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chan(n)			     Tcl Built-In Commands		       chan(n)


       chan - Read, write and manipulate channels

       chan option ?arg	arg ...?

       This  command  provides several operations for reading from, writing to
       and otherwise manipulating open channels	(such  as  have	 been  created
       with the	open and socket	commands, or the default named channels	stdin,
       stdout or stderr	which correspond to the	process's standard input, out-
       put  and	error streams respectively).  Option indicates what to do with
       the channel; any	unique abbreviation for	option	is  acceptable.	 Valid
       options are:

       chan blocked channelId
	      This  tests  whether  the	 last  input  operation	on the channel
	      called channelId failed because it would have  otherwise	caused
	      the process to block, and	returns	1 if that was the case.	It re-
	      turns 0 otherwise. Note that this	only ever returns 1  when  the
	      channel has been configured to be	non-blocking; all Tcl channels
	      have blocking turned on by default.

       chan close channelId ?direction?
	      Close and	destroy	the channel called channelId. Note  that  this
	      deletes  all existing file-events	registered on the channel.  If |
	      the direction argument (which must  be  read  or	write  or  any |
	      unique  abbreviation  of them) is	present, the channel will only |
	      be half-closed, so that it  can  go  from	 being	read-write  to |
	      write-only  or read-only respectively. If	a read-only channel is |
	      closed for reading, it is	the same as if the  channel  is	 fully |
	      closed,  and respectively	similar	for write-only channels. With- |
	      out the direction	argument, the channel is closed	for both read- |
	      ing  and	writing	 (but  only  if	those directions are currently |
	      open). It	is an error to close a read-only channel for  writing, |
	      or a write-only channel for reading.

	      As  part	of closing the channel,	all buffered output is flushed
	      to the channel's output device (only if the channel  is  ceasing
	      to  be  writable),  any buffered input is	discarded (only	if the
	      channel is ceasing to be	readable),  the	 underlying  operating
	      system  resource is closed and channelId becomes unavailable for
	      future use  (both	 only  if  the	channel	 is  being  completely

	      If  the  channel	is  blocking  and the channel is ceasing to be
	      writable,	the command  does  not	return	until  all  output  is
	      flushed.	 If the	channel	is non-blocking	and there is unflushed
	      output, the channel remains open and the command returns immedi-
	      ately;  output will be flushed in	the background and the channel
	      will be closed when all the flushing is complete.

	      If channelId is a	blocking channel for a command	pipeline  then
	      chan close waits for the child processes to complete.

	      If  the  channel is shared between interpreters, then chan close
	      makes channelId unavailable in the invoking interpreter but  has
	      no  other	 effect	 until	all  of	 the sharing interpreters have
	      closed the channel. When the last	interpreter in which the chan-
	      nel is registered	invokes	chan close (or close), the cleanup ac-
	      tions described above occur. With	half-closing,  the  half-close
	      of the channel only applies to the current interpreter's view of
	      the channel until	all channels have closed it in that  direction
	      (or  completely).	  See  the interp command for a	description of
	      channel sharing.

	      Channels are automatically fully closed when an  interpreter  is
	      destroyed	 and when the process exits.  Channels are switched to
	      blocking mode, to	ensure that all	output	is  correctly  flushed
	      before the process exits.

	      The  command  returns an empty string, and may generate an error
	      if an error occurs while flushing	output.	 If  a	command	 in  a
	      command  pipeline	created	with open returns an error, chan close
	      generates	an error (similar to the exec command.)

	      Note that	half-closes of sockets and command pipelines can  have |
	      important	 side  effects	because	they result in a shutdown() or |
	      close() of the underlying	system resource, which can change  how |
	      other processes or systems respond to the	Tcl program.

       chan configure channelId	?optionName? ?value? ?optionName value?...
	      Query  or	 set  the  configuration  options of the channel named

	      If no optionName or value	arguments are  supplied,  the  command
	      returns  a  list	containing alternating option names and	values
	      for the channel.	If optionName is supplied but  no  value  then
	      the  command  returns the	current	value of the given option.  If
	      one or more pairs	of optionName and value	are supplied, the com-
	      mand  sets each of the named options to the corresponding	value;
	      in this case the return value is an empty	string.

	      The options described below are supported	for all	 channels.  In
	      addition,	 each  channel	type may add options that only it sup-
	      ports. See the manual entry for the command  that	 creates  each
	      type  of channel for the options supported by that specific type
	      of channel. For example, see the manual  entry  for  the	socket
	      command for additional options for sockets, and the open command
	      for additional options for serial	devices.

	      -blocking	boolean
		     The -blocking option determines whether I/O operations on
		     the  channel can cause the	process	to block indefinitely.
		     The value of the option must be a proper  boolean	value.
		     Channels  are  normally in	blocking mode; if a channel is
		     placed into non-blocking mode it will affect  the	opera-
		     tion  of the chan gets, chan read,	chan puts, chan	flush,
		     and chan close commands; see the documentation for	 those
		     commands for details.  For	non-blocking mode to work cor-
		     rectly, the application must be using the Tcl event  loop
		     (e.g.  by	calling	 Tcl_DoOneEvent	 or invoking the vwait

	      -buffering newValue
		     If	newValue is full then the I/O system will buffer  out-
		     put  until	 its internal buffer is	full or	until the chan
		     flush command is invoked. If newValue is line,  then  the
		     I/O  system will automatically flush output for the chan-
		     nel whenever a newline character is output.  If  newValue
		     is	 none,	the  I/O system	will flush automatically after
		     every output operation.  The default is for -buffering to
		     be	set to full except for channels	that connect to	termi-
		     nal-like devices; for these channels the initial  setting
		     is	 line.	 Additionally,	stdin and stdout are initially
		     set to line, and stderr is	set to none.

	      -buffersize newSize
		     Newvalue must be an integer; its value is used to set the
		     size  of  buffers,	 in  bytes, subsequently allocated for
		     this channel to store input or output. Newvalue must be a
		     number  of	 no more than one million, allowing buffers of
		     up	to one million bytes in	size.

	      -encoding	name
		     This option is used to specify the	encoding of the	 chan-
		     nel  as  one  of the named	encodings returned by encoding
		     names or the special value	binary,	so that	the  data  can
		     be	converted to and from Unicode for use in Tcl.  For in-
		     stance, in	order for Tcl to read characters from a	 Japa-
		     nese  file	 in  shiftjis and properly process and display
		     the contents, the encoding	 would	be  set	 to  shiftjis.
		     Thereafter,  when	reading	from the channel, the bytes in
		     the Japanese file would be	converted to Unicode  as  they
		     are read.	Writing	is also	supported - as Tcl strings are
		     written to	the channel they will  automatically  be  con-
		     verted to the specified encoding on output.

		     If	a file contains	pure binary data (for instance,	a JPEG
		     image), the encoding for the channel should be configured
		     to	 be binary.  Tcl will then assign no interpretation to
		     the data in the file and simply read or write raw	bytes.
		     The  Tcl  binary  command	can be used to manipulate this
		     byte-oriented data.  It is	 usually  better  to  set  the
		     -translation  option  to binary when you want to transfer
		     binary data, as this turns	off the	other automatic	inter-
		     pretations	of the bytes in	the stream as well.

		     The  default  encoding  for  newly	opened channels	is the
		     same platform- and	locale-dependent system	encoding  used
		     for interfacing with the operating	system,	as returned by
		     encoding system.

	      -eofchar char

	      -eofchar {inChar outChar}
		     This option supports DOS file systems that	use  Control-z
		     (\x1a) as an end of file marker.  If char is not an empty
		     string, then this character signals end-of-file  when  it
		     is	encountered during input.  For output, the end-of-file
		     character is output when the channel is closed.  If  char
		     is	the empty string, then there is	no special end of file
		     character marker.	For read-write channels, a two-element
		     list  specifies the end of	file marker for	input and out-
		     put, respectively.	 As a convenience,  when  setting  the
		     end-of-file  character  for  a read-write channel you can
		     specify a single value that will apply  to	 both  reading
		     and  writing.  When querying the end-of-file character of
		     a read-write channel, a two-element list will  always  be
		     returned.	 The  default  value for -eofchar is the empty
		     string in all cases except	for files under	 Windows.   In
		     that  case	 the  -eofchar is Control-z (\x1a) for reading
		     and the empty string for writing.	The  acceptable	 range
		     for  -eofchar  values  is	\x01 - \x7f; attempting	to set
		     -eofchar to a value outside of this range	will  generate
		     an	error.

	      -translation mode

	      -translation {inMode outMode}
		     In	 Tcl  scripts  the end of a line is always represented
		     using a single newline character (\n).  However,  in  ac-
		     tual  files  and  devices the end of a line may be	repre-
		     sented differently	on different platforms,	 or  even  for
		     different devices on the same platform.  For example, un-
		     der UNIX newlines are used	in files, whereas carriage-re-
		     turn-linefeed sequences are normally used in network con-
		     nections.	On input (i.e.,	with chan gets and chan	 read)
		     the  Tcl I/O system automatically translates the external
		     end-of-line representation	into newline characters.  Upon
		     output  (i.e., with chan puts), the I/O system translates
		     newlines to the external end-of-line representation.  The
		     default  translation  mode,  auto,	handles	all the	common
		     cases automatically, but the -translation option provides
		     explicit control over the end of line translations.

		     The  value	 associated with -translation is a single item
		     for read-only and write-only channels.  The  value	 is  a
		     two-element list for read-write channels; the read	trans-
		     lation mode is the	first element of  the  list,  and  the
		     write  translation	mode is	the second element.  As	a con-
		     venience, when setting the	translation mode for  a	 read-
		     write  channel  you  can specify a	single value that will
		     apply to both reading and	writing.   When	 querying  the
		     translation  mode	of a read-write	channel, a two-element
		     list will always be returned.  The	following  values  are
		     currently supported:

		     auto   As	the input translation mode, auto treats	any of
			    newline (lf), carriage return  (cr),  or  carriage
			    return  followed by	a newline (crlf) as the	end of
			    line representation.  The end of line  representa-
			    tion  can  even  change from line-to-line, and all
			    cases are translated to a newline.	As the	output
			    translation	mode, auto chooses a platform specific
			    representation; for	sockets	on all	platforms  Tcl
			    chooses crlf, for all Unix flavors,	it chooses lf,
			    and	for the	various	flavors	of Windows it  chooses
			    crlf.   The	 default  setting  for -translation is
			    auto for both input	and output.

		     binary No end-of-line translations	are  performed.	  This
			    is nearly identical	to lf mode, except that	in ad-
			    dition binary mode also sets the end-of-file char-
			    acter  to the empty	string (which disables it) and
			    sets the encoding to binary	(which disables	encod-
			    ing	 filtering).   See the description of -eofchar
			    and	-encoding for more information.

		     cr	    The	end of a line in the underlying	file or	device
			    is represented by a	single carriage	return charac-
			    ter.  As the input translation mode, cr mode  con-
			    verts  carriage returns to newline characters.  As
			    the	output translation mode,  cr  mode  translates
			    newline characters to carriage returns.

		     crlf   The	end of a line in the underlying	file or	device
			    is represented by a	carriage return	character fol-
			    lowed  by  a  linefeed  character.	 As  the input
			    translation	mode, crlf mode	converts  carriage-re-
			    turn-linefeed sequences to newline characters.  As
			    the	output translation mode, crlf mode  translates
			    newline characters to carriage-return-linefeed se-
			    quences.  This mode	is typically used  on  Windows
			    platforms and for network connections.

		     lf	    The	end of a line in the underlying	file or	device
			    is represented  by	a  single  newline  (linefeed)
			    character.	 In  this  mode	 no translations occur
			    during either input	or output.  This mode is typi-
			    cally used on UNIX platforms.

       chan copy inputChan outputChan ?-size size? ?-command callback?
	      Copy  data  from	the  channel  inputChan,  which	must have been
	      opened for reading, to the channel outputChan, which  must  have
	      been  opened  for	 writing.  The chan copy command leverages the
	      buffering	in the Tcl I/O system to avoid	extra  copies  and  to
	      avoid  buffering too much	data in	main memory when copying large
	      files to slow destinations like network sockets.

	      The chan copy command transfers data from	inputChan until	end of
	      file  or size bytes or characters	have been transferred; size is
	      in bytes if the two channels are using the same encoding,	and is
	      in  characters  otherwise.   If no -size argument	is given, then
	      the copy goes until end of file. All  the	 data  read  from  in-
	      putChan  is  copied to outputChan.  Without the -command option,
	      chan copy	blocks until the copy is complete and returns the num-
	      ber  of  bytes  or  characters  (using the same rules as for the
	      -size option) written to outputChan.

	      The -command argument makes chan copy work  in  the  background.
	      In  this case it returns immediately and the callback is invoked
	      later when the copy completes.  The callback is called with  one
	      or  two  additional arguments that indicates how many bytes were
	      written to outputChan.  If an error occurred  during  the	 back-
	      ground  copy, the	second argument	is the error string associated
	      with the error.  With a background copy, it is not necessary  to
	      put  inputChan  or  outputChan  into non-blocking	mode; the chan
	      copy command takes care of that automatically.  However,	it  is
	      necessary	 to enter the event loop by using the vwait command or
	      by using Tk.

	      You are not allowed to do	other I/O operations with inputChan or
	      outputChan  during  a background chan copy.  If either inputChan
	      or outputChan get	closed while the copy is in progress, the cur-
	      rent  copy  is stopped and the command callback is not made.  If
	      inputChan	is closed, then	all data already queued	for outputChan
	      is written out.

	      Note  that  inputChan  can  become  readable during a background
	      copy.  You should	turn off any chan event	or fileevent  handlers
	      during a background copy so those	handlers do not	interfere with
	      the copy.	 Any I/O attempted by a	chan event or  fileevent  han-
	      dler will	get a "channel busy" error.

	      Chan copy	translates end-of-line sequences in inputChan and out-
	      putChan according	to the -translation option for these  channels
	      (see chan	configure above).  The translations mean that the num-
	      ber of bytes read	from inputChan can be different	than the  num-
	      ber  of  bytes  written to outputChan.  Only the number of bytes
	      written to outputChan is reported, either	as the return value of
	      a	 synchronous  chan copy	or as the argument to the callback for
	      an asynchronous chan copy.

	      Chan copy	obeys the encodings and	character translations config-
	      ured  for	 the channels. This means that the incoming characters
	      are converted internally first UTF-8 and then into the  encoding
	      of the channel chan copy writes to (see chan configure above for
	      details on the -encoding and -translation	options).  No  conver-
	      sion  is	done  if  both channels	are set	to encoding binary and
	      have matching translations. If only the output channel is	set to
	      encoding	binary the system will write the internal UTF-8	repre-
	      sentation	of the incoming	characters. If only the	input  channel
	      is set to	encoding binary	the system will	assume that the	incom-
	      ing bytes	are valid UTF-8	characters and convert them  according
	      to  the  output  encoding. The behaviour of the system for bytes
	      which are	not valid UTF-8	characters is undefined	in this	case.

       chan create mode	cmdPrefix
	      This subcommand creates a	new script  level  channel  using  the
	      command  prefix  cmdPrefix  as  its handler. Any such channel is
	      called a reflected channel. The specified	command	 prefix,  cmd-
	      Prefix, must be a	non-empty list,	and should provide the API de-
	      scribed in the refchan manual page. The handle of	the new	 chan-
	      nel  is  returned	 as the	result of the chan create command, and
	      the channel is open. Use either close or chan  close  to	remove
	      the channel.

	      The  argument  mode  specifies  if the new channel is opened for
	      reading, writing,	or both. It has	to be a	list containing	any of
	      the  strings "read" or "write".  The list	must have at least one
	      element, as a channel you	can neither write  to  nor  read  from
	      makes  no	 sense.	 The  handler command for the new channel must
	      support the chosen mode, or an error is thrown.

	      The command prefix is executed in	the global namespace,  at  the
	      top  of  call stack, following the appending of arguments	as de-
	      scribed in the refchan manual page. Command  resolution  happens
	      at  the time of the call.	Renaming the command, or destroying it
	      means that the next call of a handler method may	fail,  causing
	      the  channel  command  invoking the handler to fail as well. De-
	      pending on the subcommand	being invoked, the error  message  may
	      not be able to explain the reason	for that failure.

	      Every  channel  created  with this subcommand knows which	inter-
	      preter it	was created in,	and only  ever	executes  its  handler
	      command in that interpreter, even	if the channel was shared with
	      and/or was moved into a different	 interpreter.  Each  reflected
	      channel  also  knows  the	thread it was created in, and executes
	      its handler command only in that thread, even if the channel was
	      moved  into  a  different	thread.	To this	end all	invocations of
	      the handler are forwarded	to the original	thread by posting spe-
	      cial events to it. This means that the original thread (i.e. the
	      thread that executed the chan create command) must have  an  ac-
	      tive  event  loop,  i.e. it must be able to process such events.
	      Otherwise	the thread sending them	will block indefinitely. Dead-
	      lock may occur.

	      Note  that this permits the creation of a	channel	whose two end-
	      points live in two different threads,  providing	a  stream-ori-
	      ented  bridge between these threads. In other words, we can pro-
	      vide a way for regular stream communication between threads  in-
	      stead of having to send commands.

	      When  a  thread  or interpreter is deleted, all channels created
	      with this	subcommand and using this thread/interpreter as	 their
	      computing	 base  are  deleted  as	well, in all interpreters they
	      have been	shared with or moved into, and in whatever thread they
	      have been	transferred to.	While this pulls the rug out under the
	      other thread(s) and/or interpreter(s), this cannot  be  avoided.
	      Trying to	use such a channel will	cause the generation of	a reg-
	      ular error about unknown channel handles.

	      This subcommand is safe  and  made  accessible  to  safe	inter-
	      preters.	 While	it arranges for	the execution of arbitrary Tcl
	      code the system also makes sure that the code is always executed
	      within the safe interpreter.

       chan eof	channelId
	      Test  whether  the  last	input  operation on the	channel	called
	      channelId	failed because the end of the data stream was reached,
	      returning	1 if end-of-file was reached, and 0 otherwise.

       chan event channelId event ?script?
	      Arrange  for  the	 Tcl  script  script to	be installed as	a file
	      event handler to be called whenever the channel called channelId
	      enters  the state	described by event (which must be either read-
	      able or writable); only one such handler may  be	installed  per
	      event per	channel	at a time.  If script is the empty string, the
	      current handler is deleted (this also happens if the channel  is
	      closed  or  the interpreter deleted).  If	script is omitted, the
	      currently	installed script is returned (or an empty string if no
	      such  handler  is	installed).  The callback is only performed if
	      the event	loop is	being serviced (e.g. via vwait or update).

	      A	file event handler is  a  binding  between  a  channel	and  a
	      script,  such  that the script is	evaluated whenever the channel
	      becomes readable or writable.  File event	handlers are most com-
	      monly  used to allow data	to be received from another process on
	      an event-driven basis, so	that the receiver can continue to  in-
	      teract  with  the	 user or with other channels while waiting for
	      the data to arrive.  If an application invokes chan gets or chan
	      read  on	a  blocking channel when there is no input data	avail-
	      able, the	process	will block; until the input data  arrives,  it
	      will  not	 be able to service other events, so it	will appear to
	      the user to "freeze up".	With chan event, the process can  tell
	      when data	is present and only invoke chan	gets or	chan read when
	      they will	not block.

	      A	channel	is considered to be readable if	there is  unread  data
	      available	 on  the underlying device.  A channel is also consid-
	      ered to be readable if there is unread data in an	input  buffer,
	      except in	the special case where the most	recent attempt to read
	      from the channel was a chan gets call that could not find	a com-
	      plete  line  in the input	buffer.	 This feature allows a file to
	      be read a	line at	a time in non-blocking mode using  events.   A
	      channel  is  also	considered to be readable if an	end of file or
	      error condition is present on the	underlying file	or device.  It
	      is important for script to check for these conditions and	handle
	      them appropriately; for example, if there	is  no	special	 check
	      for  end	of file, an infinite loop may occur where script reads
	      no data, returns,	and is immediately invoked again.

	      A	channel	is considered to be writable if	at least one  byte  of
	      data  can	 be  written  to the underlying	file or	device without
	      blocking,	or if an error condition is present on the  underlying
	      file or device.  Note that client	sockets	opened in asynchronous
	      mode become writable when	they become connected or if  the  con-
	      nection fails.

	      Event-driven  I/O	 works best for	channels that have been	placed
	      into non-blocking	mode with  the	chan  configure	 command.   In
	      blocking mode, a chan puts command may block if you give it more
	      data than	the underlying file or device can accept, and  a  chan
	      gets or chan read	command	will block if you attempt to read more
	      data than	is ready; no events will be processed while  the  com-
	      mands  block.   In  non-blocking	mode chan puts,	chan read, and
	      chan gets	never block.

	      The script for a file event is executed at global	level (outside
	      the  context  of	any Tcl	procedure) in the interpreter in which
	      the chan event command was invoked.  If an  error	 occurs	 while
	      executing	 the  script  then  the	command	registered with	interp
	      bgerror is used to report	the  error.   In  addition,  the  file
	      event  handler  is  deleted if it	ever returns an	error; this is
	      done in order to prevent infinite	loops due to buggy handlers.

       chan flush channelId
	      Ensures that all pending output for the channel called channelId
	      is written.

	      If  the  channel is in blocking mode the command does not	return
	      until all	the buffered output has	been flushed to	 the  channel.
	      If  the  channel is in non-blocking mode,	the command may	return
	      before all buffered output has been flushed; the remainder  will
	      be  flushed  in the background as	fast as	the underlying file or
	      device is	able to	absorb it.

       chan gets channelId ?varName?
	      Reads the	next line from the channel called channelId.  If  var-
	      Name  is	not  specified,	 the result of the command will	be the
	      line that	has been read (without a trailing  newline  character)
	      or an empty string upon end-of-file or, in non-blocking mode, if
	      the data available is exhausted. If varName  is  specified,  the
	      line  that  has been read	will be	written	to the variable	called
	      varName and result will be the number of	characters  that  have
	      been  read  or -1	if end-of-file was reached or, in non-blocking
	      mode, if the data	available is exhausted.

	      If an end-of-file	occurs while part way through reading a	 line,
	      the  partial  line  will	be returned (or	written	into varName).
	      When varName is not specified, the end-of-file case can be  dis-
	      tinguished  from	an  empty line using the chan eof command, and
	      the partial-line-but-non-blocking	case can be distinguished with
	      the chan blocked command.

       chan names ?pattern?
	      Produces	a  list	of all channel names. If pattern is specified,
	      only those channel names that match it (according	to  the	 rules
	      of string	match) will be returned.

       chan pending mode channelId
	      Depending	on whether mode	is input or output, returns the	number
	      of bytes of input	or output  (respectively)  currently  buffered
	      internally  for channelId	(especially useful in a	readable event
	      callback to impose application-specific  limits  on  input  line
	      lengths  to  avoid  a potential denial-of-service	attack where a
	      hostile user crafts an extremely	long  line  that  exceeds  the
	      available	 memory	 to buffer it).	 Returns -1 if the channel was
	      not opened for the mode in question.

       chan pipe
	      Creates a	standalone pipe	whose read-  and  write-side  channels |
	      are  returned  as	 a 2-element list, the first element being the |
	      read side	and the	second the write side. Can be useful  e.g.  to |
	      redirect	separately  stderr and stdout from a subprocess. To do |
	      this, spawn with "2>@" or	">@" redirection  operators  onto  the |
	      write  side of a pipe, and then immediately close	it in the par- |
	      ent. This	is necessary to	get an EOF on the read side  once  the |
	      child has	exited or otherwise closed its output.		       |

	      Note that	the pipe buffering semantics can vary at the operating |
	      system level substantially; it is	not  safe  to  assume  that  a |
	      write  performed	on the output side of the pipe will appear in- |
	      stantly to the input side. This is a fundamental difference  and |
	      Tcl cannot conceal it. The overall stream	semantics are compati- |
	      ble, so blocking reads and writes	will not see most of the  dif- |
	      ferences,	 but the details of what exactly gets written when are |
	      not. This	is most	likely to show up  when	 using	pipelines  for |
	      testing;	care  should  be taken to ensure that deadlocks	do not |
	      occur and	that potential short reads are allowed for.	       |

       chan pop	channelId
	      Removes the topmost transformation from the  channel  channelId, |
	      if  there	is any.	If there are no	transformations	added to chan- |
	      nelId, this is equivalent	to chan	close of that channel. The re- |
	      sult  is	normally the empty string, but can be an error in some |
	      situations (i.e. where the underlying system  stream  is	closed |
	      and that results in an error).

       chan postevent channelId	eventSpec
	      This  subcommand is used by command handlers specified with chan
	      create. It notifies the channel represented by the handle	 chan-
	      nelId  that  the event(s)	listed in the eventSpec	have occurred.
	      The argument has to be a list containing any of the strings read
	      and write. The list must contain at least	one element as it does
	      not make sense to	invoke the command if there are	no  events  to

	      Note  that this subcommand can only be used with channel handles
	      that were	created/opened by chan create. All other channels will
	      cause this subcommand to report an error.

	      As  only	the  Tcl level of a channel, i.e. its command handler,
	      should post events to it we also restrict	the usage of this com-
	      mand  to	the  interpreter  that	created	 the channel. In other
	      words, posting events to a reflected channel from	an interpreter
	      that  does  not  contain it's implementation is not allowed. At-
	      tempting to post an event	from any other interpreter will	 cause
	      this subcommand to report	an error.

	      Another  restriction  is	that it	is not possible	to post	events
	      that the I/O core	has not	registered an interest in.  Trying  to
	      do  so  will cause the method to throw an	error. See the command
	      handler method watch described in	refchan, the document specify-
	      ing the API of command handlers for reflected channels.

	      This  command  is	safe and made accessible to safe interpreters.
	      It can trigger the execution of chan event handlers, whether  in
	      the  current  interpreter	 or  in	 other	interpreters  or other
	      threads, even where the event is posted from a safe  interpreter
	      and  listened  for by a trusted interpreter. Chan	event handlers
	      are always executed in the interpreter that set them up.

       chan push channelId cmdPrefix
	      Adds a new transformation	on top of the channel  channelId.  The |
	      cmdPrefix	 argument  describes a list of one or more words which |
	      represent	a handler that will be used to implement the transfor- |
	      mation. The command prefix must provide the API described	in the |
	      transchan	manual page.  The result of this subcommand is a  han- |
	      dle  to  the  transformation.  Note that it is important to make |
	      sure that	the transformation is capable of supporting the	 chan- |
	      nel  mode	that it	is used	with or	this can make the channel nei- |
	      ther readable nor	writable.

       chan puts ?-nonewline? ?channelId? string
	      Writes string to the channel named channelId followed by a  new-
	      line  character.	A trailing newline character is	written	unless
	      the optional flag	-nonewline is given. If	channelId is  omitted,
	      the string is written to the standard output channel, stdout.

	      Newline  characters in the output	are translated by chan puts to
	      platform-specific	end-of-line sequences according	 to  the  cur-
	      rently configured	value of the -translation option for the chan-
	      nel (for example,	on PCs newlines	 are  normally	replaced  with
	      carriage-return-linefeed sequences; see chan configure above for

	      Tcl buffers output internally, so	characters written  with  chan
	      puts  may	 not  appear immediately on the	output file or device;
	      Tcl will normally	delay output until the buffer is full  or  the
	      channel  is  closed.  You	can force output to appear immediately
	      with the chan flush command.

	      When the output buffer fills up, the chan	puts command will nor-
	      mally  block  until  all the buffered data has been accepted for
	      output by	the operating system.  If channelId is in non-blocking
	      mode then	the chan puts command will not block even if the oper-
	      ating system cannot accept the data.  Instead, Tcl continues  to
	      buffer  the  data	and writes it in the background	as fast	as the
	      underlying file or device	can accept it.	The  application  must
	      use  the	Tcl event loop for non-blocking	output to work;	other-
	      wise Tcl never finds out that the	file or	device	is  ready  for
	      more  output  data.   It	is  possible  for an arbitrarily large
	      amount of	data to	be buffered  for  a  channel  in  non-blocking
	      mode,  which  could  consume a large amount of memory.  To avoid
	      wasting memory, non-blocking I/O should normally be used	in  an
	      event-driven  fashion with the chan event	command	(do not	invoke
	      chan puts	unless you have	recently  been	notified  via  a  file
	      event that the channel is	ready for more output data).

       chan read channelId ?numChars?

       chan read ?-nonewline? channelId
	      In  the first form, the result will be the next numChars charac-
	      ters read	from the channel named channelId; if numChars is omit-
	      ted,  all	characters up to the point when	the channel would sig-
	      nal a failure (whether an	end-of-file, blocked  or  other	 error
	      condition)  are read. In the second form (i.e. when numChars has
	      been omitted) the	flag -nonewline	may be given to	indicate  that
	      any  trailing newline in the string that has been	read should be

	      If channelId is in non-blocking mode, chan read may not read  as
	      many  characters as requested: once all available	input has been
	      read, the	command	will return the	data that is available	rather
	      than  blocking  for more input.  If the channel is configured to
	      use a multi-byte encoding, then there may	actually be some bytes
	      remaining	 in  the  internal buffers that	do not form a complete
	      character.  These	bytes will not be returned  until  a  complete
	      character	 is  available or end-of-file is reached.  The -nonew-
	      line switch is ignored if	the command  returns  before  reaching
	      the end of the file.

	      Chan  read  translates  end-of-line  sequences in	the input into
	      newline characters according to the -translation option for  the
	      channel  (see  chan configure above for a	discussion on the ways
	      in which chan configure will alter input).

	      When reading from	a serial port, most applications  should  con-
	      figure the serial	port channel to	be non-blocking, like this:

		     chan configure channelId -blocking	0.

	      Then  chan  read	behaves	 much like described above.  Note that
	      most serial ports	are comparatively slow;	it is entirely	possi-
	      ble  to  get a readable event for	each character read from them.
	      Care must	be taken when  using  chan  read  on  blocking	serial

	      chan read	channelId numChars
		     In	 this  form  chan read blocks until numChars have been
		     received from the serial port.

	      chan read	channelId
		     In	this form chan read blocks until the reception of  the
		     end-of-file  character,  see  chan	configure -eofchar. If
		     there no end-of-file character has	 been  configured  for
		     the channel, then chan read will block forever.

       chan seek channelId offset ?origin?
	      Sets  the	 current  access  position  within the underlying data
	      stream for the channel named channelId to	be offset bytes	 rela-
	      tive  to	origin.	 Offset	must be	an integer (which may be nega-
	      tive) and	origin must be one of the following:

	      start	The new	access position	will be	offset bytes from  the
			start of the underlying	file or	device.

	      current	The  new access	position will be offset	bytes from the
			current	access position; a negative offset  moves  the
			access	position  backwards  in	the underlying file or

	      end	The new	access position	will be	offset bytes from  the
			end  of	 the file or device.  A	negative offset	places
			the access position before the end of file, and	a pos-
			itive  offset places the access	position after the end
			of file.

	      The origin argument defaults to start.

	      Chan seek	flushes	all buffered output for	the channel before the
	      command  returns,	 even  if the channel is in non-blocking mode.
	      It also discards any buffered and	unread	input.	 This  command
	      returns an empty string.	An error occurs	if this	command	is ap-
	      plied to channels	whose underlying file or device	does not  sup-
	      port seeking.

	      Note that	offset values are byte offsets,	not character offsets.
	      Both chan	seek and chan tell operate  in	terms  of  bytes,  not
	      characters, unlike chan read.

       chan tell channelId
	      Returns  a  number giving	the current access position within the
	      underlying data stream for the  channel  named  channelId.  This
	      value  returned is a byte	offset that can	be passed to chan seek
	      in order to set the channel to a particular position.  Note that
	      this  value is in	terms of bytes,	not characters like chan read.
	      The value	returned is -1 for channels that do not	support	 seek-

       chan truncate channelId ?length?
	      Sets the byte length of the underlying data stream for the chan-
	      nel named	channelId to be	length (or to the current byte	offset
	      within  the  underlying  data  stream if length is omitted). The
	      channel is flushed before	truncation.

       This opens a file using a known encoding	(CP1252, a very	common	encod-
       ing  on	Windows), searches for a string, rewrites that part, and trun-
       cates the file after a further two lines.

	      set f [open somefile.txt r+]
	      chan configure $f	-encoding cp1252
	      set offset 0

	      #	Search for string "FOOBAR" in the file
	      while {[chan gets	$f line] >= 0} {
		  set idx [string first	FOOBAR $line]
		  if {$idx > -1} {
		      #	Found it; rewrite line

		      chan seek	$f [expr {$offset + $idx}]
		      chan puts	-nonewline $f BARFOO

		      #	Skip to	end of following line, and truncate
		      chan gets	$f
		      chan gets	$f
		      chan truncate $f

		      #	Stop searching the file	now

		  # Save offset	of start of next line for later
		  set offset [chan tell	$f]
	      chan close $f

       A network server	that does echoing of its  input	 line-by-line  without
       preventing servicing of other connections at the	same time.

	      #	This is	a very simple logger...
	      proc log {message} {
		  chan puts stdout $message

	      #	This is	called whenever	a new client connects to the server
	      proc connect {chan host port} {
		  set clientName [format <%s:%d> $host $port]
		  log "connection from $clientName"
		  chan configure $chan -blocking 0 -buffering line
		  chan event $chan readable [list echoLine $chan $clientName]

	      #	This is	called whenever	either at least	one byte of input
	      #	data is	available, or the channel was closed by	the client.
	      proc echoLine {chan clientName} {
		  chan gets $chan line
		  if {[chan eof	$chan]}	{
		      log "finishing connection	from $clientName"
		      chan close $chan
		  } elseif {![chan blocked $chan]} {
		      #	Didn't block waiting for end-of-line
		      log "$clientName - $line"
		      chan puts	$chan $line

	      #	Create the server socket and enter the event-loop to wait
	      #	for incoming connections...
	      socket -server connect 12345
	      vwait forever

       close(n),   eof(n),   fblocked(n),  fconfigure(n),  fcopy(n),  file(n),
       fileevent(n), flush(n), gets(n),	open(n),  puts(n),  read(n),  seek(n),
       socket(n), tell(n), refchan(n), transchan(n)

       channel,	input, output, events, offset

Tcl				      8.5			       chan(n)


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