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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 identi-
       fier 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 specified.

       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 initial access position  to the end of the file.

       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-dependent;  its use is discouraged (it is better to use the fconfig-
		      ure 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 integer) 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 command to change the  -translation  option
       of  the channel to binary before transferring any binary data.  This is in contrast to the
       ``b'' character 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 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 pipe-
       line 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.

SERIAL COMMUNICATIONS										  |
       If fileName refers to a serial port, then the specified serial port is opened and initial- |
       ized  in a platform-dependent manner.  Acceptable values 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 configuration options  spe- |
       cific 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 let- |
	      ters: 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 handshake control. Note |
	      that not all handshake types maybe supported by your  operating  system.	The  type |
	      parameter is case-independent.							  |

	      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 hard- |
	      ware handshake dtrdsr is available only under Windows.  There is no  default  hand- |
	      shake  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 opera- |
	      tions. It specifies the maximum interval between the reception of two bytes in mil- |
	      liseconds.  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 cur- |
	      rent 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  Windows  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 buffers 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 outgoing 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 pur- |
       poses.											  |

       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 worksta- |
	      tion 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 workstation is	ready  to |
	      establish  a  link.  DTR	is  often enabled automatically 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" sig- |
	      nal.										  |

       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 communications |
	      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  reli- |
       able  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 prob- |
       lem.  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 practi- |
		 cally 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) buffer and/or setup a lower(1) interrupt threshold value.	  |

       RXPARITY 										  |
		 A parity error has been detected by your UART.  Wrong parity settings with fcon- |
		 figure -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 fcon- |
		 figure -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 notation only works for serial ports from 1 to
	      9, if the system happens 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 interactions between  the
	      real console, if one is present, and a command 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 the pipe is closed.  This behavior occurs
	      whether the command 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 prob-
	      lems 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 applications that receive standard input from
	      a  pipe  and  send  standard 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 interactions between the
	      real console, if one is present, and a command 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 application, no output is visible on the console until the  the
	      pipe  is closed.	These problems only occur because both Tcl and the child applica-
	      tion 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 pipeline 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 com-
	      mand pipeline 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 Macintosh.

	      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 interactions 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 pipeline 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 spe-
       cific to command pipelines about executing applications on the various platforms

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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