Inter-Client Exchange Library

X Consortium Standard

Ralph Mor

X Consortium

X Version 11, Release 7.7

Version 1.0

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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.

                       A callback to be invoked when authentication has
protocol_setup_proc    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.

                  If True authentication is over, and the function should clean
clean_up          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.

                  If the authentication procedure encounters an error during
error_string_ret  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.

                   Returns a null-terminated error message, if any. The
error_string_ret   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.

                 Returns a null-terminated error message, if any. The
error_string_ret 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);

                 Specifies the port identification for the address(es) to be
port_id          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.

                 Returns a null-terminated error message, if any. The
error_string_ret 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.

                  Returns a null-terminated error message, if any. The
error_string_ret  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 B32;

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.

              Returns a pointer to the ICE output buffer that points
pdata         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.

          If a lock already exists that is the specified dead seconds old, it
dead      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;

