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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:



  

  
Open the file for reading only; the file must already exist. This is the
      default value if access is not specified.

  

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

  

  
Open the file for writing only. Truncate it if it exists. If it does not
      exist, create a new file.

  

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

  

  
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.

  

  
Open the file for reading and writing. If the file does not exist, create
      a new empty file. Set the initial access 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 binary option, making the channel
    suitable for reading or writing of binary data.


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



  

  
Open the file for reading only.

  

  
Open the file for writing only.

  

  
Open the file for both reading and writing.

  

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

  

  
Configure the opened channel with the -translation binary
    option.

  

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

  

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

  

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

  

  
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.

  

  
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.






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
    pipeline returns an error (according to the definition in exec), a
    Tcl error is generated when close is called on the channel unless the
    pipeline 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 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 specific to serial ports (where
  supported):



  

  
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.

  

  
(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 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.





  

  
(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.

  

  
(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.

  

  
(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 option
      cannot be queried.

  

  
(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.

  

  
(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.

  

  
(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).

  

  

  

  
(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.

  

  
(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 purposes.



  

  
Transmitted Data: Outgoing serial data.

  

  
Received Data:Incoming serial data.

  

  
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.

  

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

  

  
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.

  

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

  

  
Data Carrier Detect: This line becomes active when a modem detects
      a “Carrier” signal.

  

  
Ring Indicator: Goes active when the modem detects an incoming
      call.

  

  
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 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.



  

  
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.

  

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

  

  
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.

  

  
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.

  

  
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.

  

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








PORTABILITY ISSUES



  

  
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 alternate 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
  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 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.

Files opened in the “a” mode or with the
    APPEND flag set are implemented by seeking immediately before each
    write, which is not an atomic operation and does not carry the guarantee of
    strict appending that is present on POSIX platforms.





  

  
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 specific to command pipelines about
    executing applications on the various platforms






EXAMPLES


Open a file for writing, forcing it to be created and raising an
    error if it already exists.






set myNewFile [open filename.txt {WRONLY CREAT EXCL}]




Open a file for writing as a log file.






set myLogFile [open filename.log "a"]
fconfigure $myLogFile -buffering line




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"
}




Open a command pipeline and read binary data from it. Note the
    unusual form with “|[list” that handles non-trivial edge cases
    with arguments that potentially have spaces in.






set fl [open |[list create_image_data $input] "rb"]
set binData [read $fl]
close $fl








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







  

    8.3
    Tcl