Tcl Built-In Commands open(1T)
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 doesn't exist, create a
new file.
w] Open the file for reading and writing. Trun-
cate it if it exists. If it doesn't exist,
create a new file.
a Open the file for writing only. If the file
doesn't 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 doesn't exist, create a new empty
file. Set the initial access position to
the end of the file.
In the second form, access consists of a list of any of the
following flags, all of which have the standard POSIX
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Tcl Built-In Commands open(1T)
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.
APEND Set the file pointer to the end of the file
prior to each write.
CREAT Create the file if it doesn't 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.
NOCTY If the file is a terminal device, this flag
prevents the file from becoming the control-
ling 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 nonblock-
ing mode). For details refer to your system
documentation on the open system call's
ONONBLOCK 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.
Note that if you are going to be reading or writing binary
data from the channel created by this command, you should
use the fconfigure command to change the -translation option
of the channel to binary before transferring any binary
data. This is in contrast to the ``b'' character passed as
part of the equivalent of the access parameter to some ver-
sions of the C library fopen() function.
COMAND PIPELINES
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Tcl Built-In Commands open(1T)
If the first character of fileName is ``'' then the remain-
ing characters of fileName are treated as a list of argu-
ments 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 pipe-
line is directed to the current standard output unless over-
ridden 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 com-
mand 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 pipe-
lines so other processing may happen at the same time as
running the command in the background.
SERIAL COMUNICATIONS
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
ISUES section.
The fconfigure command can be used to query and set addi-
tional 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
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Tcl Built-In Commands open(1T)
handshake types maybe supported by your operating sys-
tem. 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 represent-
ing 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 out-
put to high and the DTR output to low. The BREAK con-
dition (see below) is enabled and disabled with {BREAK
1} and {BREAK 0} respectively. It's not a good idea to
change the RTS (or DTR) signal with active hardware
handshake rtscts (or dtrdsr). The result is unpredict-
able. The -ttycontrol option cannot be queried.
-ttys-
tatus
(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
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Tcl Built-In Commands open(1T)
operating system default should be DC1 (0x11) and DC3
(0x13) representing the ASCI standard XON and XOF
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. Espe-
cially at higher communication rates the default input
buffer size of 4096 bytes can overrun for latent sys-
tems. The first form specifies the input buffer size,
in the second form both input and output buffers are
defined.
-laster-
ror
(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 inter-
face 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 fol-
lowing 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)
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Tcl Built-In Commands open(1T)
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 works-
tation 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.
EROR CODES (Windows only)
A lot of different errors may occur during serial read
operations or during event polling in background. The exter-
nal device may have been switched off, the data lines may be
noisy, system buffers may overrun or your mode settings may
be wrong. That's 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 -laster-
ror may help to locate the problem. The following error
codes may be returned.
RXOVER
Windows input buffer overrun. The data comes
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Tcl Built-In Commands open(1T)
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.
TXFUL
Windows output buffer overrun. Complement to
RXOVER. This error should practically not happen,
because Tcl cares about the output buffer status.
OVER-
RUN
UART buffer overrun (hardware) with data lost.
The data comes faster than the system driver
receives it. Check your advanced serial port set-
tings to enable the FIFO (16550) buffer and/or
setup a lower(1) interrupt threshold value.
RXPAR-
ITY
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 ISUES
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 consider-
ably slower on Windows 95 and Windows 98.
Windows NT
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
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Tcl Built-In Commands open(1T)
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 compet-
ing 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 applica-
tion 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 exe-
cute 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 console, if one
is present, and a command pipeline that uses standard
input or output. If a command pipeline is opened for
reading from a 32-bit application, some of the keys-
trokes entered at the console will be sent to the com-
mand pipeline and some will be sent to the Tcl evalua-
tor. If a command pipeline is opened for writing to a
32-bit application, no output is visible on the console
until the pipe is closed. These problems only occur
because both Tcl and the child application are compet-
ing 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 com-
mand 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 synchronously,
as described above.
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Tcl Built-In Commands open(1T)
Macintosh
Opening a serial port is not currently implemented
under Macintosh.
Opening a command pipeline is not supported under
Macintosh, since applications do not support the con-
cept of standard input or output.
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 stan-
dard input, but is redirected from a file, then the
above problem does not occur.
See the PORTABILITY ISUES 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 thisfiledoesnotexist"]
set data [read $fl]
if {[catch {close $fl} err]} {
puts "ls command failed: $err"
}
SEE ALSO
file(1T), close(1T), filename(1T), fconfigure(1T), gets(1T),
read(1T), puts(1T), exec(1T), pid(1T), fopen(3TCL)
KEYWORDS
access mode, append, create, file, non-blocking, open, per-
missions, pipeline, process, serial
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Tcl Built-In Commands open(1T)
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
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