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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_ChannelClose2Proc,  Tcl_ChannelInputProc,  Tcl_Chan-
       nelOutputProc,	  Tcl_ChannelWideSeekProc,    Tcl_ChannelTruncateProc,
       Tcl_ChannelSetOptionProc,  Tcl_ChannelGetOptionProc,  Tcl_ChannelWatch-
       Proc,  Tcl_ChannelGetHandleProc,	 Tcl_ChannelFlushProc, Tcl_ChannelHan-
       dlerProc, Tcl_ChannelThreadActionProc, Tcl_IsChannelShared, Tcl_IsChan-
       nelRegistered, Tcl_CutChannel,  Tcl_SpliceChannel,  Tcl_IsChannelExist-
       ing,   Tcl_ClearChannelHandlers,	  Tcl_GetChannelThread,	  Tcl_Channel-
       Buffered	- procedures for creating and manipulating channels

SYNOPSIS
       #include	<tcl.h>

       Tcl_Channel
       Tcl_CreateChannel(typePtr, channelName, instanceData, mask)

       void *
       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								       2
       Tcl_RemoveChannelMode(interp, channel, mode)			       2

       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)

       Tcl_CutChannel(channel)

       Tcl_SpliceChannel(channel)

       Tcl_ClearChannelHandlers(channel)

       int
       Tcl_ChannelBuffered(channel)

       const char *
       Tcl_ChannelName(typePtr)

       Tcl_ChannelTypeVersion
       Tcl_ChannelVersion(typePtr)

       Tcl_DriverBlockModeProc *
       Tcl_ChannelBlockModeProc(typePtr)

       Tcl_DriverClose2Proc *
       Tcl_ChannelClose2Proc(typePtr)

       Tcl_DriverInputProc *
       Tcl_ChannelInputProc(typePtr)

       Tcl_DriverOutputProc *
       Tcl_ChannelOutputProc(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.

       void *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.

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

       Tcl_Size	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_RemoveChannelMode removes an	access privilege from the channel, ei- 2
       ther  TCL_READABLE  or  TCL_WRITABLE,  and returns a regular Tcl	result 2
       code, TCL_OK, or	TCL_ERROR. The function	throws an error	if  either  an 2
       invalid	mode is	specified or the result	of the removal would be	an in- 2
       accessible channel. In that case	an error message is left in the	interp 2
       argument, if not	NULL.

       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 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 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 {
		      const char *typeName;
		      Tcl_ChannelTypeVersion version;
		      void *closeProc; /* Not used any more*/
		      Tcl_DriverInputProc *inputProc;
		      Tcl_DriverOutputProc *outputProc;
		      void *seekProc; /* Not used any more */
		      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-
       Close2Proc, Tcl_ChannelInputProc,  Tcl_ChannelOutputProc,  Tcl_Channel-
       WideSeekProc,   Tcl_ChannelThreadActionProc,   Tcl_ChannelTruncateProc,
       Tcl_ChannelSetOptionProc,  Tcl_ChannelGetOptionProc,  Tcl_ChannelWatch-
       Proc,  Tcl_ChannelGetHandleProc,	 Tcl_ChannelFlushProc, or Tcl_Channel-
       HandlerProc.

       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_5 is the minimum supported.

       This value can be retrieved with	Tcl_ChannelVersion.

   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(
		      void *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.

   CLOSE2PROC
       The  close2Proc	field contains the address of a	function called	by the
       generic layer to	clean up driver-related	information when  the  channel
       is closed. Close2Proc must match	the following prototype:

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

       If  flags is 0, the instanceData	argument is the	same as	the value pro-
       vided to	Tcl_CreateChannel 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  in-
       voked on	this instance after calling the	closeProc. If the close	opera-
       tion  is	 successful,  the  procedure  should return zero; otherwise it
       should return a nonzero POSIX error code. In addition, if an error  oc-
       curs  and  interp is not	NULL, the procedure should store an error mes-
       sage in the interpreter's result.

       Alternatively, channels that support closing the	read and  write	 sides
       independently  may  accept  other  flag values than 0.  Then 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.   In	all cases, the
       close2Proc function should return zero if the close operation was  suc-
       cessful;	otherwise it should return a nonzero POSIX error code.	In ad-
       dition, if an error occurs and interp is	not NULL, the procedure	should
       store an	error message in the interpreter's result.

       The close2Proc value can	be retrieved with Tcl_ChannelClose2Proc, which
       returns a pointer to the	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(
		      void *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(
		      void *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.

   WIDESEEKPROC
       The wideSeekProc	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. WideSeekProc must match the  follow-
       ing prototype:

	      typedef long long	Tcl_DriverWideSeekProc(
		      void *instanceData,
		      long 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.

       The wideSseekProc value can be retrieved	with  Tcl_ChannelWideSeekProc,
       which returns a pointer to the function.

   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(
		      void *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(
		      void *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(
		      void *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(
		      void *instanceData,
		      int direction,
		      void **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(
		      void *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(
		      void *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(
		      void *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(
		      void *instanceData,
		      long long	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.

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)

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