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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  (or
       other  pending  tasks  like  a  write  flush should be performed).  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 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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