open(n)
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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
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 initial access
position to the end of the file.
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-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.
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 com-
mand to change the -translation option of the channel to binary before
transferring any binary data. This is in contrast to the ``b'' charac-
ter 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 charac-
ters 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
error is generated when close is called on the channel (similar to the
close command.)
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,par- |
ity,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 |
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. |
-ttysta- |
tus | |
(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 xof- |
fChar} | |
(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 out- |
Size} | |
(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 |
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 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 outgo- |
ing 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(out- |
put) | |
Transmitted Data: Outgoing serial data. |
RXD(input) ||
Received Data:Incoming serial data. |
RTS(out- |
put) | |
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(out- |
put) | |
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 |
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 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'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. |
OVER- |
RUN | |
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. |
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 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 nota-
tion only works for serial ports from 1 to 9, if the system hap-
pens 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 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
until the pipe is closed. This behavior occurs whether the com-
mand 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 appli-
cations that receive standard input from a pipe and send stan-
dard 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 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 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 applica-
tion, 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 pipe-
line 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 pipe-
line 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 Macin-
tosh.
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 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
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(n), close(n), filename(n), fconfigure(n), gets(n), read(n),
puts(n), exec(n), pid(n), fopen(3)
KEYWORDS
access mode, append, create, file, non-blocking, open, permissions,
pipeline, process, serial
Tcl 8.3 open(n)
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