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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_SetDefaultTranslation,  Tcl_SetChannel‐
       BufferSize, Tcl_NotifyChannel, Tcl_BadChannelOption  -  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_GetChannelFlags(channel)

       Tcl_SetDefaultTranslation(channel, transMode)

       int
       Tcl_GetChannelBufferSize(channel)

       Tcl_SetChannelBufferSize(channel, size)

       Tcl_NotifyChannel(channel, mask)					       │

       int								       │
       Tcl_BadChannelOption(interp, optionName, optionList)		       │

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_SetDefaultTranslation sets the  default  end	 of  line  translation
       mode. This mode will be installed as the translation mode for the chan‐
       nel if an attempt is made to output on the channel while it is still in
       TCL_TRANSLATE_AUTO  mode.  For a description of end of line translation
       modes, see the manual entry for fconfigure.

       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
       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_ChannelType;					       │

       The driver must provide implementations for all functions except block‐
       ModeProc, seekProc, setOptionProc,  and	getOptionProc,	which  may  be
       specified  as  NULL to indicate that the channel does not support seek‐
       ing.  Other functions that can not be  implemented  for	this  type  of
       device  should return EINVAL when invoked to indicate that they are not
       implemented.

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.

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.

CLOSEPROC
       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
       interp->result.

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.

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.

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_SeekChannel 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.

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.

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.

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.							       │

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.

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.

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

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

Tcl				      8.0		  Tcl_CreateChannel(3)
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