open(n) manual page
Table of Contents
open - Open a file-based or command pipeline
channel
open fileName
open fileName access
open fileName access permissions
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.
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.
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.
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.
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).
- 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
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"
}
file(n)
, close(n)
, filename(n)
, fconfigure(n)
, gets(n)
, read(n)
,
puts(n)
, exec(n)
, pid(n)
, fopen(3)
access mode, append, create,
file, non-blocking, open, permissions, pipeline, process, serial
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