Provided by: tcl8.4-doc_8.4.20-7_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 doesn't exist, create a
new file.
w+ Open the file for reading and writing. Truncate 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 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.
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.
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.
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 versions of the C library fopen() function.
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's 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'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 -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 (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 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. 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 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.
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 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 keystrokes 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 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 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 synchronously, 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 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"
}
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