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. 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.
- 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 concept 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 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