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

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NAME

       open - Open a file-based or command pipeline channel

SYNOPSIS

       open fileName
       open fileName access
       open fileName access permissions
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DESCRIPTION

       This command opens a file, serial port, or command pipeline and returns a channel identifier that may be used in future invocations of com- |
       mands 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 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 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 pipeline is taken from the current standard input unless overridden  by  the
       command.

SERIAL COMMUNICATIONS

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

CONFIGURATION OPTIONS
																   |
       The fconfigure command can be used to query and set the following configuration option for open serial ports:				   |

       -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.		   |

       -pollinterval msec															   |
	      This  option,  available	only  on  Windows  for serial ports, 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 often than 10 msec (the default).						   |

       -lasterror																   |
	      This  option  is	available only on Windows for serial ports, and is query only (will only be reported when directly requested).	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 (e.g. FRAME RXOVER).

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  Win- |
	      dows 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, key- |
	      strokes 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 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 applica- |
	      tions 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 application are competing for the console at the same time.  If the command pipe- |
	      line 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  command  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 out- |
	      put.																   |

       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.											   |

	      When running Tcl interactively, there may be some strange interactions between the console, if one is present, and a  command  pipe- |
	      line  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  appli- |
	      cation are competing for the console at the same time.  If the command pipeline is started from a script, so that Tcl is not access- |
	      ing 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 specific to command pipelines about executing appli- |
       cations on the various platforms 													   |

SEE ALSO
																	   |
       file(n), close(n), filename(n), fconfigure(n), gets(n), read(n), puts(n), exec(n), fopen(1)						   |

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

Tcl									7.6								   open(n)