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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
		      exists.  If it doesn't exist, create a new file.

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

       a	      Open  the	 file  for  writing only.  If the file doesn't
		      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
		      doesn't exist, create a new empty file.  Set the initial
		      access position  to the end of the file.

       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.

       CREAT	      Create the file if it  doesn't  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.

       Note  that  if  you are going to be reading or writing binary data from
       the channel created by this command, you should use the fconfigure com‐
       mand  to change the -translation option of the channel to binary before
       transferring any binary data.  This is in contrast to the ``b'' charac‐
       ter  passed  as	part of the equivalent of the access parameter to some
       versions of the C library fopen() function.

COMMAND PIPELINES
       If the first character of fileName is ``|'' then the remaining  charac‐
       ters  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
       error 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 │
	      activates	 hardware  handshake.  Hardware	 handshake signals are │
	      described 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 │
	      between  the  reception  of two bytes in milliseconds.  For Unix │
	      systems the  granularity	is  100	 milliseconds.	 The  -timeout │
	      option  does  not	 affect write operations or nonblocking reads. │
	      This option 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's not a good idea to change │
	      the RTS (or DTR) signal with active  hardware  handshake	rtscts │
	      (or  dtrdsr).   The  result  is  unpredictable.  The -ttycontrol │
	      option 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 │
	      default 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 │
	      between 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 │
	      default).							       │

       -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 │
	      input 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 │
	      error 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 │
       serial 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, │
       because	some  signal  names (DCD, RI) come from modems. Of course your │
       external 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 │
	      detects 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's  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 │
		 error	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 (all versions)
	      Valid  values for fileName to open a serial port are of the form
	      comX:, where X is a number, generally from 1 to 4.   This	 nota‐
	      tion only works for serial ports from 1 to 9, if the system hap‐
	      pens to have more than four.  An attempt to open a  serial  port
	      that  does  not  exist or has a number greater than 9 will fail.
	      An alternate form of opening serial ports is to use the filename
	      \\.\comX,	 where	X  is  any number that corresponds to a serial
	      port; please note that this method  is  considerably  slower  on
	      Windows 95 and Windows 98.

       Windows NT
	      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
	      until the pipe is closed.	 This behavior occurs whether the com‐
	      mand pipeline is executing 16-bit or 32-bit applications.	 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 command pipeline does not use standard
	      input or output, but is redirected from or to a file,  then  the
	      above problems do not occur.

       Windows 95
	      A command pipeline that executes a 16-bit DOS application cannot
	      be opened for both reading and writing, since 16-bit DOS	appli‐
	      cations  that  receive standard input from a pipe and send stan‐
	      dard output to a pipe run synchronously.	Command pipelines that
	      do  not  execute	16-bit DOS applications run asynchronously and
	      can be opened for both reading and writing.

	      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 from a 32-bit application, some
	      of the keystrokes 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 to  a  32-bit  applica‐
	      tion,  no	 output	 is  visible  on the console until 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 command pipeline
	      does not use standard input or output, but is redirected from or
	      to a file, then the above problems do not occur.

	      Whether  or not Tcl is running interactively, if a command pipe‐
	      line is opened for reading from a 16-bit	DOS  application,  the
	      call to open will not return until end-of-file has been received
	      from the command pipeline's standard output.  If a command pipe‐
	      line  is opened for writing to a 16-bit DOS application, no data
	      will be sent to the command pipeline's standard output until the
	      pipe is actually closed.	This problem occurs because 16-bit DOS
	      applications are run synchronously, as described above.

       Macintosh
	      Opening a serial port is not currently implemented under	Macin‐
	      tosh.

	      Opening  a  command  pipeline  is not supported under Macintosh,
	      since applications do not support the concept of standard	 input
	      or output.

       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)
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