Provided by: tcl8.6-doc_8.6.13+dfsg-2_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