open

Section: Tcl Built-In Commands (n)
Updated: 8.3
Page Index

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 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"
}

KEYWORDS

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