open

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 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 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
                      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, 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
                      binary 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-
                      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.

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):

       -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
              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.

       -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 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.

       -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
              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 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
              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” 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
              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.

       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)
                 buffer 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
              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.

       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 specific to command pipelines about executing
       applications 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)