open(3tcl) Tcl Built-In Commands open(3tcl)
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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 speci-
fied.
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 ex-
ists. 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 ac-
cess 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 bi-
nary option, making the channel suitable for reading or writing of bi-
nary 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 bi-
nary 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-depen-
dent; 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 inte-
ger) 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 pipe-
line returns an error (according to the definition in exec), a Tcl er-
ror is generated when close is called on the channel unless the pipe-
line 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 val-
ues 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 configu-
ration 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 hand-
shake control. Note that not all handshake types maybe supported
by your operating system. The type parameter is case-indepen-
dent.
If type is none then any handshake is switched off. rtscts ac-
tivates hardware handshake. Hardware handshake signals are de-
scribed 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 be-
tween the reception of two bytes in milliseconds. For Unix sys-
tems the granularity is 100 milliseconds. The -timeout option
does not affect write operations or nonblocking reads. This op-
tion 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 op-
tion 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 de-
fault 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 be-
tween 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 de-
fault).
-sysbuffer inSize
-sysbuffer {inSize outSize}
(Windows only). This option is used to change the size of Win-
dows 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 in-
put buffer size, in the second form both input and output buf-
fers 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 er-
ror 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 se-
rial 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 outgo-
ing data, status lines and handshaking. Here we are using the terms
workstation for your computer and modem for the external device, be-
cause some signal names (DCD, RI) come from modems. Of course your ex-
ternal 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 work-
station is ready to establish a link. DTR is often enabled auto-
matically 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 de-
tects 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 communi-
cations 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 er-
ror 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) buf-
fer 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 alter-
nate 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 inter-
actions between the real console, if one is present, and a com-
mand 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 un-
til 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 com-
mand pipeline does not use standard input or output, but is
redirected from or to a file, then the above problems do not oc-
cur.
Files opened in the “a” mode or with the APPEND flag set are im-
plemented 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 inter-
actions 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 pipe-
line 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 applica-
tions 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)
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