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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
							addresses   of	proce‐
							dures  that   can   be
							called	to perform I/O
							and other functions on
							the channel.

       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
							assigned 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
							invoked.

       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
							indicates 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
       upper layer to enable C and Tcl programs to perform  input  and	output
       using  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
       instances  of  that  type of channel. For example, the Tcl open command
       creates 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,
       below.

       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 ten and one million, allow‐
       ing buffers of ten bytes 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
       (result == 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, and close2Proc,
       in addition to  flushProc,  handlerProc,	 threadActionProc,  and	 trun‐
       cateProc.   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
       instantiation.  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
       returns 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
       instance after calling the closeProc. If the close  operation  is  suc‐
       cessful, 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
       operation  was  successful;  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 interpreter's result.

       The  closeProc and close2Proc values can be retrieved with Tcl_Channel‐
       CloseProc or Tcl_ChannelClose2Proc,  which  return  a  pointer  to  the
       respective 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
       device.	 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
       entry 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
       returns 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
       options.

       If the option value is successfully modified  to	 the  new  value,  the
       function	 returns  TCL_OK.   It	should call Tcl_BadChannelOption which
       itself returns TCL_ERROR if the optionName is  unrecognized.   If  new‐
       Value  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
       returns 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
       returns	TCL_ERROR  if the optionName is unrecognized. 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.

       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
       returns 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
       notice  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  channel  drivers or sources of callbacks by invoking Tcl_Notify‐
       Channel 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,
       according  the  direction argument.  The handle should be stored in the
       location referred to by handlePtr, and TCL_OK should be	returned.   If
       the  channel is not open for the specified direction, or if the channel
       implementation does not use device handles, the function should	return
       TCL_ERROR.

       This  value  can	 be  retrieved	with  Tcl_ChannelGetHandleProc,	 which
       returns 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
       returns 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
       option  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)
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