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Tcl_CreateChannel(3)	    Tcl	Library	Procedures	  Tcl_CreateChannel(3)

______________________________________________________________________________

NAME
       Tcl_CreateChannel,    Tcl_GetChannelInstanceData,   Tcl_GetChannelType,
       Tcl_GetChannelName,	Tcl_GetChannelHandle,	   Tcl_GetChannelMode,
       Tcl_GetChannelBufferSize,  Tcl_SetChannelBufferSize, Tcl_NotifyChannel,
       Tcl_BadChannelOption, Tcl_ChannelName, Tcl_ChannelVersion, Tcl_Channel-
       BlockModeProc,  Tcl_ChannelCloseProc,  Tcl_ChannelClose2Proc, Tcl_Chan-
       nelInputProc, Tcl_ChannelOutputProc, Tcl_ChannelSeekProc,  Tcl_Channel-
       WideSeekProc,	 Tcl_ChannelTruncateProc,    Tcl_ChannelSetOptionProc,
       Tcl_ChannelGetOptionProc,    Tcl_ChannelWatchProc,     Tcl_ChannelGetH-
       andleProc,   Tcl_ChannelFlushProc,   Tcl_ChannelHandlerProc,  Tcl_Chan-
       nelThreadActionProc,   Tcl_IsChannelShared,    Tcl_IsChannelRegistered,
       Tcl_CutChannel,	      Tcl_SpliceChannel,	Tcl_IsChannelExisting,
       Tcl_ClearChannelHandlers, Tcl_GetChannelThread,	Tcl_ChannelBuffered  -
       procedures for creating and manipulating	channels

SYNOPSIS
       #include	<tcl.h>

       Tcl_Channel
       Tcl_CreateChannel(typePtr, channelName, instanceData, mask)

       ClientData
       Tcl_GetChannelInstanceData(channel)

       const Tcl_ChannelType *
       Tcl_GetChannelType(channel)

       const char *
       Tcl_GetChannelName(channel)

       int
       Tcl_GetChannelHandle(channel, direction,	handlePtr)

       Tcl_ThreadId
       Tcl_GetChannelThread(channel)

       int
       Tcl_GetChannelMode(channel)

       int
       Tcl_GetChannelBufferSize(channel)

       Tcl_SetChannelBufferSize(channel, size)

       Tcl_NotifyChannel(channel, mask)

       int
       Tcl_BadChannelOption(interp, optionName,	optionList)

       int
       Tcl_IsChannelShared(channel)

       int
       Tcl_IsChannelRegistered(interp, channel)

       int
       Tcl_IsChannelExisting(channelName)

       void
       Tcl_CutChannel(channel)

       void
       Tcl_SpliceChannel(channel)

       void
       Tcl_ClearChannelHandlers(channel)

       int
       Tcl_ChannelBuffered(channel)

       const char *
       Tcl_ChannelName(typePtr)

       Tcl_ChannelTypeVersion
       Tcl_ChannelVersion(typePtr)

       Tcl_DriverBlockModeProc *
       Tcl_ChannelBlockModeProc(typePtr)

       Tcl_DriverCloseProc *
       Tcl_ChannelCloseProc(typePtr)

       Tcl_DriverClose2Proc *
       Tcl_ChannelClose2Proc(typePtr)

       Tcl_DriverInputProc *
       Tcl_ChannelInputProc(typePtr)

       Tcl_DriverOutputProc *
       Tcl_ChannelOutputProc(typePtr)

       Tcl_DriverSeekProc *
       Tcl_ChannelSeekProc(typePtr)

       Tcl_DriverWideSeekProc *
       Tcl_ChannelWideSeekProc(typePtr)

       Tcl_DriverThreadActionProc *
       Tcl_ChannelThreadActionProc(typePtr)

       Tcl_DriverTruncateProc *
       Tcl_ChannelTruncateProc(typePtr)

       Tcl_DriverSetOptionProc *
       Tcl_ChannelSetOptionProc(typePtr)

       Tcl_DriverGetOptionProc *
       Tcl_ChannelGetOptionProc(typePtr)

       Tcl_DriverWatchProc *
       Tcl_ChannelWatchProc(typePtr)

       Tcl_DriverGetHandleProc *
       Tcl_ChannelGetHandleProc(typePtr)

       Tcl_DriverFlushProc *
       Tcl_ChannelFlushProc(typePtr)

       Tcl_DriverHandlerProc *
       Tcl_ChannelHandlerProc(typePtr)

ARGUMENTS
       const Tcl_ChannelType *typePtr (in)		Points	to a structure
							containing   the   ad-
							dresses	 of procedures
							that can be called  to
							perform	 I/O and other
							functions on the chan-
							nel.

       const char *channelName (in)			The name of this chan-
							nel,  such  as	file3;
							must  not be in	use by
							any other channel. Can
							be NULL, in which case
							the channel is created
							without	a name.	If the
							created	channel	is as-
							signed	to  one	of the
							standard      channels
							(stdin,	   stdout   or
							stderr), the  assigned
							channel	 name  will be
							the name of the	 stan-
							dard channel.

       ClientData instanceData (in)			Arbitrary     one-word
							value to be associated
							with   this   channel.
							This value  is	passed
							to procedures in type-
							Ptr when they are  in-
							voked.

       int mask	(in)					OR-ed  combination  of
							TCL_READABLE	   and
							TCL_WRITABLE  to indi-
							cate whether a channel
							is     readable	   and
							writable.

       Tcl_Channel channel (in)				The channel to operate
							on.

       int direction (in)				TCL_READABLE means the
							input	 handle	    is
							wanted;	  TCL_WRITABLE
							means the output  han-
							dle is wanted.

       ClientData *handlePtr (out)			Points to the location
							where the desired  OS-
							specific handle	should
							be stored.

       int size	(in)					The size, in bytes, of
							buffers	to allocate in
							this channel.

       int mask	(in)					An  OR-ed  combination
							of	 TCL_READABLE,
							TCL_WRITABLE	   and
							TCL_EXCEPTION that in-
							dicates	 events	  that
							have  occurred on this
							channel.

       Tcl_Interp *interp (in)				Current	  interpreter.
							(can be	NULL)

       const char *optionName (in)			Name  of  the  invalid
							option.

       const char *optionList (in)			Specific options  list
							(space	     separated
							words, without "-") to
							append to the standard
							generic	options	 list.
							Can    be   NULL   for
							generic	options	 error
							message	only.
______________________________________________________________________________

DESCRIPTION
       Tcl  uses a two-layered channel architecture. It	provides a generic up-
       per layer to enable C and Tcl programs to perform input and output  us-
       ing  the	 same  APIs  for a variety of files, devices, sockets etc. The
       generic C APIs are described in the manual entry	for  Tcl_OpenFileChan-
       nel.

       The lower layer provides	type-specific channel drivers for each type of
       device supported	on each	platform.  This	manual entry describes	the  C
       APIs  used  to  communicate between the generic layer and the type-spe-
       cific channel drivers.  It also explains	how new	types of channels  can
       be added	by providing new channel drivers.

       Channel	drivers	consist	of a number of components: First, each channel
       driver provides a  Tcl_ChannelType  structure  containing  pointers  to
       functions implementing the various operations used by the generic layer
       to communicate with the channel driver. The  Tcl_ChannelType  structure
       and  the	 functions  referenced	by  it	are  described	in the section
       TCL_CHANNELTYPE,	below.

       Second, channel drivers usually provide a Tcl  command  to  create  in-
       stances of that type of channel.	For example, the Tcl open command cre-
       ates channels that use the file and command channel  drivers,  and  the
       Tcl  socket  command  creates channels that use TCP sockets for network
       communication.

       Third, a	channel	driver optionally provides a C function	to open	 chan-
       nel  instances  of  that	type. For example, Tcl_OpenFileChannel opens a
       channel that uses the file channel driver, and Tcl_OpenTcpClient	 opens
       a channel that uses the TCP network protocol.  These creation functions
       typically use Tcl_CreateChannel internally to open the channel.

       To add a	new type of channel you	must implement a C API or a  Tcl  com-
       mand  that  opens  a  channel by	invoking Tcl_CreateChannel.  When your
       driver calls Tcl_CreateChannel it passes	in a Tcl_ChannelType structure
       describing  the	driver's  I/O procedures.  The generic layer will then
       invoke the functions referenced in that structure to perform operations
       on the channel.

       Tcl_CreateChannel opens a new channel and associates the	supplied type-
       Ptr and instanceData with it. The channel is opened in the  mode	 indi-
       cated  by  mask.	 For a discussion of channel drivers, their operations
       and the Tcl_ChannelType structure, see the section TCL_CHANNELTYPE, be-
       low.

       Tcl_CreateChannel  interacts  with the code managing the	standard chan-
       nels. Once a standard channel was initialized either through a call  to
       Tcl_GetStdChannel  or a call to Tcl_SetStdChannel closing this standard
       channel will cause the next call	to Tcl_CreateChannel to	make  the  new
       channel	the  new  standard channel too.	See Tcl_StandardChannels for a
       general treatise	about standard channels	and the	behavior  of  the  Tcl
       library with regard to them.

       Tcl_GetChannelInstanceData  returns  the	 instance data associated with
       the channel in channel. This is the same	as the	instanceData  argument
       in the call to Tcl_CreateChannel	that created this channel.

       Tcl_GetChannelType  returns  a pointer to the Tcl_ChannelType structure
       used by the channel in the channel argument. This is the	 same  as  the
       typePtr	argument  in  the  call	to Tcl_CreateChannel that created this
       channel.

       Tcl_GetChannelName returns a string containing the name associated with
       the  channel,  or NULL if the channelName argument to Tcl_CreateChannel
       was NULL.

       Tcl_GetChannelHandle places the OS-specific  device  handle  associated
       with  channel for the given direction in	the location specified by han-
       dlePtr and returns TCL_OK.  If the channel does not have	a device  han-
       dle  for	 the  specified	direction, then	TCL_ERROR is returned instead.
       Different channel drivers will return different types of	handle.	 Refer
       to  the manual entries for each driver to determine what	type of	handle
       is returned.

       Tcl_GetChannelThread returns the	id of the  thread  currently  managing
       the  specified  channel.	This allows channel drivers to send their file
       events to the correct event queue even for a multi-threaded core.

       Tcl_GetChannelMode returns an OR-ed  combination	 of  TCL_READABLE  and
       TCL_WRITABLE, indicating	whether	the channel is open for	input and out-
       put.

       Tcl_GetChannelBufferSize	returns	the size, in bytes, of	buffers	 allo-
       cated  to store input or	output in channel. If the value	was not	set by
       a previous call to Tcl_SetChannelBufferSize, described below, then  the
       default value of	4096 is	returned.

       Tcl_SetChannelBufferSize	 sets the size,	in bytes, of buffers that will
       be allocated in subsequent operations on	the channel to store input  or
       output. The size	argument should	be between one and one million,	allow-
       ing buffers of one byte to one million bytes. If	size is	 outside  this
       range, Tcl_SetChannelBufferSize sets the	buffer size to 4096.

       Tcl_NotifyChannel  is  called  by  a  channel driver to indicate	to the
       generic layer that the events specified by mask have  occurred  on  the
       channel.	  Channel  drivers  are	responsible for	invoking this function
       whenever	the channel handlers need to be	called for the	channel.   See
       WATCHPROC below for more	details.

       Tcl_BadChannelOption  is	 called	 from driver specific setOptionProc or
       getOptionProc to	generate a complete error message.

       Tcl_ChannelBuffered returns the number  of  bytes  of  input  currently
       buffered	in the internal	buffer (push back area)	of the channel itself.
       It does not report about	the data in the	overall	buffers	for the	 stack
       of channels the supplied	channel	is part	of.

       Tcl_IsChannelShared  checks  the	 refcount of the specified channel and
       returns whether the channel was shared among multiple interpreters (re-
       sult == 1) or not (result == 0).

       Tcl_IsChannelRegistered	checks whether the specified channel is	regis-
       tered in	the given interpreter (result == 1) or not (result == 0).

       Tcl_IsChannelExisting checks whether a channel with the specified  name
       is registered in	the (thread)-global list of all	channels (result == 1)
       or not (result == 0).

       Tcl_CutChannel removes the specified channel  from  the	(thread)global
       list of all channels (of	the current thread).  Application to a channel
       still registered	in some	interpreter is not allowed.  Also notifies the
       driver  if  the	Tcl_ChannelType	 version  is TCL_CHANNEL_VERSION_4 (or
       higher),	and Tcl_DriverThreadActionProc is defined for it.

       Tcl_SpliceChannel adds the specified channel to the (thread)global list
       of all channels (of the current thread).	 Application to	a channel reg-
       istered in some interpreter is not allowed.  Also notifies  the	driver
       if  the	Tcl_ChannelType	 version is TCL_CHANNEL_VERSION_4 (or higher),
       and Tcl_DriverThreadActionProc is defined for it.

       Tcl_ClearChannelHandlers	removes	all channel handlers and event scripts
       associated  with	 the  specified	 channel, thus shutting	down all event
       processing for this channel.

TCL_CHANNELTYPE
       A channel driver	provides a  Tcl_ChannelType  structure	that  contains
       pointers	 to functions that implement the various operations on a chan-
       nel; these operations are invoked as needed by the generic layer.   The
       structure  was versioned	starting in Tcl	8.3.2/8.4 to correct a problem
       with stacked channel drivers.  See the OLD CHANNEL TYPES	section	 below
       for details about the old structure.

       The Tcl_ChannelType structure contains the following fields:

	      typedef struct Tcl_ChannelType {
		      const char *typeName;
		      Tcl_ChannelTypeVersion version;
		      Tcl_DriverCloseProc *closeProc;
		      Tcl_DriverInputProc *inputProc;
		      Tcl_DriverOutputProc *outputProc;
		      Tcl_DriverSeekProc *seekProc;
		      Tcl_DriverSetOptionProc *setOptionProc;
		      Tcl_DriverGetOptionProc *getOptionProc;
		      Tcl_DriverWatchProc *watchProc;
		      Tcl_DriverGetHandleProc *getHandleProc;
		      Tcl_DriverClose2Proc *close2Proc;
		      Tcl_DriverBlockModeProc *blockModeProc;
		      Tcl_DriverFlushProc *flushProc;
		      Tcl_DriverHandlerProc *handlerProc;
		      Tcl_DriverWideSeekProc *wideSeekProc;
		      Tcl_DriverThreadActionProc *threadActionProc;
		      Tcl_DriverTruncateProc *truncateProc;
	      }	Tcl_ChannelType;

       It  is  not necessary to	provide	implementations	for all	channel	opera-
       tions.  Those which are not necessary may be set	to NULL	in the struct:
       blockModeProc,  seekProc,  setOptionProc, getOptionProc,	getHandleProc,
       and close2Proc, in addition to  flushProc,  handlerProc,	 threadAction-
       Proc,  and truncateProc.	 Other functions that cannot be	implemented in
       a meaningful way	should return EINVAL when called, to indicate that the
       operations   they   represent   are   not  available.  Also  note  that
       wideSeekProc can	be NULL	if seekProc is.

       The user	should only use	the above structure  for  Tcl_ChannelType  in-
       stantiation.   When  referencing	fields in a Tcl_ChannelType structure,
       the following functions should be used to obtain	the values:  Tcl_Chan-
       nelName,	  Tcl_ChannelVersion,  Tcl_ChannelBlockModeProc,  Tcl_Channel-
       CloseProc, Tcl_ChannelClose2Proc, Tcl_ChannelInputProc, Tcl_ChannelOut-
       putProc,	   Tcl_ChannelSeekProc,	  Tcl_ChannelWideSeekProc,   Tcl_Chan-
       nelThreadActionProc, Tcl_ChannelTruncateProc, Tcl_ChannelSetOptionProc,
       Tcl_ChannelGetOptionProc,     Tcl_ChannelWatchProc,    Tcl_ChannelGetH-
       andleProc, Tcl_ChannelFlushProc,	or Tcl_ChannelHandlerProc.

       The change to the structures was	made in	such a way that	standard chan-
       nel  types  are	binary	compatible.   However,	channel	types that use
       stacked channels	(i.e. TLS, Trf)	have new versions to correspond	to the
       above change since the previous code for	stacked	channels had problems.

   TYPENAME
       The  typeName  field  contains a	null-terminated	string that identifies
       the type	of the device  implemented  by	this  driver,  e.g.   file  or
       socket.

       This  value  can	 be  retrieved	with  Tcl_ChannelName, which returns a
       pointer to the string.

   VERSION
       The version field should	be set to the version of  the  structure  that
       you   require.	TCL_CHANNEL_VERSION_2	is  the	 minimum  recommended.
       TCL_CHANNEL_VERSION_3 must be set to specify the	 wideSeekProc  member.
       TCL_CHANNEL_VERSION_4  must be set to specify the threadActionProc mem-
       ber (includes wideSeekProc).   TCL_CHANNEL_VERSION_5  must  be  set  to
       specify	the  truncateProc members (includes wideSeekProc and threadAc-
       tionProc).  If it is not	set to any of these, then this Tcl_ChannelType
       is  assumed  to have the	original structure.  See OLD CHANNEL TYPES for
       more details.  While Tcl	will recognize and function with either	struc-
       tures,  stacked	channels  must be of at	least TCL_CHANNEL_VERSION_2 to
       function	correctly.

       This value can be retrieved with	Tcl_ChannelVersion, which returns  one
       of TCL_CHANNEL_VERSION_5, TCL_CHANNEL_VERSION_4,	TCL_CHANNEL_VERSION_3,
       TCL_CHANNEL_VERSION_2 or	TCL_CHANNEL_VERSION_1.

   BLOCKMODEPROC
       The blockModeProc field contains	the address of a  function  called  by
       the  generic  layer to set blocking and nonblocking mode	on the device.
       BlockModeProc should match the following	prototype:

	      typedef int Tcl_DriverBlockModeProc(
		      ClientData instanceData,
		      int mode);

       The instanceData	is the same as the value passed	 to  Tcl_CreateChannel
       when   this   channel   was  created.   The  mode  argument  is	either
       TCL_MODE_BLOCKING or TCL_MODE_NONBLOCKING to set	the device into	block-
       ing  or nonblocking mode. The function should return zero if the	opera-
       tion was	successful, or a nonzero POSIX error  code  if	the  operation
       failed.

       If  the	operation  is successful, the function can modify the supplied
       instanceData to record that the channel entered blocking	or nonblocking
       mode  and  to implement the blocking or nonblocking behavior.  For some
       device types, the blocking and nonblocking behavior can be  implemented
       by  the underlying operating system; for	other device types, the	behav-
       ior must	be emulated in the channel driver.

       This value can be retrieved with	 Tcl_ChannelBlockModeProc,  which  re-
       turns a pointer to the function.

       A  channel  driver  not	supplying a blockModeProc has to be very, very
       careful.	It has to tell the generic layer exactly which	blocking  mode
       is acceptable to	it, and	should this also document for the user so that
       the blocking mode of the	channel	is  not	 changed  to  an  unacceptable
       value. Any confusion here may lead the interpreter into a (spurious and
       difficult to find) deadlock.

   CLOSEPROC AND CLOSE2PROC
       The closeProc field contains the	address	of a function  called  by  the
       generic	layer  to clean	up driver-related information when the channel
       is closed. CloseProc must match the following prototype:

	      typedef int Tcl_DriverCloseProc(
		      ClientData instanceData,
		      Tcl_Interp *interp);

       The instanceData	argument is the	same as	the value provided to Tcl_Cre-
       ateChannel  when	 the  channel was created. The function	should release
       any storage maintained by the channel  driver  for  this	 channel,  and
       close  the  input  and output devices encapsulated by this channel. All
       queued output will have been flushed to the device before this function
       is called, and no further driver	operations will	be invoked on this in-
       stance after calling the	closeProc. If the close	operation is  success-
       ful,  the  procedure  should  return zero; otherwise it should return a
       nonzero POSIX error code. In addition, if an error occurs and interp is
       not  NULL,  the	procedure  should store	an error message in the	inter-
       preter's	result.

       Alternatively, channels that support closing the	read and  write	 sides
       independently may set closeProc to TCL_CLOSE2PROC and set close2Proc to
       the address of a	function that matches the following prototype:

	      typedef int Tcl_DriverClose2Proc(
		      ClientData instanceData,
		      Tcl_Interp *interp,
		      int flags);

       The close2Proc will be called with flags	set to an OR'ed	combination of
       TCL_CLOSE_READ  or  TCL_CLOSE_WRITE  to indicate	that the driver	should
       close the read and/or write side	of the channel.	  The  channel	driver
       may  be	invoked	 to perform additional operations on the channel after
       close2Proc is called to close one or both sides	of  the	 channel.   If
       flags  is  0  (zero), the driver	should close the channel in the	manner
       described above for closeProc.  No further operations will  be  invoked
       on this instance	after close2Proc is called with	all flags cleared.  In
       all cases, the close2Proc function should return	zero if	the close  op-
       eration	was successful;	otherwise it should return a nonzero POSIX er-
       ror code. In addition, if an error occurs and interp is not  NULL,  the
       procedure should	store an error message in the interpreter's result.

       The  closeProc and close2Proc values can	be retrieved with Tcl_Channel-
       CloseProc or Tcl_ChannelClose2Proc, which return	a pointer to  the  re-
       spective	function.

   INPUTPROC
       The  inputProc  field  contains the address of a	function called	by the
       generic layer to	read data from the file	or device and store it	in  an
       internal	buffer.	InputProc must match the following prototype:

	      typedef int Tcl_DriverInputProc(
		      ClientData instanceData,
		      char *buf,
		      int bufSize,
		      int *errorCodePtr);

       InstanceData  is	the same as the	value passed to	Tcl_CreateChannel when
       the channel was created.	 The buf argument points to an array of	 bytes
       in which	to store input from the	device,	and the	bufSize	argument indi-
       cates how many bytes are	available at buf.

       The errorCodePtr	argument points	to an integer variable provided	by the
       generic layer. If an error occurs, the function should set the variable
       to a POSIX error	code that identifies the error that occurred.

       The function should read	data from the input device encapsulated	by the
       channel	and store it at	buf.  On success, the function should return a
       nonnegative integer indicating how many bytes were read from the	 input
       device  and  stored at buf. On error, the function should return	-1. If
       an error	occurs after some data has been	read  from  the	 device,  that
       data is lost.

       If  inputProc  can determine that the input device has some data	avail-
       able but	less than requested by	the  bufSize  argument,	 the  function
       should  only  attempt  to  read as much data as is available and	return
       without blocking. If the	input device has no data available  whatsoever
       and  the	 channel is in nonblocking mode, the function should return an
       EAGAIN error. If	the input device has no	data available whatsoever  and
       the  channel  is	 in  blocking  mode, the function should block for the
       shortest	possible time until at least one byte of data can be read from
       the  device; then, it should return as much data	as it can read without
       blocking.

       This value can be retrieved with	Tcl_ChannelInputProc, which returns  a
       pointer to the function.

   OUTPUTPROC
       The  outputProc	field contains the address of a	function called	by the
       generic layer to	transfer data from an internal buffer  to  the	output
       device.	OutputProc must	match the following prototype:

	      typedef int Tcl_DriverOutputProc(
		      ClientData instanceData,
		      const char *buf,
		      int toWrite,
		      int *errorCodePtr);

       InstanceData  is	the same as the	value passed to	Tcl_CreateChannel when
       the channel was created.	The buf	argument contains an array of bytes to
       be  written  to the device, and the toWrite argument indicates how many
       bytes are to be written from the	buf argument.

       The errorCodePtr	argument points	to an integer variable provided	by the
       generic	layer.	If an error occurs, the	function should	set this vari-
       able to a POSIX error code that identifies the error.

       The function should write the data at buf to the	output device encapsu-
       lated by	the channel. On	success, the function should return a nonnega-
       tive integer indicating how many	bytes were written to the  output  de-
       vice.   The  return  value  is normally the same	as toWrite, but	may be
       less in some cases such as if the output	operation is interrupted by  a
       signal.	If  an error occurs the	function should	return -1.  In case of
       error, some data	may have been written to the device.

       If the channel is nonblocking and the output device is unable to	absorb
       any data	whatsoever, the	function should	return -1 with an EAGAIN error
       without writing any data.

       This value can be retrieved with	Tcl_ChannelOutputProc, which returns a
       pointer to the function.

   SEEKPROC AND	WIDESEEKPROC
       The  seekProc  field  contains  the address of a	function called	by the
       generic layer to	move the access	point at  which	 subsequent  input  or
       output  operations  will	 be applied. SeekProc must match the following
       prototype:

	      typedef int Tcl_DriverSeekProc(
		      ClientData instanceData,
		      long offset,
		      int seekMode,
		      int *errorCodePtr);

       The instanceData	argument is the	same as	the value given	to Tcl_Create-
       Channel	when  this  channel was	created.  Offset and seekMode have the
       same meaning as for the Tcl_Seek	procedure (described in	the manual en-
       try for Tcl_OpenFileChannel).

       The errorCodePtr	argument points	to an integer variable provided	by the
       generic layer for returning errno values	from the function.  The	 func-
       tion should set this variable to	a POSIX	error code if an error occurs.
       The function should store an EINVAL error code if the channel type does
       not implement seeking.

       The  return value is the	new access point or -1 in case of error. If an
       error occurred, the function should not move the	access point.

       If there	is a non-NULL seekProc field, the wideSeekProc field may  con-
       tain  the  address of an	alternative function to	use which handles wide
       (i.e. larger than 32-bit)  offsets,  so	allowing  seeks	 within	 files
       larger  than 2GB.  The wideSeekProc will	be called in preference	to the
       seekProc, but both must be defined  if  the  wideSeekProc  is  defined.
       WideSeekProc must match the following prototype:

	      typedef Tcl_WideInt Tcl_DriverWideSeekProc(
		      ClientData instanceData,
		      Tcl_WideInt offset,
		      int seekMode,
		      int *errorCodePtr);

       The  arguments  and  return values mean the same	thing as with seekProc
       above, except that the type of offsets and the return type are  differ-
       ent.

       The seekProc value can be retrieved with	Tcl_ChannelSeekProc, which re-
       turns a pointer to the function,	and similarly the wideSeekProc can  be
       retrieved with Tcl_ChannelWideSeekProc.

   SETOPTIONPROC
       The  setOptionProc  field  contains the address of a function called by
       the generic layer to set	a channel type specific	option on  a  channel.
       setOptionProc must match	the following prototype:

	      typedef int Tcl_DriverSetOptionProc(
		      ClientData instanceData,
		      Tcl_Interp *interp,
		      const char *optionName,
		      const char *newValue);

       optionName  is  the  name  of an	option to set, and newValue is the new
       value for that option, as a string. The instanceData is the same	as the
       value  given  to	 Tcl_CreateChannel  when this channel was created. The
       function	should do whatever channel type	specific action	is required to
       implement the new value of the option.

       Some options are	handled	by the generic code and	this function is never
       called to set them, e.g.	-blockmode. Other options are specific to each
       channel type and	the setOptionProc procedure of the channel driver will
       get called to implement them. The  setOptionProc	 field	can  be	 NULL,
       which  indicates	 that  this channel type supports no type specific op-
       tions.

       If the option value is successfully modified  to	 the  new  value,  the
       function	returns	TCL_OK.	 It should call	Tcl_BadChannelOption which it-
       self returns TCL_ERROR if the optionName	is unrecognized.  If  newValue
       specifies  a  value for the option that is not supported	or if a	system
       call error occurs, the function should leave an error  message  in  the
       result  field of	interp if interp is not	NULL. The function should also
       call Tcl_SetErrno to store an appropriate POSIX error code.

       This value can be retrieved with	 Tcl_ChannelSetOptionProc,  which  re-
       turns a pointer to the function.

   GETOPTIONPROC
       The  getOptionProc  field  contains the address of a function called by
       the generic layer to get	the value of a channel type specific option on
       a channel. getOptionProc	must match the following prototype:

	      typedef int Tcl_DriverGetOptionProc(
		      ClientData instanceData,
		      Tcl_Interp *interp,
		      const char *optionName,
		      Tcl_DString *optionValue);

       OptionName  is the name of an option supported by this type of channel.
       If the option name is not NULL, the function stores its current	value,
       as  a  string, in the Tcl dynamic string	optionValue.  If optionName is
       NULL, the function stores in optionValue	an  alternating	 list  of  all
       supported  options  and their current values.  On success, the function
       returns TCL_OK.	It should call Tcl_BadChannelOption which  itself  re-
       turns TCL_ERROR if the optionName is unrecognized. If a system call er-
       ror occurs, the function	should leave an	error message in the result of
       interp  if interp is not	NULL. The function should also call Tcl_SetEr-
       rno to store an appropriate POSIX error code.

       Some options are	handled	by the generic code and	this function is never
       called to retrieve their	value, e.g. -blockmode.	Other options are spe-
       cific to	each channel type and the getOptionProc	procedure of the chan-
       nel  driver  will get called to implement them. The getOptionProc field
       can be NULL, which indicates that this channel type  supports  no  type
       specific	options.

       This  value  can	 be retrieved with Tcl_ChannelGetOptionProc, which re-
       turns a pointer to the function.

   WATCHPROC
       The watchProc field contains the	address	of a function  called  by  the
       generic	layer to initialize the	event notification mechanism to	notice
       events of interest on this channel.  WatchProc should match the follow-
       ing prototype:

	      typedef void Tcl_DriverWatchProc(
		      ClientData instanceData,
		      int mask);

       The  instanceData  is the same as the value passed to Tcl_CreateChannel
       when this channel was created. The mask argument	is an  OR-ed  combina-
       tion  of	 TCL_READABLE,	TCL_WRITABLE  and  TCL_EXCEPTION; it indicates
       events the caller is interested in noticing on this channel.

       The function should initialize device type specific mechanisms  to  no-
       tice  when an event of interest is present on the channel.  When	one or
       more of the designated events occurs on the channel, the	channel	driver
       is  responsible	for  calling  Tcl_NotifyChannel	 to inform the generic
       channel module.	The driver should take care not	to starve other	 chan-
       nel  drivers  or	sources	of callbacks by	invoking Tcl_NotifyChannel too
       frequently.  Fairness can be insured by using the Tcl  event  queue  to
       allow  the channel event	to be scheduled	in sequence with other events.
       See the description of Tcl_QueueEvent for details on how	 to  queue  an
       event.

       This  value can be retrieved with Tcl_ChannelWatchProc, which returns a
       pointer to the function.

   GETHANDLEPROC
       The getHandleProc field contains	the address of a  function  called  by
       the  generic  layer to retrieve a device-specific handle	from the chan-
       nel.  GetHandleProc should match	the following prototype:

	      typedef int Tcl_DriverGetHandleProc(
		      ClientData instanceData,
		      int direction,
		      ClientData *handlePtr);

       InstanceData is the same	as the value passed to Tcl_CreateChannel  when
       this channel was	created. The direction argument	is either TCL_READABLE
       to retrieve the handle used for input, or TCL_WRITABLE to retrieve  the
       handle used for output.

       If the channel implementation has device-specific handles, the function
       should retrieve the appropriate handle associated with the channel, ac-
       cording the direction argument.	The handle should be stored in the lo-
       cation referred to by handlePtr,	and TCL_OK should be returned.	If the
       channel	is not open for	the specified direction, or if the channel im-
       plementation does not use device	handles, the  function	should	return
       TCL_ERROR.

       This  value  can	 be retrieved with Tcl_ChannelGetHandleProc, which re-
       turns a pointer to the function.

   FLUSHPROC
       The flushProc field is currently	reserved for future use.  It should be
       set to NULL.  FlushProc should match the	following prototype:

	      typedef int Tcl_DriverFlushProc(
		      ClientData instanceData);

       This  value can be retrieved with Tcl_ChannelFlushProc, which returns a
       pointer to the function.

   HANDLERPROC
       The handlerProc field contains the address of a function	called by  the
       generic	layer to notify	the channel that an event occurred.  It	should
       be defined for stacked channel drivers that  wish  to  be  notified  of
       events  that  occur  on	the underlying (stacked) channel.  HandlerProc
       should match the	following prototype:

	      typedef int Tcl_DriverHandlerProc(
		      ClientData instanceData,
		      int interestMask);

       InstanceData is the same	as the value passed to Tcl_CreateChannel  when
       this  channel was created.  The interestMask is an OR-ed	combination of
       TCL_READABLE or TCL_WRITABLE; it	indicates what type of event  occurred
       on this channel.

       This  value can be retrieved with Tcl_ChannelHandlerProc, which returns
       a pointer to the	function.

   THREADACTIONPROC
       The threadActionProc field contains the address of the function	called
       by  the	generic	 layer	when a channel is created, closed, or going to
       move to a different thread, i.e.	whenever thread-specific driver	 state
       might  have  to	initialized  or	 updated.  It can be NULL.  The	action
       TCL_CHANNEL_THREAD_REMOVE is used to notify the driver that  it	should
       update  or  remove any thread-specific data it might be maintaining for
       the channel.

       The action TCL_CHANNEL_THREAD_INSERT is used to notify the driver  that
       it  should  update  or  initialize any thread-specific data it might be
       maintaining using the calling thread as the associate. See Tcl_CutChan-
       nel and Tcl_SpliceChannel for more detail.

	      typedef void Tcl_DriverThreadActionProc(
		      ClientData instanceData,
		      int action);

       InstanceData  is	the same as the	value passed to	Tcl_CreateChannel when
       this channel was	created.

       These values can	be retrieved with  Tcl_ChannelThreadActionProc,	 which
       returns a pointer to the	function.

   TRUNCATEPROC
       The  truncateProc  field	contains the address of	the function called by
       the generic layer when a	channel	is truncated to	some length. It	can be
       NULL.

	      typedef int Tcl_DriverTruncateProc(
		      ClientData instanceData,
		      Tcl_WideInt length);

       InstanceData  is	the same as the	value passed to	Tcl_CreateChannel when
       this channel was	created, and length is the new length of the  underly-
       ing  file, which	should not be negative.	The result should be 0 on suc-
       cess or an errno	code (suitable for use with Tcl_SetErrno) on failure.

       These values can	be retrieved with Tcl_ChannelTruncateProc,  which  re-
       turns a pointer to the function.

TCL_BADCHANNELOPTION
       This  procedure generates a "bad	option"	error message in an (optional)
       interpreter.  It	is used	by channel drivers when	an invalid Set/Get op-
       tion  is	 requested.  Its purpose is to concatenate the generic options
       list to the specific ones and factorize the generic options error  mes-
       sage string.

       It always returns TCL_ERROR

       An error	message	is generated in	interp's result	value to indicate that
       a command was invoked with a bad	option.	 The message has the form
		  bad option "blah": should be one of
		  <...generic options...>+<...specific options...>
       so you get for instance:
		  bad option "-blah": should be	one of -blocking,
		  -buffering, -buffersize, -eofchar, -translation,
		  -peername, or	-sockname
       when called with	optionList equal to "peername sockname"

       "blah" is the optionName	argument and "<specific	options>" is  a	 space
       separated  list of specific option words.  The function takes good care
       of inserting minus signs	before each option, commas after, and an  "or"
       before the last option.

OLD CHANNEL TYPES
       The  original  (8.3.1 and below)	Tcl_ChannelType	structure contains the
       following fields:

	      typedef struct Tcl_ChannelType {
		  const	char *typeName;
		  Tcl_DriverBlockModeProc *blockModeProc;
		  Tcl_DriverCloseProc *closeProc;
		  Tcl_DriverInputProc *inputProc;
		  Tcl_DriverOutputProc *outputProc;
		  Tcl_DriverSeekProc *seekProc;
		  Tcl_DriverSetOptionProc *setOptionProc;
		  Tcl_DriverGetOptionProc *getOptionProc;
		  Tcl_DriverWatchProc *watchProc;
		  Tcl_DriverGetHandleProc *getHandleProc;
		  Tcl_DriverClose2Proc *close2Proc;
	      }	Tcl_ChannelType;

       It is still possible to create channel with the above  structure.   The
       internal	 channel code will determine the version.  It is imperative to
       use the new Tcl_ChannelType structure if	you  are  creating  a  stacked
       channel driver, due to problems with the	earlier	stacked	channel	imple-
       mentation (in 8.2.0 to 8.3.1).

       Prior to	8.4.0 (i.e. during the later releases of 8.3 and early part of
       the  8.4	development cycle) the Tcl_ChannelType structure contained the
       following fields:

	      typedef struct Tcl_ChannelType {
		  const	char *typeName;
		  Tcl_ChannelTypeVersion version;
		  Tcl_DriverCloseProc *closeProc;
		  Tcl_DriverInputProc *inputProc;
		  Tcl_DriverOutputProc *outputProc;
		  Tcl_DriverSeekProc *seekProc;
		  Tcl_DriverSetOptionProc *setOptionProc;
		  Tcl_DriverGetOptionProc *getOptionProc;
		  Tcl_DriverWatchProc *watchProc;
		  Tcl_DriverGetHandleProc *getHandleProc;
		  Tcl_DriverClose2Proc *close2Proc;
		  Tcl_DriverBlockModeProc *blockModeProc;
		  Tcl_DriverFlushProc *flushProc;
		  Tcl_DriverHandlerProc	*handlerProc;
		  Tcl_DriverTruncateProc *truncateProc;
	      }	Tcl_ChannelType;

       When the	above structure	is registered as a channel type,  the  version
       field should always be TCL_CHANNEL_VERSION_2.

SEE ALSO
       Tcl_Close(3),	      Tcl_OpenFileChannel(3),	      Tcl_SetErrno(3),
       Tcl_QueueEvent(3), Tcl_StackChannel(3), Tcl_GetStdChannel(3)

KEYWORDS
       blocking, channel driver, channel registration, channel type, nonblock-
       ing

Tcl				      8.4		  Tcl_CreateChannel(3)

NAME | SYNOPSIS | ARGUMENTS | DESCRIPTION | TCL_CHANNELTYPE | TCL_BADCHANNELOPTION | OLD CHANNEL TYPES | SEE ALSO | KEYWORDS

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