Server.

      Bool  _XimWrite
      XIM im
      INT16 len
      XPointer data

im

  Specifies XIM structure address.

len

  Specifies the length of writing data.

data

  Specifies the writing data.

This function writes the data to the IM Server, regardless of the contents. The number of bytes is passed to len. The writing data is passed to data. If data is sent successfully, the function returns True. Refer to "The Input Method Protocol" for the contents of the writing data. The Alternative Entry for this function is:

      Bool _XimWrite
      XIM im
      INT16 len
      XPointer data

im

  Specifies XIM structure address.

len

  Specifies the length of writing data.

data

  Specifies the writing data.

#### Reading data

The following function is called when Xlib waits for response from IM server synchronously.

      Bool  _XimRead
      XIM im
      XPointer read_buf
      int buf_len
      int *ret_len

im

  Specifies XIM structure address.

read_buf

  Specifies the buffer to store data.

buf_len

  Specifies the size of the buffer

ret_len

  Specifies the length of stored data.

This function stores the read data in read_buf, which size is specified as buf_len. The size of data is set to ret_len. This function return True, if the data is read normally or reading data is completed.

The Alternative Entry for this function is:

      Bool  _XimRead
      XIM im
      INT16 *ret_len
      XPointer buf
      int buf_len
      Bool (*predicate)()
      XPointer predicate_arg

im

  Specifies XIM structure address.

ret_len

  Specifies the size of the data buffer.

buf

  Specifies the buffer to store data.

buf_len

  Specifies the length of buffer.

predicate

  Specifies the predicate for the XIM data.

predicate_arg

  Specifies the predicate specific data.

The predicate procedure indicates whether the data is for the XIM or not. len This function stores the read data in buf, which size is specified as buf_len. The size of data is set to ret_len. If preedicate() returns True, this function returns True. If not, it calls the registered callback function.

The procedure and its arguments are:

      void (*predicate)
      XIM im
      INT16 len
      XPointer data
      XPointer predicate_arg

im

  Specifies XIM structure address.

len

  Specifies the size of the data buffer.

data

  Specifies the buffer to store data.

predicate_arg

  Specifies the predicate specific data.

#### Flushing buffer

The following function is called when Xlib needs to flush the data.

      void (*flush)
      XIM im

im

  Specifies XIM structure address.

This function must flush the data stored in internal buffer on the transport layer. If data transfer is completed, the function returns True. The Alternative Entry for this function is:

      void  _XimFlush
      XIM im

im

  Specifies XIM structure address.

#### Registering asynchronous data handler

Xlib needs to handle asynchronous response from IM Server. This is because some of the XIM data occur asynchronously to X events.

Those data will be handled in the Filter, and the Filter will call asynchronous data handler in the protocol layer. Then it calls dispatchers in the transport layer. The dispatchers are implemented by the protocol layer. This function must store the information and prepare for later call of the dispatchers using `_XimCallDispatcher`.

When multiple dispatchers are registered, they will be called sequentially in order of registration, on arrival of asynchronous data. The register_dispatcher is declared as following:

      Bool (*register_dispatcher)
      XIM im
      Bool (*dispatcher)()
      XPointer call_data

im

  Specifies XIM structure address.

dispatcher

  Specifies the dispatcher function to register.

call_data

  Specifies a parameter for the dispatcher.

The dispatcher is a function of the following type:

      Bool (*dispatcher)
      XIM im
      INT16 len
      XPointer data
      XPointer call_data

im

  Specifies XIM structure address.

len

  Specifies the size of the data buffer.

data

  Specifies the buffer to store data.

call_data

  Specifies a parameter passed to the register_dispatcher.

The dispatcher is provided by the protocol layer. They are called once for every asynchronous data, in order of registration. If the data is used, it must return True. otherwise, it must return False.

If the dispatcher function returns True, the Transport Layer assume that the data has been processed by the upper layer. The Alternative Entry for this function is:

      Bool  _XimRegisterDispatcher
      XIM im
      Bool (*dispatcher)()
      XPointer call_data

im

  Specifies XIM structure address.

dispatcher

  Specifies the dispatcher function to register.

call_data

  Specifies a parameter for the dispatcher.

#### Calling dispatcher

The following function is used to call the registered dispatcher function, when the asynchronous response from IM Server has arrived.

      Bool (*call_dispatcher)
      XIM im
      INT16 len
      XPointer data

im

  Specifies XIM structure address.

len

  Specifies the size of data buffer.

data

  Specifies the buffer to store data.

The call_dispatcher must call the dispatcher function, in order of their registration. len and data are the data passed to register_dispatcher.

The return values are checked at each invocation, and if it finds True, it immediately return with true for its return value.

It is depend on the upper layer whether the read data is XIM Protocol packet unit or not. The Alternative Entry for this function is:

      Bool  _XimCallDispatcher
      XIM im
      INT16 len
      XPointer call_data

### Sample implementations for the Transport Layer

Sample implementations for the transporter using the X connection is described here.

#### X Transport

At the beginning of the X Transport connection for the XIM transport mechanism, two different windows must be created either in an Xlib XIM or in an IM Server, with which the Xlib and the IM Server exchange the XIM transports by using the ClientMessage events and Window Properties. In the following, the window created by the Xlib is referred as the "client communication window", and on the other hand, the window created by the IM Server is referred as the "IMS communication window".

##### Connection

In order to establish a connection, a communication window is created. A ClientMessage in the following event's format is sent to the owner window of XIM_SERVER selection, which the IM Server has created.

Refer to "The Input Method Protocol" for the XIM_SERVER atom.

###### The ClientMessage sent to the IMS window.

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_CONNECT", false)
int | format | 32
long | data.1[0] | client communication window ID
long | data.1[1] | client-major-transport-version(*1)
long | data.1[2] | client-major-transport-version(*1)

In order to establish the connection (to notify the IM Server communication window), the IM Server sends a ClientMessage in the following event's format to the client communication window.

###### The ClientMessage sent by IM Server.

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_CONNECT", false)
int | format | 32
long | data.1[0] | client communication window ID
long | data.1[1] | client-major-transport-version(*1)
long | data.1[2] | client-major-transport-version(*1)
long | data.1[3] | dividing size between ClientMessage and Property(*2)

(*1) major/minor-transport-version

The read/write method is decided by the combination of major/minor-transport-version, as follows:

###### The read/write method and the major/minor-transport-version

Transport-version | read/write
--- | ---
major | minor |
0 | 0 | only-CM & Property-with-CM
1 | only-CM & multi-CM
2 | only-CM & multi-CM & Property-with-CM
1 | 0 | PropertyNotify
2 | 0 | only-CM & PropertyNotify
1 | only-CM & multi-CM & PropertyNotify

    only-CM            :    data is sent via a ClientMessage
    multi-CM           :    data is sent via multiple ClientMessages
    Property-with-CM   :    data is written in Property, and its Atom
                            is send via ClientMessage
    PropertyNotify     :    data is written in Property, and its Atom
                            is send via PropertyNotify

The method to decide major/minor-transport-version is as follows:

- The client sends 0 as major/minor-transport-version to the IM Server. The client must support all methods in Table 4-3. The client may send another number as major/minor-transport-version to use other method than the above in the future.

- The IM Server sends its major/minor-transport-version number to the client. The client sends data using the method specified by the IM Server.

- If major/minor-transport-version number is not available, it is regarded as 0.

(*2) dividing size between ClientMessage and Property

If data is sent via both of multi-CM and Property, specify the dividing size between ClientMessage and Property. The data, which is smaller than this size, is sent via multi-CM (or only-CM), and the data, which is lager than this size, is sent via Property.

##### read/write

The data is transferred via either ClientMessage or Window Property in the X Window System.

###### Format for the data from the Client to the IM Server

ClientMessage

If data is sent via ClientMessage event, the format is as follows:

###### The ClientMessage event's format (first or middle)

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_MOREDATA", False)
int | format | 8
char | data.b[20] | (read/write DATA : 20 byte)

###### The ClientMessage event's format (only or last)

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_PROTOCOL", False)
int | format | 8
char | data.b[20] | (read/write DATA : MAX 20 byte) If the data is smaller than 20 bytes, all data other than available data must be 0.

Property

In the case of large data, data will be sent via the Window Property for the efficiency. There are the following two methods to notify Property, and transport-version is decided which method is used.

- The XChangeProperty function is used to store data in the client communication window, and Atom of the stored data is notified to the IM Server via ClientMessage event.

- The XChangeProperty function is used to store data in the client communication window, and Atom of the stored data is notified to the IM Server via PropertyNotify event.

The arguments of the XChangeProperty are as follows:

###### The XChangeProperty event's format

Argument | Contents
--- | ---
Display | *display | The display to which connects
Window | window | IMS communication window ID
Atom | property | read/write property Atom (*1)
int | format | 8
int | mode | PropModeAppend
u_char | *data | read/write DATA
int | nelements | length of DATA

(*1) The read/write property ATOM allocates the following strings by `XInternAtom`. "_clientXXX"

The client changes the property with the mode of PropModeAppend and the IM Server will read it with the delete mode i.e. (delete = True).

If Atom is notified via ClientMessage event, the format of the ClientMessage is as follows:

###### The ClientMessage event's format to send Atom of property

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_PROTOCOL", False)
int | format | 8
long | data.1[0] | length of read/write property Atom
long | data.1[1] | read/write property Atom

###### Format for the data from the IM Server to the Client

ClientMessage

The format of the ClientMessage is as follows:

###### The ClientMessage event's format (first or middle)

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_MOREDATA", False)
int | format | 8
char | data.b[20] | (read/write DATA : 20 byte)

###### The ClientMessage event's format (only or last)

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_PROTOCOL", False)
int | format | 8
char | data.b[20] | (read/write DATA : MAX 20 byte) (*1)

(*1) If the data size is smaller than 20 bytes, all data other than available data must be 0.

Property

In the case of large data, data will be sent via the Window Property for the efficiency. There are the following two methods to notify Property, and transport-version is decided which method is used.

- The XChangeProperty function is used to store data in the IMS communication window, and Atom of the property is sent via the ClientMessage event.

- The XChangeProperty function is used to store data in the IMS communication window, and Atom of the property is sent via PropertyNotify event.

The arguments of the XChangeProperty are as follows:

###### The XChangeProperty event's format

Argument | Contents
--- | ---
Display | *display | The display to which connects
Window | window | IMS communication window ID
Atom | property | read/write property Atom (*1)
int | format | 8
int | mode | PropModeAppend
u_char | *data | read/write DATA
int | nelements | length of DATA

(*1) The read/write property ATOM allocates some strings, which are not allocated by the client, by `XInternAtom`.

The IM Server changes the property with the mode of PropModeAppend and the client reads it with the delete mode, i.e. (delete = True).

If Atom is notified via ClientMessage event, the format of the ClientMessage is as follows:

###### The ClientMessage event's format to send Atom of property

Structure Member | Contents
--- | ---
int | type | ClientMessage
u_long | serial | Set by the X Window System
Bool | send_event | Set by the X Window System
Display | *display | The display to which connects
Window | window | IMS Window ID
Atom | message_type | XInternAtom(display, "_XIM_PROTOCOL", False)
int | format | 8
long | data.1[0] | length of read/write property Atom
long | data.1[1] | read/write property Atom

##### Closing Connection

If the client disconnect with the IM Server, shutdown function should free the communication window properties and etc..

### References

[1] Masahiko Narita and Hideki Hiura, "The Input Method Protocol"