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, cre
ate 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
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 con
trolling terminal of the process.
NONBLOCK Prevents the process from blocking while
opening the file, and possibly in subse
quent 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 sys
tem 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, permis
sions (an integer) is used to set the permissions for the
new file in conjunction with the process's file mode cre
ation 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
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 out
put 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.
SERIAL COMMUNICATIONS
If fileName refers to a serial port, then the specified |
serial port is opened and initialized in a platform-depen |
dent manner. Acceptable values for the fileName to use to |
open a serial port are described in the PORTABILITY ISSUES |
section. |
CONFIGURATION OPTIONS |
The fconfigure command can be used to query and set the |
following configuration option for open serial ports: |
-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 connec |
tion speed. Parity is one of the following let |
ters: 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. |
-pollinterval |
msec ||
This option, available only on Windows for serial |
ports, 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 often than 10 msec (the default). |
-lasterror ||
This option is available only on Windows for serial |
ports, and is query only (will only be reported |
when directly requested). In case of a serial com |
munication error, read or puts returns a general |
Tcl file I/O error. fconfigure -lasterror can be |
called to get a list of error details (e.g. FRAME |
RXOVER).
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 |
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 |
to the Tcl evaluator. If a command pipeline is |
opened for writing to a 32-bit application, no out |
put is visible on the console until the 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 redi |
rected 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 actu |
ally 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. |
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 read |
ing, 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. |
additional information not specific to command pipelines |
about executing applications on the various platforms |
SEE ALSO |
file(n), close(n), filename(n), fconfigure(n), gets(n), |
read(n), puts(n), exec(n), fopen(1) |
KEYWORDS |
access mode, append, create, file, non-blocking, open, |
permissions, pipeline, process, serial |
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