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

______________________________________________________________________________

NAME
       open - Open a file-based	or command pipeline channel

SYNOPSIS
       open fileName
       open fileName access
       open fileName access permissions
______________________________________________________________________________

DESCRIPTION
       This command opens a file, serial port, or command pipeline and returns
       a channel identifier that may be	used in	future invocations of commands
       like  read, puts, and close.  If	the first character of fileName	is not
       | then the command opens	a file:	fileName gives the name	of the file to
       open,  and it must conform to the conventions described in the filename
       manual entry.

       The access argument, if present,	indicates the way in  which  the  file
       (or  command pipeline) is to be accessed.  In the first form access may
       have any	of the following values:

       r	      Open the file for	reading	only; the  file	 must  already
		      exist. This is the default value if access is not	speci-
		      fied.

       r+	      Open the file for	both reading  and  writing;  the  file
		      must already exist.

       w	      Open  the	 file for writing only.	 Truncate it if	it ex-
		      ists.  If	it does	not exist, create a new	file.

       w+	      Open the file for	reading	and writing.  Truncate	it  if
		      it exists.  If it	does not exist,	create a new file.

       a	      Open  the	 file  for writing only.  If the file does not
		      exist, create a new empty	file.  Set the file pointer to
		      the end of the file prior	to each	write.

       a+	      Open the file for	reading	and writing.  If the file does
		      not exist, create	a new empty file.  Set the initial ac-
		      cess position  to	the end	of the file.

       All  of the legal access	values above may have the character b added as
       the second or third character in	the value to indicate that the	opened
       channel should be configured as if with the fconfigure -translation bi-
       nary option, making the channel suitable	for reading or writing of  bi-
       nary data.

       In  the	second form, access consists of	a list of any of the following
       flags, all of which have	the standard POSIX meanings.  One of the flags
       must be either RDONLY, WRONLY or	RDWR.

       RDONLY	      Open the file for	reading	only.

       WRONLY	      Open the file for	writing	only.

       RDWR	      Open the file for	both reading and writing.

       APPEND	      Set  the	file  pointer  to the end of the file prior to
		      each write.

       BINARY	      Configure	the opened channel with	the  -translation  bi-
		      nary option.

       CREAT	      Create  the  file	 if it does not	already	exist (without
		      this flag	it is an error for the file not	to exist).

       EXCL	      If CREAT is also specified, an error is returned if  the
		      file already exists.

       NOCTTY	      If the file is a terminal	device,	this flag prevents the
		      file from	 becoming  the	controlling  terminal  of  the
		      process.

       NONBLOCK	      Prevents	the  process  from  blocking while opening the
		      file, and	possibly in subsequent	I/O  operations.   The
		      exact behavior of	this flag is system- and device-depen-
		      dent;  its use is	discouraged (it	is better to  use  the
		      fconfigure  command  to put a file in nonblocking	mode).
		      For details refer	to your	system	documentation  on  the
		      open system call's O_NONBLOCK flag.

       TRUNC	      If the file exists it is truncated to zero length.

       If  a  new file is created as part of opening it, permissions (an inte-
       ger) is used to set the permissions for the  new	 file  in  conjunction
       with  the  process's  file mode creation	mask.  Permissions defaults to
       0666.

COMMAND	PIPELINES
       If the first character of fileName is "|" then the remaining characters
       of  fileName are	treated	as a list of arguments that describe a command
       pipeline	to invoke, in the same style as	the arguments  for  exec.   In
       this case, the channel identifier returned by open may be used to write
       to the command's	input pipe or read from	its output pipe, depending  on
       the  value  of  access.	 If  write-only	access is used (e.g. access is
       "w"), then standard output for the pipeline is directed to the  current
       standard	 output	unless overridden by the command.  If read-only	access
       is used (e.g. access is "r"), standard input for	the pipeline is	 taken
       from  the current standard input	unless overridden by the command.  The
       id of the spawned process is accessible through the pid command,	 using
       the channel id returned by open as argument.

       If  the	command	(or one	of the commands) executed in the command pipe-
       line returns an error (according	to the definition in exec), a Tcl  er-
       ror  is	generated when close is	called on the channel unless the pipe-
       line is in non-blocking mode then no exit status	is returned (a	silent
       close with -blocking 0).

       It is often useful to use the fileevent command with pipelines so other
       processing may happen at	the same time as running the  command  in  the
       background.

SERIAL COMMUNICATIONS
       If  fileName refers to a	serial port, then the specified	serial port is
       opened and initialized in a platform-dependent manner.  Acceptable val-
       ues  for	the fileName to	use to open a serial port are described	in the
       PORTABILITY ISSUES section.

       The fconfigure command can be used to query and set additional configu-
       ration options specific to serial ports (where supported):

       -mode baud,parity,data,stop
	      This option is a set of 4	comma-separated	values:	the baud rate,
	      parity, number of	data bits, and number of stop  bits  for  this
	      serial  port.   The baud rate is a simple	integer	that specifies
	      the connection speed.  Parity is one of the  following  letters:
	      n,  o,  e,  m,  s; respectively signifying the parity options of
	      "none", "odd", "even", "mark", or	"space".  Data is  the	number
	      of data bits and should be an integer from 5 to 8, while stop is
	      the number of stop bits and should be the	integer	1 or 2.

       -handshake type
	      (Windows and Unix). This option is used to setup automatic hand-
	      shake control. Note that not all handshake types maybe supported
	      by your operating	system.	The type  parameter  is	 case-indepen-
	      dent.

	      If  type is none then any	handshake is switched off.  rtscts ac-
	      tivates hardware handshake. Hardware handshake signals  are  de-
	      scribed  below.	For  software  handshake xonxoff the handshake
	      characters can be	redefined with -xchar.	An additional hardware
	      handshake	 dtrdsr	 is available only under Windows.  There is no
	      default handshake	configuration, the initial  value  depends  on
	      your operating system settings.  The -handshake option cannot be
	      queried.

       -queue (Windows and Unix). The -queue option can	only be	 queried.   It
	      returns  a  list of two integers representing the	current	number
	      of bytes in the input and	output queue respectively.

       -timeout	msec
	      (Windows and Unix). This option is used to set the  timeout  for
	      blocking	read operations. It specifies the maximum interval be-
	      tween the	reception of two bytes in milliseconds.	 For Unix sys-
	      tems  the	 granularity is	100 milliseconds.  The -timeout	option
	      does not affect write operations or nonblocking reads.  This op-
	      tion cannot be queried.

       -ttycontrol {signal boolean signal boolean ...}
	      (Windows	and  Unix). This option	is used	to setup the handshake
	      output lines (see	below) permanently or to send a	BREAK over the
	      serial line.  The	signal names are case-independent.  {RTS 1 DTR
	      0} sets the RTS output to	high and the DTR output	to  low.   The
	      BREAK  condition (see below) is enabled and disabled with	{BREAK
	      1} and {BREAK 0} respectively.  It is not	a good idea to	change
	      the  RTS	(or  DTR) signal with active hardware handshake	rtscts
	      (or dtrdsr).  The	result is unpredictable.  The -ttycontrol  op-
	      tion cannot be queried.

       -ttystatus
	      (Windows	and  Unix). The	-ttystatus option can only be queried.
	      It returns the current modem status and handshake	input  signals
	      (see  below).  The result	is a list of signal,value pairs	with a
	      fixed order, e.g.	{CTS 1 DSR 0 RING 1 DCD	0}.  The signal	 names
	      are returned upper case.

       -xchar {xonChar xoffChar}
	      (Windows	and  Unix). This option	is used	to query or change the
	      software handshake characters. Normally the operating system de-
	      fault should be DC1 (0x11) and DC3 (0x13)	representing the ASCII
	      standard XON and XOFF characters.

       -pollinterval msec
	      (Windows only). This option is used to set the maximum time  be-
	      tween  polling  for  fileevents.	This affects the time interval
	      between checking for events throughout the Tcl interpreter  (the
	      smallest	value  always wins).  Use this option only if you want
	      to poll the serial port more or less often than 10 msec (the de-
	      fault).

       -sysbuffer inSize

       -sysbuffer {inSize outSize}
	      (Windows	only).	This option is used to change the size of Win-
	      dows system buffers for a	serial channel.	Especially  at	higher
	      communication  rates the default input buffer size of 4096 bytes
	      can overrun for latent systems. The first	form specifies the in-
	      put  buffer  size, in the	second form both input and output buf-
	      fers are defined.

       -lasterror
	      (Windows only). This option is query only.  In case of a	serial
	      communication error, read	or puts	returns	a general Tcl file I/O
	      error.  fconfigure -lasterror can	be called to get a list	of er-
	      ror  details.  See below for an explanation of the various error
	      codes.

SERIAL PORT SIGNALS
       RS-232 is the most commonly used	standard electrical interface for  se-
       rial  communications.  A	 negative  voltage  (-3V..-12V)	 define	a mark
       (on=1) bit and a	positive voltage (+3..+12V) define a space (off=0) bit
       (RS-232C).  The following signals are specified for incoming and	outgo-
       ing data, status	lines and handshaking. Here we	are  using  the	 terms
       workstation  for	 your  computer	and modem for the external device, be-
       cause some signal names (DCD, RI) come from modems. Of course your  ex-
       ternal device may use these signal lines	for other purposes.

       TXD(output)
	      Transmitted Data:	Outgoing serial	data.

       RXD(input)
	      Received Data:Incoming serial data.

       RTS(output)
	      Request  To Send:	This hardware handshake	line informs the modem
	      that your	workstation is ready to	receive	data. Your workstation
	      may  automatically  reset	this signal to indicate	that the input
	      buffer is	full.

       CTS(input)
	      Clear To Send: The complement to RTS. Indicates that  the	 modem
	      is ready to receive data.

       DTR(output)
	      Data  Terminal Ready: This signal	tells the modem	that the work-
	      station is ready to establish a link. DTR	is often enabled auto-
	      matically	whenever a serial port is opened.

       DSR(input)
	      Data  Set	 Ready:	 The  complement to DTR. Tells the workstation
	      that the modem is	ready to establish a link.

       DCD(input)
	      Data Carrier Detect: This	line becomes active when a  modem  de-
	      tects a "Carrier"	signal.

       RI(input)
	      Ring  Indicator:	Goes active when the modem detects an incoming
	      call.

       BREAK  A	BREAK condition	is not a hardware signal line, but  a  logical
	      zero  on the TXD or RXD lines for	a long period of time, usually
	      250 to 500 milliseconds.	Normally a receive  or	transmit  data
	      signal stays at the mark (on=1) voltage until the	next character
	      is transferred. A	BREAK is sometimes used	to reset the  communi-
	      cations  line  or	 change	 the  operating	mode of	communications
	      hardware.

ERROR CODES (Windows only)
       A lot of	different errors may occur during serial  read	operations  or
       during  event  polling in background. The external device may have been
       switched	off, the data lines may	be noisy, system buffers  may  overrun
       or  your	 mode  settings	may be wrong.  That is why a reliable software
       should always catch serial read operations.  In cases of	an  error  Tcl
       returns	a general file I/O error.  Then	fconfigure -lasterror may help
       to locate the problem.  The following error codes may be	returned.

       RXOVER	 Windows input buffer overrun. The data	comes faster than your
		 scripts reads it or your system is overloaded.	Use fconfigure
		 -sysbuffer to avoid a temporary bottleneck and/or  make  your
		 script	faster.

       TXFULL	 Windows output	buffer overrun.	Complement to RXOVER. This er-
		 ror should practically	not happen, because  Tcl  cares	 about
		 the output buffer status.

       OVERRUN	 UART  buffer  overrun	(hardware)  with  data lost.  The data
		 comes faster than the system driver receives it.  Check  your
		 advanced serial port settings to enable the FIFO (16550) buf-
		 fer and/or setup a lower(1) interrupt threshold value.

       RXPARITY	 A parity error	has been detected by your UART.	 Wrong	parity
		 settings with fconfigure -mode	or a noisy data	line (RXD) may
		 cause this error.

       FRAME	 A stop-bit error has been detected by your UART.  Wrong  mode
		 settings with fconfigure -mode	or a noisy data	line (RXD) may
		 cause this error.

       BREAK	 A BREAK condition has been detected by	your UART (see above).

PORTABILITY ISSUES
       Windows
	      Valid values for fileName	to open	a serial port are of the  form
	      comX, where X is a number, generally from	1 to 9.	 A legacy form
	      accepted as well is comX:. This notation only works  for	serial
	      ports  from  1 to	9.  An attempt to open a serial	port that does
	      not exist	or has a number	greater	than 9 will fail.   An	alter-
	      nate  form  of  opening  serial  ports  is  to  use the filename
	      //./comX,	where X	is any number that  corresponds	 to  a	serial
	      port.

	      When running Tcl interactively, there may	be some	strange	inter-
	      actions between the real console,	if one is present, and a  com-
	      mand  pipeline that uses standard	input or output.  If a command
	      pipeline is opened for reading, some of the lines	entered	at the
	      console  will  be	 sent to the command pipeline and some will be
	      sent to the Tcl evaluator.  If a command pipeline	is opened  for
	      writing, keystrokes entered into the console are not visible un-
	      til the pipe is closed.  These problems only occur because  both
	      Tcl  and	the child application are competing for	the console at
	      the same time.  If  the  command	pipeline  is  started  from  a
	      script, so that Tcl is not accessing the console,	or if the com-
	      mand pipeline does not use standard  input  or  output,  but  is
	      redirected from or to a file, then the above problems do not oc-
	      cur.

       Unix
	      Valid values for fileName	to open	a serial port are generally of
	      the  form	 /dev/ttyX,  where  X  is  a or	b, but the name	of any
	      pseudo-file that maps to a serial	port may  be  used.   Advanced
	      configuration  options  are only supported for serial ports when
	      Tcl is built to use the POSIX serial interface.

	      When running Tcl interactively, there may	be some	strange	inter-
	      actions  between	the  console, if one is	present, and a command
	      pipeline that uses standard input.  If  a	 command  pipeline  is
	      opened  for  reading,  some  of the lines	entered	at the console
	      will be sent to the command pipeline and some will  be  sent  to
	      the  Tcl	evaluator.   This problem only occurs because both Tcl
	      and the child application	are competing for the console  at  the
	      same time.  If the command pipeline is started from a script, so
	      that Tcl is not accessing	the console, or	if the	command	 pipe-
	      line does	not use	standard input,	but is redirected from a file,
	      then the above problem does not occur.

       See the PORTABILITY ISSUES section of the exec command  for  additional
       information  not	specific to command pipelines about executing applica-
       tions on	the various platforms

EXAMPLE
       Open a command pipeline and catch any errors:

	      set fl [open "| ls this_file_does_not_exist"]
	      set data [read $fl]
	      if {[catch {close	$fl} err]} {
		  puts "ls command failed: $err"
	      }

SEE ALSO
       file(n),	 close(n),  filename(n),  fconfigure(n),   gets(n),   read(n),
       puts(n),	exec(n), pid(n), fopen(3)

KEYWORDS
       access  mode,  append,  create,	file, non-blocking, open, permissions,
       pipeline, process, serial

Tcl				      8.3			       open(n)

NAME | SYNOPSIS | DESCRIPTION | COMMAND PIPELINES | SERIAL COMMUNICATIONS | SERIAL PORT SIGNALS | ERROR CODES (Windows only) | PORTABILITY ISSUES | EXAMPLE | SEE ALSO | KEYWORDS

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