Provided by: tcl8.4-doc_8.4.20-8_all 
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.
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