X Transport Interface

X Consortium Standard

Stuart Anderson

NCR Corporation

Ralph Mor

X Consortium

Alan Coopersmith

Oracle Corp.

X Version 11, Release 7.7

Version 1.2

Copyright © 1993, 1994 NCR Corporation - Dayton, Ohio, USA

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Table of Contents

The X Transport Interface
1. Purposes and Goals
2. Overview of the Interface
3. Definition of Address Specification Format
4. Internal Data Structures

    Xtransport
    XtransConnInfo

5. Exposed Transport Independent API

    Core Interface API
    Utility API

6. Transport Option Definition
7. Hidden Transport Dependent API
8. Configuration
9. Transport Specific Definitions
10. Implementation Notes

The X Transport Interface

Designed by Stuart Anderson (NCR) with help from Ralph Mor (X Consortium)

Note

This documentation does not completely match the implementation in R6 (as a
result of some late changes made in the code). Specifically, support was added
for font server cloning, and conditional compliation was introduced for client
vs. server code.

Chapter 1. Purposes and Goals

The X Transport Interface is intended to combine all system and transport
specific code into a single place in the source tree. This API should be used
by all libraries, clients and servers of the X Window System. Use of this API
should allow the addition of new types of transports and support for new
platforms without making any changes to the source except in the X Transport
Interface code.

This interface should solve the problem of multiple #ifdef TRANSPORT and #ifdef
PLATFORM statements scattered throughout the source tree.

This interface should provide enough functionality to support all types of
protocols, including connection oriented protocols such as X11 and FS, and
connection-less oriented protocols such as XDMCP.

Chapter 2. Overview of the Interface

The interface provides an API for use by applications. The functions in this
API perform work that is common to all transports and systems, such as parsing
an address into a host and port number. The functions in this API call
transport specific functions that are contained in a table whose contents are
defined at compile time. This table contains an entry for each type of
transport. Each entry is a record containing mostly pointers to function that
implements the interface for the given transport.

This API does not provide an abstraction for select() or poll(). These
functions are themselves transport independent, so an additional interface is
not needed for these functions. It is also unclear how such an interface would
affect performance.

Chapter 3. Definition of Address Specification Format

Addresses are specified in the following syntax,

protocol/host:port

where protocol specifies a protocol family or an alias for a protocol family. A
definition of common protocol families is given in a later section.

The host part specifies the name of a host or other transport dependent entity
that could be interpreted as a Network Service Access Point (NSAP).

The port part specifies the name of a Transport Service Access Point (TSAP).
The format of the TSAP is defined by the underlying transport implementation,
but it is represented using a string format when it is part of an address.

Chapter 4. Internal Data Structures

Table of Contents

Xtransport
XtransConnInfo

There are two major data structures associated with the transport independent
portion of this interface. Additional data structures may be used internally by
each transport.

Xtransport

Each transport supported has an entry in the transport table. The transport
table is an array of Xtransport records. Each record contains all the entry
points for a single transport. This record is defined as:

typedef struct _Xtransport {

    const char *TransName;
    int  flags;

    XtransConnInfo (*OpenCOTSClient)(
     struct _Xtransport *,    /* transport */
     const char *,            /* protocol */
     const char *,            /* host */
     const char *             /* port */
    );

    XtransConnInfo (*OpenCOTSServer)(
     struct _Xtransport *,    /* transport */
     const char *,            /* protocol */
     const char *,            /* host */
     const char *             /* port */
    );

    int     (*SetOption)(
     XtransConnInfo,          /* connection */
     int,               /* option */
     int               /* arg */
    );

    int     (*CreateListener)(
     XtransConnInfo,          /* connection */
     const char *,            /* port */
     int                      /* flags */
    );

    int     (*ResetListener)(
     XtransConnInfo          /* connection */
    );

    XtransConnInfo (*Accept)(
     XtransConnInfo          /* connection */
    );

    int     (*Connect)(
     XtransConnInfo,          /* connection */
     const char *,            /* host */
     const char *             /* port */
    );

    int     (*BytesReadable)(
     XtransConnInfo,          /* connection */
     BytesReadable_t *     /* pend */
    );

    int     (*Read)(
     XtransConnInfo,          /* connection */
     char *,               /* buf */
     int               /* size */
    );

    int     (*Write)(
     XtransConnInfo,          /* connection */
     const char *,            /* buf */
     int               /* size */
    );

    int     (*Readv)(
     XtransConnInfo,          /* connection */
     struct iovec *,          /* buf */
     int               /* size */
    );

    int     (*Writev)(
     XtransConnInfo,          /* connection */
     struct iovec *,          /* buf */
     int               /* size */
    );

    int     (*Disconnect)(
     XtransConnInfo          /* connection */
    );

    int     (*Close)(
     XtransConnInfo          /* connection */
    );

} Xtransport;

The flags field can contain an OR of the following masks:

TRANS_ALIAS    indicates that this record is providing an alias, and should not
               be used to create a listener.

TRANS_LOCAL    indicates that this is a LOCALCONN transport.

TRANS_ABSTRACT indicates that a local connection transport uses the abstract
               socket namespace.

Some additional flags may be set in the flags field by the library while it is
running:

TRANS_DISABLED indicates that this transport has been disabled.

TRANS_NOLISTEN indicates that servers should not open new listeners using this
               transport.

TRANS_NOUNLINK set by a transport backend to indicate that the endpoints for
               its connection should not be unlinked.

XtransConnInfo

Each connection will have an opaque XtransConnInfo transport connection object
allocated for it. This record contains information specific to the connection.
The record is defined as:

typedef struct _XtransConnInfo *XtransConnInfo;

struct _XtransConnInfo {
    struct _Xtransport     *transptr;
    char     *priv;
    int          flags;
    int          fd;
    int          family;
    char     *addr;
    int          addrlen;
    char     *peeraddr;
    int          peeraddrlen;
};

Chapter 5. Exposed Transport Independent API

Table of Contents

Core Interface API
Utility API

This API is included in each library and server that uses it. The API may be
used by the library, but it is not added to the public API for that library.
This interface is simply an implementation facilitator. This API contains a low
level set of core primitives, and a few utility functions that are built on top
of the primitives. The utility functions exist to provide a more familiar
interface that can be used to port existing code.

A macro is defined in Xtrans.h for TRANS(func) that creates a unique function
name depending on where the code is compiled. For example, when built for Xlib,
TRANS(OpenCOTSClient) becomes _X11TransOpenCOTSClient.

All failures are considered fatal, and the connection should be closed and
re-established if desired. In most cases, however, the value of errno will be
available for debugging purposes.

Core Interface API

  • XtransConnInfo TRANS(OpenCOTSClient)(const char *address);

    This function creates a Connection-Oriented Transport that is suitable for
    use by a client. The parameter address contains the full address of the
    server to which this endpoint will be connected. This function returns an
    opaque transport connection object on success, or NULL on failure.

  • XtransConnInfo TRANS(OpenCOTSServer)(const char *address);

    This function creates a Connection-Oriented Transport that is suitable for
    use by a server. The parameter address contains the full address to which
    this server will be bound. This function returns an opaque transport
    connection object on success, or NULL on failure.

  • int TRANS(SetOption)(XtransConnInfo connection, int option, int arg);

    This function sets transport options, similar to the way setsockopt() and
    ioctl() work. The parameter connection is an endpoint that was obtained
    from _XTransOpen*() functions. The parameter option contains the option
    that will be set. The actual values for option are defined in a later
    section. The parameter arg can be used to pass in an additional value that
    may be required by some options. This function returns 0 on success and -1
    on failure.

    Note

    Based on current usage, the complimentary function TRANS(GetOption) is not
    necessary.

  • int TRANS(CreateListener)(XtransConnInfo connection, const char *port, int
    flags);

    This function sets up the server endpoint for listening. The parameter
    connection is an endpoint that was obtained from TRANS(OpenCOTSServer)().
    The parameter port specifies the port to which this endpoint should be
    bound for listening. If port is NULL, then the transport may attempt to
    allocate any available TSAP for this connection. If the transport cannot
    support this, then this function will return a failure. The flags parameter
    can be set to ADDR_IN_USE_ALLOWED to allow the call to the underlying
    binding function to fail with a EADDRINUSE error without causing the TRANS
    (CreateListener) function itself to fail. This function return 0 on success
    and -1 on failure.

  • int TRANS(ResetListener)(XtransConnInfo connection);

    When a server is restarted, certain listen ports may need to be reset. For
    example, unix domain needs to check that the file used for communication
    has not been deleted. If it has, it must be recreated. The parameter
    connection is an opened and bound endpoint that was obtained from TRANS
    (OpenCOTSServer)() and passed to TRANS(CreateListener)(). This function
    will return one of the following values: TRANS_RESET_NOOP, 
    TRANS_RESET_NEW_FD, or TRANS_RESET_FAILURE.

  • XtransConnInfo TRANS(Accept)(XtransConnInfo connection);

    Once a connection indication is received, this function can be called to
    accept the connection. The parameter connection is an opened and bound
    endpoint that was obtained from TRANS(OpenCOTSServer)() and passed to TRANS
    (CreateListener)(). This function will return a new opaque transport
    connection object upon success, NULL otherwise.

  • int TRANS(Connect)(XtransConnInfo connection, const char *address);

    This function creates a connection to a server. The parameter connection is
    an endpoint that was obtained from TRANS(OpenCOTSClient)(). The parameter
    address specifies the TSAP to which this endpoint should connect. If the
    protocol is included in the address, it will be ignored. This function
    returns 0 on success and -1 on failure.

  • int TRANS(BytesReadable)(XtransConnInfo connection, BytesReadable_t *pend);

    This function provides the same functionality as the BytesReadable macro.

  • int TRANS(Read)(XtransConnInfo connection, char *buf, int size);

    This function will return the number of bytes requested on a COTS
    connection, and will return the minimum of the number bytes requested.

  • int TRANS(Write)(XtransConnInfo connection, const char *buf, int size);

    This function will write the requested number of bytes on a COTS
    connection.

  • int TRANS(Readv)(XtransConnInfo connection, struct iovec *buf, int size);

    Similar to TRANS(Read)().

  • int TRANS(Writev)(XtransConnInfo connection, struct iovec *buf, int size);

    Similar to TRANS(Write)().

  • int TRANS(Disconnect)(XtransConnInfo connection);

    This function is used when an orderly disconnect is desired. This function
    breaks the connection on the transport. It is similar to the socket
    function shutdown().

  • int TRANS(Close)(XtransConnInfo connection);

    This function closes the transport, unbinds it, and frees all resources
    that was associated with the transport. If a TRANS(Disconnect) call was not
    made on the connection, a disorderly disconnect may occur.

  • int TRANS(IsLocal)(XtransConnInfo connection);

    Returns TRUE if it is a local transport.

  • int TRANS(GetMyAddr)(XtransConnInfo connection, int *familyp, int
    *addrlenp, Xtransaddr **addrp);

    This function is similar to getsockname(). This function will allocate
    space for the address, so it must be freed by the caller. Not all
    transports will have a valid address until a connection is established.
    This function should not be used until the connection is established with
    Connect() or Accept().

  • int TRANS(GetPeerAddr)(XtransConnInfo connection, int *familyp, int
    *addrlenp, Xtransaddr **addrp);

    This function is similar to getpeername(). This function will allocate
    space for the address, so it must be freed by the caller. Not all
    transports will have a valid address until a connection is established.
    This function should not be used until the connection is established with
    Connect() or Accept().

  • int TRANS(GetConnectionNumber)(XtransConnInfo connection);

    Returns the file descriptor associated with this transport.

  • int TRANS(MakeAllCOTSServerListeners)(const char *port, int *partial_ret,
    int *count_ret, XtransConnInfo **connections_ret);

    This function should be used by most servers. It will try to establish a
    COTS server endpoint for each transport listed in the transport table.
    partial_ret will be set to True if only a partial network could be created.
    count_ret is the number of transports returned, and connections_ret is the
    list of transports.

Utility API

This section describes a few useful functions that have been implemented on top
of the Core Interface API. These functions are being provided as a convenience.

  • int TRANS(ConvertAddress)(int *familyp, int *addrlenp, Xtransaddr *addrp);

    This function converts a sockaddr based address to an X authorization based
    address (ie AF_INET, AF_UNIX to the X protocol definition (ie 
    FamilyInternet, FamilyLocal)).

Chapter 6. Transport Option Definition

The following options are defined for the TRANS(SetOption)() function. If an OS
or transport does not support any of these options, then it will silently
ignore the option.

  • TRANS_NONBLOCKING

    This option controls the blocking mode of the connection. If the argument
    is set to 1, then the connection will be set to blocking. If the argument
    is set to 0, then the connection will be set to non- blocking.

  • TRANS_CLOSEONEXEC

    This option determines what will happen to the connection when an exec is
    encountered. If the argument is set to 1, then the connection will be
    closed when an exec occurs. If the argument is set to 0, then the
    connection will not be closed when an exec occurs.

Chapter 7. Hidden Transport Dependent API

The hidden transport dependent functions are placed in the Xtransport record.
These function are similar to the Exposed Transport Independent API, but some
of the parameters and return values are slightly different. Stuff like the #
ifdef SUNSYSV should be handled inside these functions.

  • XtransConnInfo *OpenCOTSClient(struct _Xtransport *thistrans, const char
    *protocol, const char *host, const char *port);

    This function creates a Connection-Oriented Transport. The parameter
    thistrans points to an Xtransport entry in the transport table. The
    parameters protocol, host, and port, point to strings containing the
    corresponding parts of the address that was passed into TRANS
    (OpenCOTSClient)(). This function must allocate and initialize the contents
    of the XtransConnInfo structure that is returned by this function. This
    function will open the transport, and bind it into the transport namespace
    if applicable. The local address portion of the XtransConnInfo structure
    will also be filled in by this function.

  • XtransConnInfo *OpenCOTSServer(struct _Xtransport *thistrans, const char
    *protocol, const char *host, const char *port);

    This function creates a Connection-Oriented Transport. The parameter
    thistrans points to an Xtransport entry in the transport table. The
    parameters protocol, host, and port point to strings containing the
    corresponding parts of the address that was passed into TRANS
    (OpenCOTSServer)(). This function must allocate and initialize the contents
    of the XtransConnInfo structure that is returned by this function. This
    function will open the transport.

  • int SetOption(struct _Xtransport *thistrans, int option, int arg);

    This function provides a transport dependent way of implementing the
    options defined by the X Transport Interface. In the current prototype,
    this function is not being used, because all of the options defined so far
    are transport independent. This function will have to be used if a
    radically different transport type is added, or a transport dependent
    option is defined.

  • int CreateListener(struct _Xtransport *thistrans, const char *port, int
    flags);

    This function takes a transport endpoint opened for a server, and sets it
    up to listen for incoming connection requests. The parameter port contains
    the port portion of the address that was passed to the Open function. The
    parameter flags should be set to ADDR_IN_USE_ALLOWED if the underlying
    transport endpoint may be already bound and this should not be considered
    as an error. Otherwise flags should be set to 0. This is used by IPv6 code,
    where the same socket can be bound to both an IPv6 address and then to a
    IPv4 address. This function will bind the transport into the transport name
    space if applicable, and fill in the local address portion of the
    XtransConnInfo structure. The transport endpoint will then be set to listen
    for incoming connection requests.

  • int ResetListener(struct _Xtransport *thistrans);

    This function resets the transport for listening.

  • XtransConnInfo Accept(struct _Xtransport *thistrans);

    This function creates a new transport endpoint as a result of an incoming
    connection request. The parameter thistrans is the endpoint that was opened
    for listening by the server. The new endpoint is opened and bound into the
    transport’s namespace. A XtransConnInfo structure describing the new
    endpoint is returned from this function

  • int Connect(struct _Xtransport *thistrans, const char *host, const char
    *port);

    This function establishes a connection to a server. The parameters host and
    port describe the server to which the connection should be established. The
    connection will be established so that Read() and Write() call can be made.

  • int BytesReadable(struct _Xtransport *thistrans, BytesReadable_t *pend);

    This function replaces the BytesReadable() macro. This allows each
    transport to have its own mechanism for determining how much data is ready
    to be read.

  • int Read(struct _Xtransport *thistrans, char *buf, int size);

    This function reads size bytes into buf from the connection.

  • int Write(struct _Xtransport *thistrans, char *buf, int size);

    This function writes size bytes from buf to the connection.

  • int Readv(struct _Xtransport *thistrans, struct iovec *buf, int size);

    This function performs a readv() on the connection.

  • int Writev(struct _Xtransport *thistrans, struct iovec *buf, int size);

    This function performs a writev() on the connection.

  • int Disconnect(struct _Xtransport *thistrans);

    This function initiates an orderly shutdown of a connection. If a transport
    does not distinguish between orderly and disorderly disconnects, then a
    call to this function will have no affect.

  • int Close(struct _Xtransport *thistrans);

    This function will break the connection, and close the endpoint.

Chapter 8. Configuration

The implementation of each transport can be platform specific. It is expected
that existing connection types such as TCPCONN, UNIXCONN, LOCALCONN, and 
STREAMSCONN will be replaced with flags for each possible transport type.

In X11R6, the below flags to enable transport types were set in ConnectionFlags
in the vendor.cf or site.def config files.

In X11R7 modular releases, these flags are set when running configure scripts
which include the XTRANS_CONNECTION_FLAGS macro from xtrans.m4.

┌───────────┬────────────────────────┬────────────────────────────────────────┐
│#define    │configure flag          │Description                             │
├───────────┼────────────────────────┼────────────────────────────────────────┤
│TCPCONN    │--enable-tcp-transport  │Enables the INET (IPv4) Domain Socket   │
│           │                        │based transport                         │
├───────────┼────────────────────────┼────────────────────────────────────────┤
│IPv6       │--enable-ipv6           │Extends TCPCONN to enable IPv6 Socket   │
│           │                        │based transport                         │
├───────────┼────────────────────────┼────────────────────────────────────────┤
│UNIXCONN   │--enable-unix-transport │Enables the UNIX Domain Socket based    │
│           │                        │transport                               │
├───────────┼────────────────────────┼────────────────────────────────────────┤
│STREAMSCONN│Not available in X11R7  │Enables the TLI based transports        │
├───────────┼────────────────────────┼────────────────────────────────────────┤
│LOCALCONN  │--enable-local-transport│Enables the SYSV Local connection       │
│           │                        │transports                              │
├───────────┼────────────────────────┼────────────────────────────────────────┤
│DNETCONN   │Not available in X11R7  │Enables the DECnet transports           │
└───────────┴────────────────────────┴────────────────────────────────────────┘

Chapter 9. Transport Specific Definitions

┌────────┬────────────────────────────────────────────────────────────────────┐
│Protocol│                         Address Component                          │
│ Family ├──────────┬────────────────┬────────────────────────────────────────┤
│        │ protocol │      host      │                  port                  │
├────────┼──────────┼────────────────┼────────────────────────────────────────┤
│        │inet inet6│   name of an   │string containing the name of a service │
│Internet│ tcp udp  │    internet    │    or a valid port number. Example:    │
│        │          │addressable host│           "xserver0", "7100"           │
├────────┼──────────┼────────────────┼────────────────────────────────────────┤
│ DECnet │  decnet  │name of a DECnet│ string containing the complete name of │
│        │          │addressable host│      the object. Example: "X$X0"       │
├────────┼──────────┼────────────────┼────────────────────────────────────────┤
│        │          │   name of a    │                                        │
│NETware │   ipx    │    NETware     │     Not sure of the specifics yet.     │
│        │          │addressable host│                                        │
├────────┼──────────┼────────────────┼────────────────────────────────────────┤
│        │          │ name of an OSI │                                        │
│  OSI   │   osi    │  addressable   │     Not sure of the specifics yet.     │
│        │          │     host.      │                                        │
├────────┼──────────┼────────────────┼────────────────────────────────────────┤
│        │local pts │                │  String containing the port name, ie   │
│ Local  │named sco │   (ignored)    │       "xserver0", "fontserver0".       │
│        │   isc    │                │                                        │
└────────┴──────────┴────────────────┴────────────────────────────────────────┘

Chapter 10. Implementation Notes

This section refers to the prototype implementation that is being developed
concurrently with this document. This prototype has been able to flush out many
details and problems as the specification was being developed.

In X11R6, all of the source code for this interface was located in xc/lib/
xtrans.

In X11R7, all of the source code for this interface is delivered via the lib/
libxtrans modular package from X.Org, and is installed under ${prefix}/X11/
Xtrans so that other modules may find it when they build.

All functions names in the source are of the format TRANS(func)(). The TRANS()
macro is defined as

#define TRANS(func) _PROTOCOLTrans##func

PROTOCOL will be uniquely defined in each directory where this code is
compiled. PROTOCOL will be defined to be the name of the protocol that is
implemented by the library or server, such as X11, FS, and ICE.

All libraries and servers that use the X Transport Interface should have a new
file called TRANSPORTtrans.c. This file will include the transports based on
the configuration flags provided by the configure script. Below is an example
xfstrans.c for the font server.

#include "config.h"

#define FONT_t 1
#define TRANS_REOPEN 1
#define TRANS_SERVER 1

#include <X11/Xtrans/transport.c>

The source files for this interface are listed below.

Xtrans.h      Function prototypes and defines for the Transport Independent
              API.

Xtransint.h   Used by the interface implementation only. Contains the internal
              data structures.

Xtranssock.c  Socket implementation of the Transport Dependent API.

Xtranstli.c   TLI implementation of the Transport Dependent API.

Xtransdnet.c  DECnet implementation of the Transport Dependent API.

Xtranslocal.c Implementation of the Transport Dependent API for SYSV Local
              connections.

Xtrans.c      Exposed Transport Independent API Functions.

Xtransutil.c  Collection of Utility functions that use the X Transport
              Interface.

The file Xtransint.h contains much of the transport related code that was
previously in Xlibint.h and Xlibnet.h. This will make the definitions available
for all transport users. This should also obsolete the equivalent code in other
libraries.

