Provided by: tcl8.6-doc_8.6.14+dfsg-1build1_all 
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, most 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.
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.
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
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(3tcl), close(3tcl), filename(3tcl), fconfigure(3tcl), gets(3tcl), read(3tcl), puts(3tcl),
exec(3tcl), pid(3tcl), fopen(3)
KEYWORDS
access mode, append, create, file, non-blocking, open, permissions, pipeline, process, serial
Tcl 8.3 open(3tcl)