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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_Chan‐
       nelSetOptionProc,    Tcl_ChannelGetOptionProc,	 Tcl_ChannelWatchProc,
       Tcl_ChannelGetHandleProc, Tcl_ChannelFlushProc, Tcl_ChannelHandlerProc,
       - procedures for creating and manipulating channels

SYNOPSIS
       #include <tcl.h>

       Tcl_Channel
       Tcl_CreateChannel(typePtr, channelName, instanceData, mask)

       ClientData
       Tcl_GetChannelInstanceData(channel)

       Tcl_ChannelType *
       Tcl_GetChannelType(channel)

       char *
       Tcl_GetChannelName(channel)

       int
       Tcl_GetChannelHandle(channel, direction, handlePtr)

       int
       Tcl_GetChannelBufferSize(channel)

       Tcl_SetChannelBufferSize(channel, size)

       Tcl_NotifyChannel(channel, mask)

       int
       Tcl_BadChannelOption(interp, optionName, optionList)

       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_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
       Tcl_ChannelType	    *typePtr	   (in)	     Points   to  a  structure
						     containing the  addresses
						     of procedures that can be
						     called to perform I/O and
						     other  functions  on  the
						     channel.

       char		    *channelName   (in)	     The name of this channel,
						     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.

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

       int		    mask	   (in)	     OR-ed   combination    of
						     TCL_READABLE	   and
						     TCL_WRITABLE to  indicate
						     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 handle is wanted.

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

       Tcl_EolTranslation   transMode	   (in)	     The translation mode; one
						     of	    the	     constants
						     TCL_TRANSLATE_AUTO,
						     TCL_TRANSLATE_CR,
						     TCL_TRANSLATE_LF	   and
						     TCL_TRANSLATE_CRLF.

       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 indi‐
						     cates  events  that  have
						     occurred on this channel.

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

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

       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_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_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 chan. 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 set or get option
       procs to generate a complete error message.

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  section  below  for
       details about the old structure.

       The Tcl_ChannelType structure contains the following fields:
	      typedef struct Tcl_ChannelType {
		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_ChannelType;

       The driver must provide implementations for all functions except block‐
       ModeProc, seekProc, setOptionProc, getOptionProc, and close2Proc, which
       may  be specified as NULL.  Other functions that can not be implemented
       for this type of device should return EINVAL when invoked  to  indicate
       that they are not implemented, except in the case of flushProc and han‐
       dlerProc, which should specified as NULL if not otherwise defined.

       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_ChannelSetOptionProc, Tcl_ChannelGe‐ │
       tOptionProc, Tcl_ChannelWatchProc, Tcl_ChannelGetHandleProc,  Tcl_Chan‐ │
       nelFlushProc, 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 (ie: 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 TCL_CHANNEL_VERSION_2.  If it is not │
       set to this value TCL_CHANNEL_VERSION_2, then this  Tcl_ChannelType  is │
       assumed to have the older structure.  See OLD_CHANNEL for more details. │
       While Tcl will recognize and function with  either  structure,  stacked │
       channels must be of the newer style to function correctly.	       │

       This  value  can	 be  retrieved	with Tcl_ChannelVersion, which returns │
       either 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.

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.

       These value can be retrieved with Tcl_ChannelCloseProc or  Tcl_Channel‐ │
       Close2Proc, which returns 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,
		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
       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.

       This value can be retrieved with Tcl_ChannelSeekProc, 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(
		ClientData instanceData,
		Tcl_Interp *interp,
		char *optionName,
		char *optionValue);

       optionName  is the name of an option to set, and optionValue 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 option‐
       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,
		char *optionName,
		Tcl_DString *dsPtr);

       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 dsPtr.	If optionName is NULL,
       the function stores in dsPtr  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	 field
       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 occured.  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  occured │
       on this channel.							       │

       This  value can be retrieved with Tcl_ChannelHandlerProc, 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 a 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 return TCL_ERROR

       An error message is generated in interp's  result  object  to  indicate
       that  a	command	 was invoked with the 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="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
       The  original  (8.3.1 and below) Tcl_ChannelType structure contains the
       following fields:

	      typedef struct Tcl_ChannelType {
		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).

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

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

ATTRIBUTES
       See attributes(5) for descriptions of the following attributes:

       ┌────────────────────┬─────────────────┐
       │  ATTRIBUTE TYPE    │ ATTRIBUTE VALUE │
       ├────────────────────┼─────────────────┤
       │Availability	    │ SUNWTcl	      │
       ├────────────────────┼─────────────────┤
       │Interface Stability │ External	      │
       └────────────────────┴─────────────────┘
NOTES
       Source for Tcl is available in the SUNWTclS package.

Tcl				      8.3		  Tcl_CreateChannel(3)
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