Inter-Client Exchange Library

X Consortium Standard

Ralph Mor

   X Consortium

   X Version 11, Release 7.7

   Version 1.0

   Copyright © 1993, 1994, 1996 X Consortium

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   Table of Contents

   1. Overview of ICE
   2. The ICE Library - C Language Interface to ICE
   3. Intended Audience
   4. Header Files and Library Name
   5. Note on Prefixes
   6. Protocol Registration

        Callbacks for Processing Messages
        Authentication Methods

   7. ICE Connections

        Opening an ICE Connection
        Listening for ICE Connections
        Host Based Authentication for ICE Connections
        Accepting ICE Connections
        Closing ICE Connections
        Connection Watch Procedures

   8. Protocol Setup and Shutdown
   9. Processing Messages
   10. Ping
   11. Using ICElib Informational Functions
   12. ICE Messages

        Sending ICE Messages
        Reading ICE Messages

   13. Error Handling
   14. Multi-Threading Support
   15. Miscellaneous Functions
   16. Acknowledgements
   A. Authentication Utility Functions
   B. MIT-MAGIC-COOKIE-1 Authentication

Chapter 1. Overview of ICE

   There are numerous possible inter-client protocols, with many
   similarities and common needs - authentication, version
   negotiation, byte order negotiation, and so on. The
   Inter-Client Exchange (ICE) protocol is intended to provide a
   framework for building such protocols, allowing them to make
   use of common negotiation mechanisms and to be multiplexed over
   a single transport connection.

Chapter 2. The ICE Library - C Language Interface to ICE

   A client that wishes to utilize ICE must first register the
   protocols it understands with the ICE library. Each protocol is
   dynamically assigned a major opcode ranging from 1-255 (two
   clients can use different major opcodes for the same protocol).
   The next step for the client is either to open a connection
   with another client or to wait for connections made by other
   clients. Authentication may be required. A client can both
   initiate connections with other clients and be waiting for
   clients to connect to itself (a nested session manager is an
   example). Once an ICE connection is established between the two
   clients, one of the clients needs to initiate a ProtocolSetup
   in order to "activate" a given protocol. Once the other client
   accepts the ProtocolSetup (once again, authentication may be
   required), the two clients are ready to start passing messages
   specific to that protocol to each other. Multiple protocols may
   be active on a single ICE connection. Clients are responsible
   for notifying the ICE library when a protocol is no longer
   active on an ICE connection, although ICE does not define how
   each subprotocol triggers a protocol shutdown.

   The ICE library utilizes callbacks to process incoming
   messages. Using callbacks allows ProtocolSetup messages and
   authentication to happen behind the scenes. An additional
   benefit is that messages never need to be buffered up by the
   library when the client blocks waiting for a particular
   message.

Chapter 3. Intended Audience

   This document is intended primarily for implementors of
   protocol libraries layered on top of ICE. Typically,
   applications that wish to utilize ICE will make calls into
   individual protocol libraries rather than directly make calls
   into the ICE library. However, some applications will have to
   make some initial calls into the ICE library in order to accept
   ICE connections (for example, a session manager accepting
   connections from clients). But in general, protocol libraries
   should be designed to hide the inner details of ICE from
   applications.

Chapter 4. Header Files and Library Name

   The header file <X11/ICE/ICElib.h> defines all of the ICElib
   data structures and function prototypes. ICElib.h includes the
   header file <X11/ICE/ICE.h>, which defines all of the ICElib
   constants. Protocol libraries that need to read and write
   messages should include the header file <X11/ICE/ICEmsg.h>.

   Applications should link against ICElib using -lICE.

Chapter 5. Note on Prefixes

   The following name prefixes are used in the library to
   distinguish between a client that initiates a ProtocolSetup and
   a client that responds with a ProtocolReply
     * IcePo - Ice Protocol Originator
     * IcePa - Ice Protocol Acceptor

Chapter 6. Protocol Registration

   Table of Contents

   Callbacks for Processing Messages
   Authentication Methods

   In order for two clients to exchange messages for a given
   protocol, each side must register the protocol with the ICE
   library. The purpose of registration is for each side to obtain
   a major opcode for the protocol and to provide callbacks for
   processing messages and handling authentication. There are two
   separate registration functions:
     * One to handle the side that does a ProtocolSetup
     * One to handle the side that responds with a ProtocolReply

   It is recommended that protocol registration occur before the
   two clients establish an ICE connection. If protocol
   registration occurs after an ICE connection is created, there
   can be a brief interval of time in which a ProtocolSetup is
   received, but the protocol is not registered. If it is not
   possible to register a protocol before the creation of an ICE
   connection, proper precautions should be taken to avoid the
   above race condition.

   The IceRegisterForProtocolSetup function should be called for
   the client that initiates a ProtocolSetup

   int IceRegisterForProtocolSetup(const char *protocol_name,
   const char *vendor, const char *release, int version_count,
   IcePoVersionRec *version_recs, int auth_count, char
   **auth_names, IcePoAuthProc *auth_procs, IceIOErrorProc
   io_error_proc);

   protocol_name

   A string specifying the name of the protocol to register.

   vendor

   A vendor string with semantics specified by the protocol.

   release

   A release string with semantics specified by the protocol.

   version_count

   The number of different versions of the protocol supported.

   version_recs

   List of versions and associated callbacks.

   auth_count

   The number of authentication methods supported.

   auth_names

   The list of authentication methods supported.

   auth_procs

   The list of authentication callbacks, one for each
   authentication method.

   io_error_proc

   IO error handler, or NULL.

   IceRegisterForProtocolSetup returns the major opcode reserved
   or -1 if an error occurred. In order to actually activate the
   protocol, the IceProtocolSetup function needs to be called with
   this major opcode. Once the protocol is activated, all messages
   for the protocol should be sent using this major opcode.

   A protocol library may support multiple versions of the same
   protocol. The version_recs argument specifies a list of
   supported versions of the protocol, which are prioritized in
   decreasing order of preference. Each version record consists of
   a major and minor version of the protocol as well as a callback
   to be used for processing incoming messages.

   typedef struct {
   int major_version;
   int minor_version;
   IcePoProcessMsgProc process_msg_proc;
   } IcePoVersionRec;

   The IcePoProcessMsgProc callback is responsible for processing
   the set of messages that can be received by the client that
   initiated the ProtocolSetup For further information, see
   Callbacks for Processing Messages

   Authentication may be required before the protocol can become
   active. The protocol library must register the authentication
   methods that it supports with the ICE library. The auth_names
   and auth_procs arguments are a list of authentication names and
   callbacks that are prioritized in decreasing order of
   preference. For information on the IcePoAuthProc callback, see
   Authentication Methods

   The IceIOErrorProc callback is invoked if the ICE connection
   unexpectedly breaks. You should pass NULL for io_error_proc if
   not interested in being notified. For further information,
   Error Handling

   The IceRegisterForProtocolReply function should be called for
   the client that responds to a ProtocolSetup with a
   ProtocolReply

   Bool IceRegisterForProtocolReply(const char *protocol_name,
   const char *vendor, const char *release, int version_count,
   IcePoVersionRec *version_recs, int auth_count, const char
   **auth_names, IcePoAuthProc *auth_procs, IceHostBasedAuthProc
   host_based_auth_proc, IceProtocolSetupProc protocol_setup_proc,
   IceProtocolActivateProc protocol_activate_proc, IceIOErrorProc
   io_error_proc);

   protocol_name

   A string specifying the name of the protocol to register.

   vendor

   A vendor string with semantics specified by the protocol.

   release

   A release string with semantics specified by the protocol.

   version_count

   The number of different versions of the protocol supported.

   version_recs

   List of versions and associated callbacks.

   auth_count

   The number of authentication methods supported.

   auth_names

   The list of authentication methods supported.

   auth_procs

   The list of authentication callbacks, one for each
   authentication method.

   host_based_auth_proc

   Host based authentication callback.

   protocol_setup_proc

   A callback to be invoked when authentication has succeeded for
   a ProtocolSetup but before the ProtocolReply is sent.

   protocol_activate_proc

   A callback to be invoked after the ProtocolReply is sent.

   io_error_proc

   IO error handler, or NULL.

   IceRegisterForProtocolReply returns the major opcode reserved
   or -1 if an error occurred. The major opcode should be used in
   all subsequent messages sent for this protocol.

   A protocol library may support multiple versions of the same
   protocol. The version_recs argument specifies a list of
   supported versions of the protocol, which are prioritized in
   decreasing order of preference. Each version record consists of
   a major and minor version of the protocol as well as a callback
   to be used for processing incoming messages.

   typedef struct {
   int major_version;
   int minor_version;
   IcePaProcessMsgProc process_msg_proc;
   } IcePaVersionRec;

   The IcePaProcessMsgProc callback is responsible for processing
   the set of messages that can be received by the client that
   accepted the ProtocolSetup For further information, see
   Callbacks for Processing Messages

   Authentication may be required before the protocol can become
   active. The protocol library must register the authentication
   methods that it supports with the ICE library. The auth_names
   and auth_procs arguments are a list of authentication names and
   callbacks that are prioritized in decreasing order of
   preference. For information on the IcePaAuthProc, See
   Authentication Methods

   If authentication fails and the client attempting to initiate
   the ProtocolSetup has not required authentication, the
   IceHostBasedAuthProc callback is invoked with the host name of
   the originating client. If the callback returns True the
   ProtocolSetup will succeed, even though the original
   authentication failed. Note that authentication can effectively
   be disabled by registering an IceHostBasedAuthProc which always
   returns True If no host based authentication is allowed, you
   should pass NULL for host_based_auth_proc.

   Bool HostBasedAuthProc(char *host_name);

   protocol_name

   The host name of the client that sent the ProtocolSetup

   The host_name argument is a string of the form
   protocol/hostname, where protocol is one of {tcp, decnet,
   local}.

   Because ProtocolSetup messages and authentication happen behind
   the scenes via callbacks, the protocol library needs some way
   of being notified when the ProtocolSetup has completed. This
   occurs in two phases. In the first phase, the
   IceProtocolSetupProc callback is invoked after authentication
   has successfully completed but before the ICE library sends a
   ProtocolReply Any resources required for this protocol should
   be allocated at this time. If the IceProtocolSetupProc returns
   a successful status, the ICE library will send the
   ProtocolReply and then invoke the IceProtocolActivateProc
   callback. Otherwise, an error will be sent to the other client
   in response to the ProtocolSetup

   The IceProtocolActivateProc is an optional callback and should
   be registered only if the protocol library intends to generate
   a message immediately following the ProtocolReply You should
   pass NULL for protocol_activate_proc if not interested in this
   callback.

   Status ProtocolSetupProc(IceConn ice_conn, int major_version,
   int minor_version, char *vendor, char *release, IcePointer
   *client_data_ret, char **failure_reason_ret);

   protocol_name

   The ICE connection object.

   major_version

   The major version of the protocol.

   minor_version

   The minor version of the protocol.

   vendor

   The vendor string registered by the protocol originator.

   release

   The release string registered by the protocol originator.

   client_data_ret

   Client data to be set by callback.

   failure_reason_ret

   Failure reason returned.

   The pointer stored in the client_data_ret argument will be
   passed to the IcePaProcessMsgProc callback whenever a message
   has arrived for this protocol on the ICE connection.

   The vendor and release strings should be freed with free when
   they are no longer needed.

   If a failure occurs, the IceProtocolSetupProc should return a
   zero status as well as allocate and return a failure reason
   string in failure_reason_ret. The ICE library will be
   responsible for freeing this memory.

   The IceProtocolActivateProc callback is defined as follows:

   void ProtocolActivateProc(IceConn ice_conn, IcePointer
   client_data);

   ice_conn

   The ICE connection object.

   client_data

   The client data set in the IceProtocolSetupProc callback.

   The IceIOErrorProc callback is invoked if the ICE connection
   unexpectedly breaks. You should pass NULL for io_error_proc if
   not interested in being notified. For further information, see
   Error Handling

Callbacks for Processing Messages

   When an application detects that there is new data to read on
   an ICE connection (via select it calls the IceProcessMessages
   function Processing Messages When IceProcessMessages reads an
   ICE message header with a major opcode other than zero
   (reserved for the ICE protocol), it needs to call a function
   that will read the rest of the message, unpack it, and process
   it accordingly.

   If the message arrives at the client that initiated the
   ProtocolSetup the IcePoProcessMsgProc callback is invoked.

   void PoProcessMsgProc(IceConn ice_conn, IcePointer client_data,
   int opcode, unsigned long length, Bool swap, IceReplyWaitInfo
   *reply_wait, Bool *reply_ready_ret);

   ice_conn

   The ICE connection object.

   client_data

   Client data associated with this protocol on the ICE
   connection.

   opcode

   The minor opcode of the message.

   length

   The length (in 8-byte units) of the message beyond the ICE
   header.

   swap

   A flag that indicates if byte swapping is necessary.

   reply_wait

   Indicates if the invoking client is waiting for a reply.

   reply_ready_ret

   If set to True a reply is ready.

   If the message arrives at the client that accepted the
   ProtocolSetup the IcePaProcessMsgProc callback is invoked.

   void IcePaProcessMsgProc(IceConn ice_conn, IcePointer
   client_data, int opcode, unsigned long length, Bool swap);

   ice_conn

   The ICE connection object.

   client_data

   Client data associated with this protocol on the ICE
   connection.

   opcode

   The minor opcode of the message.

   length

   The length (in 8-byte units) of the message beyond the ICE
   header.

   swap

   A flag that indicates if byte swapping is necessary.

   In order to read the message, both of these callbacks should
   use the macros defined for this purpose (see Reading ICE
   Messages.). Note that byte swapping may be necessary. As a
   convenience, the length field in the ICE header will be swapped
   by ICElib if necessary.

   In both of these callbacks, the client_data argument is a
   pointer to client data that was registered at ProtocolSetup
   time. In the case of IcePoProcessMsgProc the client data was
   set in the call to IceProtocolSetup In the case of
   IcePaProcessMsgProc the client data was set in the
   IceProtocolSetupProc callback.

   The IcePoProcessMsgProc callback needs to check the reply_wait
   argument. If reply_wait is NULL , the ICE library expects the
   function to pass the message to the client via a callback. For
   example, if this is a Session Management "Save Yourself"
   message, this function should notify the client of the "Save
   Yourself" via a callback. The details of how such a callback
   would be defined are implementation-dependent.

   However, if reply_wait is not NULL , then the client is waiting
   for a reply or an error for a message it previously sent. The
   reply_wait is of type IceReplyWaitInfo

   typedef struct {
   unsigned long sequence_of_request;
   int major_opcode_of_request;
   int minor_opcode_of_request;
   IcePointer reply;
   } IceReplyWaitInfo;

   IceReplyWaitInfo contains the major/minor opcodes and sequence
   number of the message for which a reply is being awaited. It
   also contains a pointer to the reply message to be filled in
   (the protocol library should cast this IcePointer to the
   appropriate reply type). In most cases, the reply will have
   some fixed-size part, and the client waiting for the reply will
   have provided a pointer to a structure to hold this fixed-size
   data. If there is variable-length data, it would be expected
   that the IcePoProcessMsgProc callback will have to allocate
   additional memory and store pointer(s) to that memory in the
   fixed-size structure. If the entire data is variable length
   (for example., a single variable-length string), then the
   client waiting for the reply would probably just pass a pointer
   to fixed-size space to hold a pointer, and the
   IcePoProcessMsgProc callback would allocate the storage and
   store the pointer. It is the responsibility of the client
   receiving the reply to free any memory allocated on its behalf.

   If reply_wait is not NULL and IcePoProcessMsgProc has a reply
   or error to return in response to this reply_wait (that is, no
   callback was generated), then the reply_ready_ret argument
   should be set to True Note that an error should only be
   returned if it corresponds to the reply being waited for.
   Otherwise, the IcePoProcessMsgProc should either handle the
   error internally or invoke an error handler for its library.

   If reply_wait is NULL, then care must be taken not to store any
   value in reply_ready_ret, because this pointer may also be
   NULL.

   The IcePaProcessMsgProc callback, on the other hand, should
   always pass the message to the client via a callback. For
   example, if this is a Session Management "Interact Request"
   message, this function should notify the client of the
   "Interact Request" via a callback.

   The reason the IcePaProcessMsgProc callback does not have a
   reply_wait, like IcePoProcessMsgProc does, is because a process
   that is acting as a server should never block for a reply
   (infinite blocking can occur if the connecting client does not
   act properly, denying access to other clients).

Authentication Methods

   As already stated, a protocol library must register the
   authentication methods that it supports with the ICE library.
   For each authentication method, there are two callbacks that
   may be registered:
     * One to handle the side that initiates a ProtocolSetup
     * One to handle the side that accepts or rejects this request

   IcePoAuthProc is the callback invoked for the client that
   initiated the ProtocolSetup This callback must be able to
   respond to the initial "Authentication Required" message or
   subsequent "Authentication Next Phase" messages sent by the
   other client.

   IcePoAuthStatus IcePoAuthStatus (IceConn ice_conn, IcePointer
   client_data, int opcode);

   ice_conn

   The ICE connection object.

   auth_state_ptr

   A pointer to state for use by the authentication callback
   procedure.

   clean_up

   If True authentication is over, and the function should clean
   up any state it was maintaining. The last 6 arguments should be
   ignored.

   swap

   If True the auth_data may have to be byte swapped (depending on
   its contents).

   auth_datalen

   The length (in bytes) of the authenticator data.

   auth_data

   The data from the authenticator.

   reply_datalen_ret

   The length (in bytes) of the reply data returned.

   reply_data_ret

   The reply data returned.

   error_string_ret

   If the authentication procedure encounters an error during
   authentication, it should allocate and return an error string.

   Authentication may require several phases, depending on the
   authentication method. As a result, the IcePoAuthProc may be
   called more than once when authenticating a client, and some
   state will have to be maintained between each invocation. At
   the start of each ProtocolSetup *auth_state_ptr is NULL, and
   the function should initialize its state and set this pointer.
   In subsequent invocations of the callback, the pointer should
   be used to get at any state previously stored by the callback.

   If needed, the network ID of the client accepting the
   ProtocolSetup can be obtained by calling the
   IceConnectionString function.

   ICElib will be responsible for freeing the reply_data_ret and
   error_string_ret pointers with free

   The auth_data pointer may point to a volatile block of memory.
   If the data must be kept beyond this invocation of the
   callback, be sure to make a copy of it.

   The IcePoAuthProc should return one of four values:
     * IcePoAuthHaveReply - a reply is available.
     * IcePoAuthRejected - authentication rejected.
     * IcePoAuthFailed - authentication failed.
     * IcePoAuthDoneCleanup - done cleaning up.

   IcePaAuthProc is the callback invoked for the client that
   received the ProtocolSetup

   IcePoAuthStatus PoAuthStatus (IceConn ice_conn, IcePointer
   *auth_state_ptr, Bool swap, int auth_datalen, IcePointer
   auth_data, int *reply_datalen_ret, IcePointer *reply_data_ret,
   char **error_string_ret);

   ice_conn

   The ICE connection object.

   auth_state_ptr

   A pointer to state for use by the authentication callback
   procedure.

   swap

   If True auth_data may have to be byte swapped (depending on its
   contents).

   auth_datalen

   The length (in bytes) of the protocol originator authentication
   data.

   auth_data

   The authentication data from the protocol originator.

   reply_datalen_ret

   The length of the authentication data returned.

   reply_data_ret

   The authentication data returned.

   error_string_ret

   If authentication is rejected or fails, an error string is
   returned.

   Authentication may require several phases, depending on the
   authentication method. As a result, the IcePaAuthProc may be
   called more than once when authenticating a client, and some
   state will have to be maintained between each invocation. At
   the start of each ProtocolSetup auth_datalen is zero,
   *auth_state_ptr is NULL, and the function should initialize its
   state and set this pointer. In subsequent invocations of the
   callback, the pointer should be used to get at any state
   previously stored by the callback.

   If needed, the network ID of the client accepting the
   ProtocolSetup can be obtained by calling the
   IceConnectionString function.

   The auth_data pointer may point to a volatile block of memory.
   If the data must be kept beyond this invocation of the
   callback, be sure to make a copy of it.

   ICElib will be responsible for transmitting and freeing the
   reply_data_ret and error_string_ret pointers with free

   The IcePaAuthProc should return one of four values:
     * IcePaAuthContinue - continue (or start) authentication.
     * IcePaAuthAccepted - authentication accepted.
     * IcePaAuthRejected - authentication rejected.
     * IcePaAuthFailed - authentication failed.

Chapter 7. ICE Connections

   Table of Contents

   Opening an ICE Connection
   Listening for ICE Connections
   Host Based Authentication for ICE Connections
   Accepting ICE Connections
   Closing ICE Connections
   Connection Watch Procedures

   In order for two clients to establish an ICE connection, one
   client has to be waiting for connections, and the other client
   has to initiate the connection. Most clients will initiate
   connections, so we discuss that first.

Opening an ICE Connection

   To open an ICE connection with another client (that is, waiting
   for connections), use IceOpenConnection

   IceConn IceOpenConnection(char *network_ids_list, IcePointer
   context, Bool must_authenticate, int major_opcode_check, int
   error_length, char *error_string_ret);

   network_ids_list

   Specifies the network ID(s) of the other client.

   context

   A pointer to an opaque object or NULL. Used to determine if an
   ICE connection can be shared (see below).

   must_authenticate

   If True the other client may not bypass authentication.

   major_opcode_check

   Used to force a new ICE connection to be created (see below).

   error_length

   Length of the error_string_ret argument passed in.

   error_string_ret

   Returns a null-terminated error message, if any. The
   error_string_ret argument points to user supplied memory. No
   more than error_length bytes are used.

   IceOpenConnection returns an opaque ICE connection object if it
   succeeds; otherwise, it returns NULL.

   The network_ids_list argument contains a list of network IDs
   separated by commas. An attempt will be made to use the first
   network ID. If that fails, an attempt will be made using the
   second network ID, and so on. Each network ID has the following
   format:
   tcp/<hostname>:<portnumber>  or
   decnet/<hostname>::<objname> or
   local/<hostname>:<path>

   Most protocol libraries will have some sort of open function
   that should internally make a call into IceOpenConnection When
   IceOpenConnection is called, it may be possible to use a
   previously opened ICE connection (if the target client is the
   same). However, there are cases in which shared ICE connections
   are not desired.

   The context argument is used to determine if an ICE connection
   can be shared. If context is NULL, then the caller is always
   willing to share the connection. If context is not NULL, then
   the caller is not willing to use a previously opened ICE
   connection that has a different non-NULL context associated
   with it.

   In addition, if major_opcode_check contains a nonzero major
   opcode value, a previously created ICE connection will be used
   only if the major opcode is not active on the connection. This
   can be used to force multiple ICE connections between two
   clients for the same protocol.

   Any authentication requirements are handled internally by the
   ICE library. The method by which the authentication data is
   obtained is implementation-dependent. ^[1]

   After IceOpenConnection is called, the client is ready to send
   a ProtocolSetup (provided that IceRegisterForProtocolSetup was
   called) or receive a ProtocolSetup (provided that
   IceRegisterForProtocolReply was called).

Listening for ICE Connections

   Clients wishing to accept ICE connections must first call
   IceListenForConnections or IceListenForWellKnownConnections so
   that they can listen for connections. A list of opaque "listen"
   objects are returned, one for each type of transport method
   that is available (for example, Unix Domain, TCP, DECnet, and
   so on).

   Normally clients will let ICElib allocate an available name in
   each transport and return listen objects. Such a client will
   then use IceComposeNetworkIdList to extract the chosen names
   and make them available to other clients for opening the
   connection. In certain cases it may be necessary for a client
   to listen for connections on pre-arranged transport object
   names. Such a client may use IceListenForWellKnownConnections
   to specify the names for the listen objects.

   Status IceListenForConnections(int *count_ret, IceListenObj
   **listen_objs_ret, int error_length, char *error_string_ret);

   count_ret

   Returns the number of listen objects created.

   listen_objs_ret

   Returns a list of pointers to opaque listen objects.

   error_length

   The length of the error_string_ret argument passed in.

   error_string_ret

   Returns a null-terminated error message, if any. The
   error_string_ret points to user supplied memory. No more than
   error_length bytes are used.

   The return value of IceListenForConnections is zero for failure
   and a positive value for success.

   Status IceListenForWellKnownConnections(char *port_id, int
   *count_ret, IceListenObj **listen_objs_ret, int error_length,
   char *error_string_ret);

   port_id

   Specifies the port identification for the address(es) to be
   opened. The value must not contain the slash ("/"> or comma
   (".") character; thse are reserved for future use.

   count_ret

   Returns the number of listen objects created.

   listen_objs_ret

   Returns a list of pointers to opaque listen objects.

   listen_objs_ret

   Returns a list of pointers to opaque listen objects.

   error_length

   The length of the error_string_ret argument passed in.

   error_string_ret

   Returns a null-terminated error message, if any. The
   error_string_ret points to user supplied memory. No more than
   error_length bytes are used.

   IceListenForWellKnownConnections constructs a list of network
   IDs by prepending each known transport to port_id and then
   attempts to create listen objects for the result. Port_id is
   the portnumber, objname, or path portion of the ICE network ID.
   If a listen object for a particular network ID cannot be
   created the network ID is ignored. If no listen objects are
   created IceListenForWellKnownConnections returns failure.

   The return value of IceListenForWellKnownConnections is zero
   for failure and a positive value for success.

   To close and free the listen objects, use IceFreeListenObjs

   void IceFreeListenObjs(int count, IceListenObj *listen_objs);

   count

   The number of listen objects.

   listen_objs

   The listen objects.

   To detect a new connection on a listen object, use select on
   the descriptor associated with the listen object.

   To obtain the descriptor, use IceGetListenConnectionNumber

   int IceGetListenConnectionNumber(IceListenObj *listen_objs);

   listen_obj

   The listen objects.

   To obtain the network ID string associated with a listen
   object, use IceGetListenConnectionString

   char IceGetListenConnectionString(IceListenObj listen_obj);

   listen_obj

   The listen objects.

   A network ID has the following format:
   tcp/<hostname>:<portnumber>  or
   decnet/<hostname>::<objname> or
   local/<hostname>:<path>

   To compose a string containing a list of network IDs separated
   by commas (the format recognized by IceOpenConnection use
   IceComposeNetworkIdList

   char IceComposeNetworkIdList(int count, IceListenObj
   *listen_objs);

   count

   The number of listen objects.

   listen_objs

   The listen objects.

Host Based Authentication for ICE Connections

   If authentication fails when a client attempts to open an ICE
   connection and the initiating client has not required
   authentication, a host based authentication procedure may be
   invoked to provide a last chance for the client to connect.
   Each listen object has such a callback associated with it, and
   this callback is set using the IceSetHostBasedAuthProc
   function.

   void IceSetHostBasedAuthProc(IceListenObj listen_obj,
   IceHostBasedAuthProc host_based_auth_proc);

   IceListenObj

   The listen object.

   host_based_auth_proc

   The host based authentication procedure.

   By default, each listen object has no host based authentication
   procedure associated with it. Passing NULL for
   host_based_auth_proc turns off host based authentication if it
   was previously set.

   Bool HostBasedAuthProc(char *host_name);

   host_name

   The host name of the client that tried to open an ICE
   connection.

   The host_name argument is a string in the form protocol/
   hostname, where protocol is one of {tcp, decnet, local}.

   If IceHostBasedAuthProc returns True access will be granted,
   even though the original authentication failed. Note that
   authentication can effectively be disabled by registering an
   IceHostBasedAuthProc which always returns True

   Host based authentication is also allowed at ProtocolSetup
   time. The callback is specified in the
   IceRegisterForProtocolReply function (see Protocol
   Registration).

Accepting ICE Connections

   After a connection attempt is detected on a listen object
   returned by IceListenForConnections you should call
   IceAcceptConnection This returns a new opaque ICE connection
   object.

   IceConn IceAcceptConnection(IceListenObj listen_obj,
   IceAcceptStatus *status_ret);

   listen_obj

   The listen object on which a new connection was detected.

   status_ret

   Return status information.

   The status_ret argument is set to one of the following values:
     * IceAcceptSuccess - the accept operation succeeded, and the
       function returns a new connection object.
     * IceAcceptFailure - the accept operation failed, and the
       function returns NULL.
     * IceAcceptBadMalloc - a memory allocation failed, and the
       function returns NULL.

   In general, to detect new connections, you should call select
   on the file descriptors associated with the listen objects.
   When a new connection is detected, the IceAcceptConnection
   function should be called. IceAcceptConnection may return a new
   ICE connection that is in a pending state. This is because
   before the connection can become valid, authentication may be
   necessary. Because the ICE library cannot block and wait for
   the connection to become valid (infinite blocking can occur if
   the connecting client does not act properly), the application
   must wait for the connection status to become valid.

   The following pseudo-code demonstrates how connections are
   accepted:
new_ice_conn = IceAcceptConnection (listen_obj, &accept_status);
if (accept_status != IceAcceptSuccess)
{
     close the file descriptor and return
}

status = IceConnectionStatus (new_ice_conn);
time_start = time_now;

while (status == IceConnectPending)
{
     select() on {new_ice_conn, all open connections}

     for (each ice_conn in the list of open connections)
          if (data ready on ice_conn)
          {
               status = IceProcessMessages (ice_conn, NULL, NULL);
               if (status == IceProcessMessagesIOError)
                    IceCloseConnection(ice_conn);
          }
     if data ready on new_ice_conn
     {
          /*
          * IceProcessMessages is called until the connection
          * is non-pending.  Doing so handles the connection
          * setup request and any authentication requirements.
          */

          IceProcessMessages ( new_ice_conn, NULL, NULL);
          status = IceConnectionStatus (new_ice_conn);
     }
     else
     {
          if (time_now - time_start > MAX_WAIT_TIME)
               status = IceConnectRejected;
     }
}

if (status == IceConnectAccepted)
{
     Add new_ice_conn to the list of open connections
}
else
{
     IceCloseConnection
     new_ice_conn
}

   After IceAcceptConnection is called and the connection has been
   validated, the client is ready to receive a ProtocolSetup
   (provided that IceRegisterForProtocolReply was called) or send
   a ProtocolSetup (provided that IceRegisterForProtocolSetup was
   called).

Closing ICE Connections

   To close an ICE connection created with IceOpenConnection or
   IceAcceptConnection use IceCloseConnection

   IceCloseStatus IceCloseConnection(IceConn ice_conn);

   ice_conn

   The ICE connection to close.

   To actually close an ICE connection, the following conditions
   must be met:
     * The open reference count must have reached zero on this ICE
       connection. When IceOpenConnection is called, it tries to
       use a previously opened ICE connection. If it is able to
       use an existing connection, it increments the open
       reference count on the connection by one. So, to close an
       ICE connection, each call to IceOpenConnection must be
       matched with a call to IceCloseConnection The connection
       can be closed only on the last call to IceCloseConnection
     * The active protocol count must have reached zero. Each time
       a ProtocolSetup succeeds on the connection, the active
       protocol count is incremented by one. When the client no
       longer expects to use the protocol on the connection, the
       IceProtocolShutdown function should be called, which
       decrements the active protocol count by one (see Protocol
       Setup and Shutdown).
     * If shutdown negotiation is enabled on the connection, the
       client on the other side of the ICE connection must agree
       to have the connection closed.
       IceCloseConnection returns one of the following values:
     * IceClosedNow - the ICE connection was closed at this time.
       The watch procedures were invoked and the connection was
       freed.
     * IceClosedASAP - an IO error had occurred on the connection,
       but IceCloseConnection is being called within a nested
       IceProcessMessages The watch procedures have been invoked
       at this time, but the connection will be freed as soon as
       possible (when the nesting level reaches zero and
       IceProcessMessages returns a status of
       IceProcessMessagesConnectionClosed
     * IceConnectionInUse - the connection was not closed at this
       time, because it is being used by other active protocols.
     * IceStartedShutdownNegotiation - the connection was not
       closed at this time and shutdown negotiation started with
       the client on the other side of the ICE connection. When
       the connection is actually closed, IceProcessMessages will
       return a status of IceProcessMessagesConnectionClosed

   When it is known that the client on the other side of the ICE
   connection has terminated the connection without initiating
   shutdown negotiation, the IceSetShutdownNegotiation function
   should be called to turn off shutdown negotiation. This will
   prevent IceCloseConnection from writing to a broken connection.

   void IceSetShutdownNegotiation(IceConn ice_conn, Bool
   negotiate);

   ice_conn

   A valid ICE connection object.

   negotiate

   If False shutdown negotiating will be turned off.

   To check the shutdown negotiation status of an ICE connection,
   use IceCheckShutdownNegotiation

   Bool IceCheckShutdownNegotiation(IceConn ice_conn);

   ice_conn

   A valid ICE connection object.

   IceCheckShutdownNegotiation returns True if shutdown
   negotiation will take place on the connection; otherwise, it
   returns False Negotiation is on by default for a connection. It
   can only be changed with the IceSetShutdownNegotiation
   function.

Connection Watch Procedures

   To add a watch procedure that will be called each time ICElib
   opens a new connection via IceOpenConnection or
   IceAcceptConnection or closes a connection via
   IceCloseConnection use IceAddConnectionWatch

   Status IceAddConnectionWatch(IceWatchProc watch_proc,
   IcePointer client_data);

   watch_proc

   The watch procedure to invoke when ICElib opens or closes a
   connection.

   client_data

   This pointer will be passed to the watch procedure.

   The return value of IceAddConnectionWatch is zero for failure,
   and a positive value for success.

   Note that several calls to IceOpenConnection might share the
   same ICE connection. In such a case, the watch procedure is
   only invoked when the connection is first created (after
   authentication succeeds). Similarly, because connections might
   be shared, the watch procedure is called only if
   IceCloseConnection actually closes the connection (right before
   the IceConn is freed).

   The watch procedures are very useful for applications that need
   to add a file descriptor to a select mask when a new connection
   is created and remove the file descriptor when the connection
   is destroyed. Because connections are shared, knowing when to
   add and remove the file descriptor from the select mask would
   be difficult without the watch procedures.

   Multiple watch procedures may be registered with the ICE
   library. No assumptions should be made about their order of
   invocation.

   If one or more ICE connections were already created by the ICE
   library at the time the watch procedure is registered, the
   watch procedure will instantly be invoked for each of these ICE
   connections (with the opening argument set to True

   The watch procedure is of type IceWatchProc

   void WatchProc(IceConn ice_conn, IcePointer client_data, Bool
   opening, IcePointer *watch_data);

   ice_conn

   The opened or closed ICE connection. Call IceConnectionNumber
   to get the file descriptor associated with this connection.

   client_data

   Client data specified in the call to IceAddConnectionWatch

   opening

   If True the connection is being opened. If False the connection
   is being closed.

   watch_data

   Can be used to save a pointer to client data.

   If opening is True the client should set the *watch_data
   pointer to any data it may need to save until the connection is
   closed and the watch procedure is invoked again with opening
   set to False

   To remove a watch procedure, use IceRemoveConnectionWatch

   void IceRemoveConnectionWatch(IceWatchProc watch_proc,
   IcePointer client_data);

   watch_proc

   The watch procedure that was passed to IceAddConnectionWatch

   client_data

   The client_data pointer that was passed to
   IceAddConnectionWatch
     __________________________________________________________

   ^[1] The X Consortium's ICElib implementation uses an
   .ICEauthority file (see Appendix A).

Chapter 8. Protocol Setup and Shutdown

   To activate a protocol on a given ICE connection, use
   IceProtocolSetup

   IceProtocolSetupStatus IceProtocolSetup(IceConn ice_conn, int
   my_opcode, IcePointer client_data, Bool must_authenticate, int
   *major_version_ret, int *minor_version_ret, char **vendor_ret,
   char **release_ret, int error_length, char *error_string_ret);

   ice_conn

   A valid ICE connection object.

   my_opcode

   The major opcode of the protocol to be set up, as returned by
   IceRegisterForProtocolSetup

   client_data

   The client data stored in this pointer will be passed to the
   IcePoProcessMsgProc callback.

   must_authenticate

   If True the other client may not bypass authentication.

   major_version_ret

   The major version of the protocol to be used is returned.

   minor_version_ret

   The minor version of the protocol to be used is returned.

   vendor_ret

   The vendor string specified by the protocol acceptor.

   release_ret

   The release string specified by the protocol acceptor.

   error_length

   Specifies the length of the error_string_ret argument passed
   in.

   error_string_ret

   Returns a null-terminated error message, if any. The
   error_string_ret argument points to user supplied memory. No
   more than error_length bytes are used.

   The vendor_ret and release_ret strings should be freed with
   free when no longer needed.

   IceProtocolSetup returns one of the following values:
     * IceProtocolSetupSuccess - the major_version_ret,
       minor_version_ret, vendor_ret, release_ret are set.
     * IceProtocolSetupFailure or IceProtocolSetupIOError - check
       error_string_ret for failure reason. The major_version_ret,
       minor_version_ret, vendor_ret, release_ret are not set.
     * IceProtocolAlreadyActive - this protocol is already active
       on this connection. The major_version_ret,
       minor_version_ret, vendor_ret, release_ret are not set.

   To notify the ICE library when a given protocol will no longer
   be used on an ICE connection, use IceProtocolShutdown

   Status IceProtocolShutdown(IceConn ice_conn, int major_opcode);

   ice_conn

   A valid ICE connection object.

   major_opcode

   The major opcode of the protocol to shut down.

   The return value of IceProtocolShutdown is zero for failure and
   a positive value for success.

   Failure will occur if the major opcode was never registered OR
   the protocol of the major opcode was never activated on the
   connection. By activated, we mean that a ProtocolSetup
   succeeded on the connection. Note that ICE does not define how
   each sub-protocol triggers a protocol shutdown.

Chapter 9. Processing Messages

   To process incoming messages on an ICE connection, use
   IceProcessMessages

   IceProcessMessagesStatus IceProcessMessages(IceConn ice_conn,
   IceReplyWaitInfo *reply_wait, Bool *reply_ready_ret);

   ice_conn

   A valid ICE connection object.

   reply_wait

   Indicates if a reply is being waited for.

   reply_ready_ret

   If set to True on return, a reply is ready.

   IceProcessMessages is used in two ways:
     * In the first, a client may generate a message and block by
       calling IceProcessMessages repeatedly until it gets its
       reply.
     * In the second, a client calls IceProcessMessages with
       reply_wait set to NULL in response to select showing that
       there is data to read on the ICE connection. The ICE
       library may process zero or more complete messages. Note
       that messages that are not blocked for are always processed
       by invoking callbacks.

   IceReplyWaitInfo contains the major/minor opcodes and sequence
   number of the message for which a reply is being awaited. It
   also contains a pointer to the reply message to be filled in
   (the protocol library should cast this IcePointer to the
   appropriate reply type). In most cases, the reply will have
   some fixed-size part, and the client waiting for the reply will
   have provided a pointer to a structure to hold this fixed-size
   data. If there is variable-length data, it would be expected
   that the IcePoProcessMsgProc callback will have to allocate
   additional memory and store pointer(s) to that memory in the
   fixed-size structure. If the entire data is variable length
   (for example, a single variable-length string), then the client
   waiting for the reply would probably just pass a pointer to
   fixed-size space to hold a pointer, and the IcePoProcessMsgProc
   callback would allocate the storage and store the pointer. It
   is the responsibility of the client receiving the reply to free
   up any memory allocated on its behalf.
typedef struct {
     unsigned long sequence_of_request;
     int major_opcode_of_request;
     int minor_opcode_of_request;
     IcePointer reply;
} IceReplyWaitInfo;

   If reply_wait is not NULL and IceProcessMessages has a reply or
   error to return in response to this reply_wait (that is, no
   callback was generated), then the reply_ready_ret argument will
   be set to True

   If reply_wait is NULL, then the caller may also pass NULL for
   reply_ready_ret and be guaranteed that no value will be stored
   in this pointer.

   IceProcessMessages returns one of the following values:
     * IceProcessMessagesSuccess - no error occurred.
     * IceProcessMessagesIOError - an IO error occurred, and the
       caller must explicitly close the connection by calling
       IceCloseConnection
     * IceProcessMessagesConnectionClosed - the ICE connection has
       been closed (closing of the connection was deferred because
       of shutdown negotiation, or because the IceProcessMessages
       nesting level was not zero). Do not attempt to access the
       ICE connection at this point, since it has been freed.

Chapter 10. Ping

   To send a "Ping" message to the client on the other side of the
   ICE connection, use IcePing

   Status IcePing(IceConn ice_conn, IcePingReplyProc
   ping_reply_proc, IcePointer client_data);

   ice_conn

   A valid ICE connection object.

   ping_reply_proc

   The callback to invoke when the Ping reply arrives.

   client_data

   This pointer will be passed to the IcePingReplyProc callback.

   IcePing returns zero for failure and a positive value for
   success.

   When IceProcessMessages processes the Ping reply, it will
   invoke the IcePingReplyProc callback.

   void PingReplyProc(IceConn ice_conn, IcePointer client_data);

   ice_conn

   A valid ICE connection object.

   client_data

   The client data specified in the call to IcePing

Chapter 11. Using ICElib Informational Functions

   IceConnectStatus IceConnectionStatus(IceConn ice_conn);

   IceConnectionStatus returns the status of an ICE connection.
   The possible return values are:
     * IceConnectPending - the connection is not valid yet (that
       is, authentication is taking place). This is only relevant
       to connections created by IceAcceptConnection
     * IceConnectAccepted - the connection has been accepted. This
       is only relevant to connections created by
       IceAcceptConnection
     * IceConnectRejected - the connection had been rejected (that
       is, authentication failed). This is only relevant to
       connections created by IceAcceptConnection
     * IceConnectIOError - an IO error has occurred on the
       connection.

   char *IceVendor(IceConn ice_conn);

   IceVendor returns the ICE library vendor identification for the
   other side of the connection. The string should be freed with a
   call to free when no longer needed.

   char *IceRelease(IceConn ice_conn);

   IceRelease returns the release identification of the ICE
   library on the other side of the connection. The string should
   be freed with a call to free when no longer needed.

   int IceProtocolVersion(IceConn ice_conn);

   IceProtocolVersion returns the major version of the ICE
   protocol on this connection.

   int IceProtocolRevision(IceConn ice_conn);

   IceProtocolRevision returns the minor version of the ICE
   protocol on this connection.

   int IceConnectionNumber(IceConn ice_conn);

   IceConnectionNumber returns the file descriptor of this ICE
   connection.

   char *IceConnectionString(IceConn ice_conn);

   IceConnectionString returns the network ID of the client that
   accepted this connection. The string should be freed with a
   call to free when no longer needed.

   unsigned long IceLastSentSequenceNumber(IceConn ice_conn);

   IceLastSentSequenceNumber returns the sequence number of the
   last message sent on this ICE connection.

   unsigned long IceLastReceivedSequenceNumber(IceConn ice_conn);

   IceLastReceivedSequenceNumber returns the sequence number of
   the last message received on this ICE connection.

   Bool IceSwapping(IceConn ice_conn);

   IceSwapping returns True if byte swapping is necessary when
   reading messages on the ICE connection.

   IcePointer IceGetContext(IceConn ice_conn);

   IceGetContext returns the context associated with a connection
   created by IceOpenConnection

Chapter 12. ICE Messages

   Table of Contents

   Sending ICE Messages
   Reading ICE Messages

   All ICE messages have a standard 8-byte header. The ICElib
   macros that read and write messages rely on the following
   naming convention for message headers:
     CARD8      major_opcode;
     CARD8      minor_opcode;
     CARD8      data[2];
     CARD32     length;

   The 3rd and 4th bytes of the message header can be used as
   needed. The length field is specified in units of 8 bytes.

Sending ICE Messages

   The ICE library maintains an output buffer used for generating
   messages. Protocol libraries layered on top of ICE may choose
   to batch messages together and flush the output buffer at
   appropriate times.

   If an IO error has occurred on an ICE connection, all write
   operations will be ignored. For further information, see Error
   Handling.

   To get the size of the ICE output buffer, use IceGetOutBufSize

   int IceGetOutBufSize(IceConn ice_conn);

   ice_conn

   A valid ICE connection object.

   To flush the ICE output buffer, use IceFlush

   int IceFlush(IceConn ice_conn);

   ice_conn

   A valid ICE connection object.

   Note that the output buffer may be implicitly flushed if there
   is insufficient space to generate a message.

   The following macros can be used to generate ICE messages:

   IceGetHeader(IceConn ice_conn, int major_opcode, int
   minor_opcode, int header_size, <C_data_type> *pmsg);

   ice_conn

   A valid ICE connection object.

   major_opcode

   The major opcode of the message.

   minor_opcode

   The minor opcode of the message.

   header_size

   The size of the message header (in bytes).

   <C_data_type>

   The actual C data type of the message header.

   pmsg

   The message header pointer. After this macro is called, the
   library can store data in the message header.

   IceGetHeader is used to set up a message header on an ICE
   connection. It sets the major and minor opcodes of the message,
   and initializes the message's length to the length of the
   header. If additional variable length data follows, the
   message's length field should be updated.

   IceGetHeaderExtra(IceConn ice_conn, int major_opcode, int
   minor_opcode, int header_size, int extra, <C_data_type> *pmsg,
   char *pdata);

   ice_conn

   A valid ICE connection object.

   major_opcode

   The major opcode of the message.

   minor_opcode

   The minor opcode of the message.

   header_size

   The size of the message header (in bytes).

   extra

   The size of the extra data beyond the header (in 8-byte units).

   <C_data_type>

   The actual C data type of the message header.

   pmsg

   The message header pointer. After this macro is called, the
   library can store data in the message header.

   pdata

   Returns a pointer to the ICE output buffer that points
   immediately after the message header. The variable length data
   should be stored here. If there was not enough room in the ICE
   output buffer, pdata is set to NULL.

   IceGetHeaderExtra is used to generate a message with a fixed
   (and relatively small) amount of variable length data. The
   complete message must fit in the ICE output buffer.

   IceSimpleMessage(IceConn ice_conn, int major_opcode, int
   minor_opcode);

   ice_conn

   A valid ICE connection object.

   major_opcode

   The major opcode of the message.

   minor_opcode

   The minor opcode of the message.

   IceSimpleMessage is used to generate a message that is
   identical in size to the ICE header message, and has no
   additional data.

   IceErrorHeader(IceConn ice_conn, int offending_major_opcode,
   int offending_minor_opcode, int offending_sequence_num, int
   severity, int error_class, int data_length);

   ice_conn

   A valid ICE connection object.

   offending_major_opcode

   The major opcode of the protocol in which an error was
   detected.

   offending_minor_opcode

   The minor opcode of the protocol in which an error was
   detected.

   offending_sequence_num

   The sequence number of the message that caused the error.

   severity

   IceCanContinue IceFatalToProtocol or IceFatalToConnection

   error_class

   The error class.

   data_length

   Length of data (in 8-byte units) to be written after the
   header.

   IceErrorHeader sets up an error message header.

   Note that the two clients connected by ICE may be using
   different major opcodes for a given protocol. The
   offending_major_opcode passed to this macro is the major opcode
   of the protocol for the client sending the error message.

   Generic errors, which are common to all protocols, have classes
   in the range 0x8000..0xFFFF. See the Inter-Client Exchange
   Protocol standard for more details.
   IceBadMinor  0x8000
   IceBadState  0x8001
   IceBadLength 0x8002
   IceBadValue  0x8003

   Per-protocol errors have classes in the range 0x0000-0x7fff.

   To write data to an ICE connection, use the IceWriteData macro.
   If the data fits into the ICE output buffer, it is copied
   there. Otherwise, the ICE output buffer is flushed and the data
   is directly sent.

   This macro is used in conjunction with IceGetHeader and
   IceErrorHeader

   IceWriteData(IceConn ice_conn, int bytes, char *data);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of bytes to write.

   data

   The data to write.

   To write data as 16-bit quantities, use IceWriteData16

   IceWriteData16(IceConn ice_conn, int bytes, char *data);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of bytes to write.

   data

   The data to write.

   To write data as 32-bit quantities, use IceWriteData32

   IceWriteData32(IceConn ice_conn, int bytes, char *data);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of bytes to write.

   data

   The data to write.

   To write data as 32-bit quantities, use IceWriteData32

   To bypass copying data to the ICE output buffer, use
   IceSendData to directly send data over the network connection.
   If necessary, the ICE output buffer is first flushed.

   IceSendData(IceConn ice_conn, int bytes, char *data);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of bytes to send.

   data

   The data to send.

   To force 32-bit or 64-bit alignment, use IceWritePad A maximum
   of 7 pad bytes can be specified.

   IceWritePad(IceConn ice_conn, int bytes, char *data);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of bytes to write.

   data

   The number of pad bytes to write.

Reading ICE Messages

   The ICE library maintains an input buffer used for reading
   messages. If the ICE library chooses to perform nonblocking
   reads (this is implementation-dependent), then for every read
   operation that it makes, zero or more complete messages may be
   read into the input buffer. As a result, for all of the macros
   described in this section that read messages, an actual read
   operation will occur on the connection only if the data is not
   already present in the input buffer.

   To get the size of the ICE input buffer, use IceGetInBufSize

   int IceGetInBufSize(IceConn ice_conn);

   ice_conn

   A valid ICE connection object.

   When reading messages, care must be taken to check for IO
   errors. If any IO error occurs in reading any part of a
   message, the message should be thrown out. After using any of
   the macros described below for reading messages, the IceValidIO
   macro can be used to check if an IO error occurred on the
   connection. After an IO error has occurred on an ICE
   connection, all read operations will be ignored. For further
   information, see Error Handling.

   Bool IceValidIO(IceConn ice_conn);

   ice_conn

   A valid ICE connection object.

   The following macros can be used to read ICE messages.

   IceReadSimpleMessage(IceConn ice_conn, <C_data_type> *pmsg);

   ice_conn

   A valid ICE connection object.

   <C_data_type>

   The actual C data type of the message header.

   pmsg

   This pointer is set to the message header.

   IceReadSimpleMessage is used for messages that are identical in
   size to the 8-byte ICE header, but use the spare 2 bytes in the
   header to encode additional data. Note that the ICE library
   always reads in these first 8 bytes, so it can obtain the major
   opcode of the message. IceReadSimpleMessage simply returns a
   pointer to these 8 bytes; it does not actually read any data
   into the input buffer.

   For a message with variable length data, there are two ways of
   reading the message. One method involves reading the complete
   message in one pass using IceReadCompleteMessage The second
   method involves reading the message header (note that this may
   be larger than the 8-byte ICE header), then reading the
   variable length data in chunks (see IceReadMessageHeader and
   IceReadData

   IceReadCompleteMessage(IceConn ice_conn, int header_size,
   <C_data_type> *pmsg, char *pdata);

   ice_conn

   A valid ICE connection object.

   header_size

   The size of the message header (in bytes).

   <C_data_type>

   The actual C data type of the message header.

   pmsg

   This pointer is set to the message header.

   pdata

   This pointer is set to the variable length data of the message.

   If the ICE input buffer has sufficient space,
   IceReadCompleteMessage will read the complete message into the
   ICE input buffer. Otherwise, a buffer will be allocated to hold
   the variable length data. After the call, the pdata argument
   should be checked against NULL to make sure that there was
   sufficient memory to allocate the buffer.

   After calling IceReadCompleteMessage and processing the
   message, IceDisposeCompleteMessage should be called.

   IceDisposeCompleteMessage(IceConn ice_conn, char *pdata);

   ice_conn

   A valid ICE connection object.

   pdata

   The pointer to the variable length data returned in
   IceReadCompleteMessage

   If a buffer had to be allocated to hold the variable length
   data (because it did not fit in the ICE input buffer), it is
   freed here by ICElib.

   IceReadMessageHeader(IceConn ice_conn, int header_size,
   <C_data_type> *pmsg);

   ice_conn

   A valid ICE connection object.

   header_size

   The size of the message header (in bytes).

   <C_data_type>

   The actual C data type of the message header.

   pmsg

   This pointer is set to the message header.

   IceReadMessageHeader reads just the message header. The rest of
   the data should be read with the IceReadData family of macros.
   This method of reading a message should be used when the
   variable length data must be read in chunks.

   To read data directly into a user supplied buffer, use
   IceReadData

   IceReadData(IceConn ice_conn, int bytes, char *pdata);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of bytes to read.

   pdata

   The data is read into this user supplied buffer.

   To read data as 16-bit quantities, use IceReadData16

   IceReadData16(IceConn ice_conn, Bool swap, int bytes, char
   *pdata);

   ice_conn

   A valid ICE connection object.

   swap

   If True, the values will be byte swapped.

   bytes

   The number of bytes to read.

   pdata

   The data is read into this user supplied buffer.

   To read data as 32-bit quantities, use IceReadData32

   IceReadData32(IceConn ice_conn, Bool swap, int bytes, char
   *pdata);

   ice_conn

   A valid ICE connection object.

   swap

   If True, the values will be byte swapped.

   bytes

   The number of bytes to read.

   pdata

   The data is read into this user supplied buffer.

   To force 32-bit or 64-bit alignment, use IceReadPad A maximum
   of 7 pad bytes can be specified.

   IceReadPad(IceConn ice_conn, int bytes);

   ice_conn

   A valid ICE connection object.

   bytes

   The number of pad bytes.

Chapter 13. Error Handling

   There are two default error handlers in ICElib:
     * One to handle typically fatal conditions (for example, a
       connection dying because a machine crashed)
     * One to handle ICE-specific protocol errors

   These error handlers can be changed to user-supplied routines
   if you prefer your own error handling and can be changed as
   often as you like.

   To set the ICE error handler, use IceSetErrorHandler

   IceSetErrorHandler(IceConn ice_conn, int bytes);

   handler

   The ICE error handler. You should pass NULL to restore the
   default handler.

   IceSetErrorHandler returns the previous error handler.

   The ICE error handler is invoked when an unexpected ICE
   protocol error (major opcode 0) is encountered. The action of
   the default handler is to print an explanatory message to
   stderr and if the severity is fatal, call exit with a nonzero
   value. If exiting is undesirable, the application should
   register its own error handler.

   Note that errors in other protocol domains should be handled by
   their respective libraries (these libraries should have their
   own error handlers).

   An ICE error handler has the type of IceErrorHandler

   void IceErrorHandler(IceConn ice_conn, Bool swap, int
   offending_minor_opcode, unsigned long offending_sequence_num,
   int error_class, int severity, IcePointer values);

   handler

   The ICE connection object.

   swap

   A flag that indicates if the values need byte swapping.

   offending_minor_opcode

   The ICE minor opcode of the offending message.

   offending_sequence_num

   The sequence number of the offending message.

   error_class

   The error class of the offending message.

   severity

   IceCanContinue IceFatalToProtocol or IceFatalToConnection

   values

   Any additional error values specific to the minor opcode and
   class.

   The following error classes are defined at the ICE level:

   IceBadMinor
   IceBadState
   IceBadLength
   IceBadValue
   IceBadMajor
   IceNoAuth
   IceNoVersion
   IceSetupFailed
   IceAuthRejected
   IceAuthFailed
   IceProtocolDuplicate
   IceMajorOpcodeDuplicate
   IceUnknownProtocol

   For further information, see the Inter-Client Exchange Protocol
   standard.

   To handle fatal I/O errors, use IceSetIOErrorHandler

   IceIOErrorHandler IceSetIOErrorHandler(IceIOErrorHandler
   handler);

   handler

   The I/O error handler. You should pass NULL to restore the
   default handler.

   IceSetIOErrorHandler returns the previous IO error handler.

   An ICE I/O error handler has the type of IceIOErrorHandler

   void IceIOErrorHandler(IceConn ice_conn);

   ice_conn

   The ICE connection object.

   There are two ways of handling IO errors in ICElib:
     * In the first, the IO error handler does whatever is
       necessary to respond to the IO error and then returns, but
       it does not call IceCloseConnection The ICE connection is
       given a "bad IO" status, and all future reads and writes to
       the connection are ignored. The next time
       IceProcessMessages is called it will return a status of
       IceProcessMessagesIOError At that time, the application
       should call IceCloseConnection
     * In the second, the IO error handler does call
       IceCloseConnection and then uses the longjmp call to get
       back to the application's main event loop. The setjmp and
       longjmp calls may not work properly on all platforms, and
       special care must be taken to avoid memory leaks.
       Therefore, this second model is less desirable.

   Before the application I/O error handler is invoked, protocol
   libraries that were interested in being notified of I/O errors
   will have their IceIOErrorProc handlers invoked. This handler
   is set up in the protocol registration functions (see
   IceRegisterForProtocolSetup and IceRegisterForProtocolReply and
   could be used to clean up state specific to the protocol.

   void IceIOErrorProc(IceConn ice_conn);

   ice_conn

   The ICE connection object.

   Note that every IceIOErrorProc callback must return. This is
   required because each active protocol must be notified of the
   broken connection, and the application IO error handler must be
   invoked afterwards.

Chapter 14. Multi-Threading Support

   To declare that multiple threads in an application will be
   using the ICE library, use IceInitThreads

   Status IceInitThreads()

   The IceInitThreads function must be the first ICElib function a
   multi-threaded program calls. It must complete before any other
   ICElib call is made. IceInitThreads returns a nonzero status if
   and only if it was able to initialize the threads package
   successfully. It is safe to call IceInitThreads more than once,
   although the threads package will only be initialized once.

   Protocol libraries layered on top of ICElib will have to lock
   critical sections of code that access an ICE connection (for
   example, when generating messages). Two calls, which are
   generally implemented as macros, are provided:

   void IceLockConn(IceConn ice_conn);

   void IceUnlockConn(IceConn ice_conn);

   ice_conn

   The ICE connection object.

   To keep an ICE connection locked across several ICElib calls,
   applications use IceAppLockConn and IceAppUnlockConn

   void IceAppLockConn(IceConn ice_conn);

   ice_conn

   The ICE connection object.

   The IceAppLockConn function completely locks out other threads
   using the connection until IceAppUnlockConn is called. Other
   threads attempting to use ICElib calls on the connection will
   block. If the program has not previously called IceInitThreads
   IceAppLockConn has no effect.

   void IceAppUnlockConn(IceConn ice_conn);

   ice_conn

   The ICE connection object.

   The IceAppUnlockConn function allows other threads to complete
   ICElib calls on the connection that were blocked by a previous
   call to IceAppLockConn from this thread. If the program has not
   previously called IceInitThreads IceAppUnlockConn has no
   effect.

Chapter 15. Miscellaneous Functions

   To allocate scratch space (for example, when generating
   messages with variable data), use IceAllocScratch Each ICE
   connection has one scratch space associated with it. The
   scratch space starts off as empty and grows as needed. The
   contents of the scratch space is not guaranteed to be preserved
   after any ICElib function is called.

   char *IceAllocScratch(IceConn ice_conn, unsigned long size);

   ice_conn

   The ICE connection object.

   size

   The number of bytes required.

   Note that the memory returned by IceAllocScratch should not be
   freed by the caller. The ICE library will free the memory when
   the ICE connection is closed.

Chapter 16. Acknowledgements

   Thanks to Bob Scheifler for his thoughtful input on the design
   of the ICE library. Thanks also to Jordan Brown, Larry Cable,
   Donna Converse, Clive Feather, Stephen Gildea, Vania Joloboff,
   Kaleb Keithley, Stuart Marks, Hiro Miyamoto, Ralph Swick, Jim
   VanGilder, and Mike Wexler.

Appendix A. Authentication Utility Functions

   As discussed in this document, the means by which
   authentication data is obtained by the ICE library (for
   ConnectionSetup messages or ProtocolSetup messages) is
   implementation-dependent./- ^[2]

   This appendix describes some utility functions that manipulate
   an ICE authority file. The authority file can be used to pass
   authentication data between clients.

   The basic operations on the .ICEauthority file are:
     * Get file name
     * Lock
     * Unlock
     * Read entry
     * Write entry
     * Search for entry

   These are fairly low-level operations, and it is expected that
   a program, like "iceauth", would exist to add, remove, and
   display entries in the file.

   In order to use these utility functions, the
   <X11/ICE/ICEutil.h> header file must be included.

   An entry in the .ICEauthority file is defined by the following
   data structure:
typedef struct {
     char *protocol_name;
     unsigned short protocol_data_length;
     char *protocol_data;
     char *network_id;
     char *auth_name;
     unsigned short auth_data_length;
     char *auth_data;
} IceAuthFileEntry;

   The protocol_name member is either "ICE" for connection setup
   authentication or the subprotocol name, such as "XSMP". For
   each entry, protocol specific data can be specified in the
   protocol_data member. This can be used to search for old
   entries that need to be removed from the file.

   The network_id member is the network ID of the client accepting
   authentication (for example, the network ID of a session
   manager). A network ID has the following form:
   tcp/<hostname>:<portnumber>  or
   decnet/<hostname>::<objname> or
   local/<hostname>:<path>

   The auth_name member is the name of the authentication method.
   The auth_data member is the actual authentication data, and the
   auth_data_length member is the number of bytes in the data.

   To obtain the default authorization file name, use
   IceAuthFileName

   char *IceAuthFileName()

   If the ICEAUTHORITY environment variable if set, this value is
   returned. Otherwise, the default authorization file name is
   $HOME/.ICEauthority. This name is statically allocated and
   should not be freed.

   To synchronously update the authorization file, the file must
   be locked with a call to IceLockAuthFile This function takes
   advantage of the fact that the link system call will fail if
   the name of the new link already exists.

   int IceLockAuthFile(char *file_name, int retries, int timeout,
   long dead);

   file_name

   The authorization file to lock.

   retries

   The number of retries.

   timeout

   The number of seconds before each retry.

   dead

   If a lock already exists that is the specified dead seconds
   old, it is broken. A value of zero is used to unconditionally
   break an old lock.

   One of three values is returned:
     * IceAuthLockSuccess - the lock succeeded.
     * IceAuthLockError - a system error occurred, and errno may
       prove useful.
     * IceAuthLockTimeout - the specified number of retries
       failed.

   To unlock an authorization file, use IceUnlockAuthFile

   int IceUnlockAuthFile(char *file_name);

   file_name

   The authorization file to unlock.

   To read the next entry in an authorization file, use
   IceReadAuthFileEntry

   IceAuthFileEntry *IceReadAuthFileEntry(FILE *auth_file);

   auth_file

   The authorization file.

   Note that it is the responsibility of the application to open
   the file for reading before calling this function. If an error
   is encountered, or there are no more entries to read, NULL is
   returned.

   Entries should be free with a call to IceFreeAuthFileEntry

   To write an entry in an authorization file, use
   IceWriteAuthFileEntry

   Status IceWriteAuthFileEntry(FILE *auth_file, IceAuthFileEntry
   *entry);

   auth_file

   The authorization file.

   entry

   The entry to write.

   Note that it is the responsibility of the application to open
   the file for writing before calling this function. The function
   returns a nonzero status if the operation was successful.

   To search the default authorization file for an entry that
   matches a given protocol_name/network_id/auth_name tuple, use
   IceGetAuthFileEntry

   IceAuthFileEntry *IceGetAuthFileEntry(const char
   *protocol_name, const char *network_id, const char *auth_name);

   auth_file

   The name of the protocol to search on.

   network_id

   The network ID to search on.

   auth_name

   The authentication method to search on.

   If IceGetAuthFileEntry fails to find such an entry, NULL is
   returned.

   To free an entry returned by IceReadAuthFileEntry or
   IceGetAuthFileEntry use IceFreeAuthFileEntry

   void IceFreeAuthFileEntry(IceAuthFileEntry *entry);

   entry

   The entry to free.
     __________________________________________________________

   ^[2] The X Consortium's ICElib implementation assumes the
   presence of an ICE authority file.

Appendix B. MIT-MAGIC-COOKIE-1 Authentication

   The X Consortium's ICElib implementation supports a simple
   MIT-MAGIC-COOKIE-1 authentication scheme using the authority
   file utilities described in Appendix A.

   In this model, an application, such as a session manager,
   obtains a magic cookie by calling IceGenerateMagicCookie and
   then stores it in the user's local .ICEauthority file so that
   local clients can connect. In order to allow remote clients to
   connect, some remote execution mechanism should be used to
   store the magic cookie in the user's .ICEauthority file on a
   remote machine.

   In addition to storing the magic cookie in the .ICEauthority
   file, the application needs to call the IceSetPaAuthData
   function in order to store the magic cookie in memory. When it
   comes time for the MIT-MAGIC-COOKIE-1 authentication procedure
   to accept or reject the connection, it will compare the magic
   cookie presented by the requestor to the magic cookie in
   memory.

   char *IceGenerateMagicCookie(int length);

   length

   The desired length of the magic cookie.

   The magic cookie returned will be null-terminated. If memory
   can not be allocated for the magic cookie, the function will
   return NULL. Otherwise, the magic cookie should be freed with a
   call to free

   To store the authentication data in memory, use
   IceSetPaAuthData Currently, this function is only used for
   MIT-MAGIC-COOKIE-1 authentication, but it may be used for
   additional authentication methods in the future.

   void IceSetPaAuthData(int num_entries, IceAuthDataEntry
   *entries);

   num_entries

   The number of authentication data entries.

   entries

   The list of authentication data entries.

   Each entry has associated with it a protocol name (for example,
   "ICE" for ICE connection setup authentication, "XSMP" for
   session management authentication), a network ID for the
   "accepting" client, an authentication name (for example,
   MIT-MAGIC-COOKIE-1), and authentication data. The ICE library
   will merge these entries with previously set entries, based on
   the (protocol_name, network_id, auth_name) tuple.
typedef struct {
        char *protocol_name;
        char *network_id;
        char *auth_name;
        unsigned short auth_data_length;
        char *auth_data;
} IceAuthDataEntry;
