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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, cre-ate create
ate 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 with the -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 con-trolling controlling
trolling 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 non-blocking nonblocking
blocking 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 permis-sions permissions
sions 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 speci-fies specifies
fies the connection speed. Parity is one of the following letters: n, o, e, m, s; respec-tively respectively
tively 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-inde-pendent. case-independent.
pendent.
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 opera-tions operations
tions 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) perma-nently permanently
nently 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 estab-lish establish
lish 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 communica-tions communications
tions hardware.
ERROR CODES (Windows only)
A lot of different errors may occur during serial read operations or during event polling in back-ground. background.
ground. 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 sys-tem system
tem 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 hap-pen, happen,
pen, 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 (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 con-sole, console,
sole, if one is present, and a command pipeline that uses standard input or output. If a com-mand command
mand 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. 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 com-peting competing
peting 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 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 con-sole, console,
sole, if one is present, and a command pipeline that uses standard input or output. If a com-mand command
mand 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 evalua-tor. evaluator.
tor. If a command pipeline is opened for writing to a 32-bit application, no output is visi-ble visible
ble on the console 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.
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 syn-chronously, synchronously,
chronously, as described above.
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 pipe-line pipeline
line 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)
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