html/TclCmd/open.htm

<HTML><HEAD><TITLE>Tcl Built-In Commands - open manual page</TITLE></HEAD><BODY>
<DL>
<DD><A HREF="open.htm#M2" NAME="L860">NAME</A>
<DL><DD>open - Open a file-based or command pipeline channel</DL>
<DD><A HREF="open.htm#M3" NAME="L861">SYNOPSIS</A>
<DL>
<DD><B>open </B><I>fileName</I>
<DD><B>open </B><I>fileName access</I>
<DD><B>open </B><I>fileName access permissions</I>
</DL>
<DD><A HREF="open.htm#M4" NAME="L862">DESCRIPTION</A>
<DL>
<DD><A HREF="open.htm#M5" NAME="L863"><B>r</B></A>
<DD><A HREF="open.htm#M6" NAME="L864"><B>r+</B></A>
<DD><A HREF="open.htm#M7" NAME="L865"><B>w</B></A>
<DD><A HREF="open.htm#M8" NAME="L866"><B>w+</B></A>
<DD><A HREF="open.htm#M9" NAME="L867"><B>a</B></A>
<DD><A HREF="open.htm#M10" NAME="L868"><B>a+</B></A>
</DL>
<DL>
<DD><A HREF="open.htm#M11" NAME="L869"><B>RDONLY</B></A>
<DD><A HREF="open.htm#M12" NAME="L870"><B>WRONLY</B></A>
<DD><A HREF="open.htm#M13" NAME="L871"><B>RDWR</B></A>
<DD><A HREF="open.htm#M14" NAME="L872"><B>APPEND</B></A>
<DD><A HREF="open.htm#M15" NAME="L873"><B>CREAT</B></A>
<DD><A HREF="open.htm#M16" NAME="L874"><B>EXCL</B></A>
<DD><A HREF="open.htm#M17" NAME="L875"><B>NOCTTY</B></A>
<DD><A HREF="open.htm#M18" NAME="L876"><B>NONBLOCK</B></A>
<DD><A HREF="open.htm#M19" NAME="L877"><B>TRUNC</B></A>
</DL>
<DD><A HREF="open.htm#M20" NAME="L878">COMMAND PIPELINES</A>
<DD><A HREF="open.htm#M21" NAME="L879">SERIAL COMMUNICATIONS</A>
<DL>
<DD><A HREF="open.htm#M22" NAME="L880"><B>-mode</B> <I>baud</I><B>,</B><I>parity</I><B>,</B><I>data</I><B>,</B><I>stop</I></A>
<DD><A HREF="open.htm#M23" NAME="L881"><B>-handshake</B> <I>type</I></A>
<DD><A HREF="open.htm#M24" NAME="L882"><B>-queue</B></A>
<DD><A HREF="open.htm#M25" NAME="L883"><B>-timeout</B> <I>msec</I></A>
<DD><A HREF="open.htm#M26" NAME="L884"><B>-ttycontrol</B> <I>{signal boolean signal boolean ...}</I></A>
<DD><A HREF="open.htm#M27" NAME="L885"><B>-ttystatus</B></A>
<DD><A HREF="open.htm#M28" NAME="L886"><B>-xchar</B> <I>{xonChar xoffChar}</I></A>
<DD><A HREF="open.htm#M29" NAME="L887"><B>-pollinterval</B> <I>msec</I></A>
<DD><A HREF="open.htm#M30" NAME="L888"><B>-sysbuffer</B> <I>inSize</I></A>
<DD><A HREF="open.htm#M31" NAME="L889"><B>-sysbuffer</B> <I>{inSize outSize}</I></A>
<DD><A HREF="open.htm#M32" NAME="L890"><B>-lasterror</B></A>
</DL>
<DD><A HREF="open.htm#M33" NAME="L891">SERIAL PORT SIGNALS</A>
<DL>
<DD><A HREF="open.htm#M34" NAME="L892"><B>TXD(output)</B></A>
<DD><A HREF="open.htm#M35" NAME="L893"><B>RXD(input)</B></A>
<DD><A HREF="open.htm#M36" NAME="L894"><B>RTS(output)</B></A>
<DD><A HREF="open.htm#M37" NAME="L895"><B>CTS(input)</B></A>
<DD><A HREF="open.htm#M38" NAME="L896"><B>DTR(output)</B></A>
<DD><A HREF="open.htm#M39" NAME="L897"><B>DSR(input)</B></A>
<DD><A HREF="open.htm#M40" NAME="L898"><B>DCD(input)</B></A>
<DD><A HREF="open.htm#M41" NAME="L899"><B>RI(input)</B></A>
<DD><A HREF="open.htm#M42" NAME="L900"><B>BREAK</B></A>
</DL>
<DD><A HREF="open.htm#M43" NAME="L901">ERROR CODES (Windows only)</A>
<DL>
<DD><A HREF="open.htm#M44" NAME="L902"><B>RXOVER</B></A>
<DD><A HREF="open.htm#M45" NAME="L903"><B>TXFULL</B></A>
<DD><A HREF="open.htm#M46" NAME="L904"><B>OVERRUN</B></A>
<DD><A HREF="open.htm#M47" NAME="L905"><B>RXPARITY</B></A>
<DD><A HREF="open.htm#M48" NAME="L906"><B>FRAME</B></A>
<DD><A HREF="open.htm#M49" NAME="L907"><B>BREAK</B></A>
</DL>
<DD><A HREF="open.htm#M50" NAME="L908">PORTABILITY ISSUES</A>
<DL>
<DD><A HREF="open.htm#M51" NAME="L909"><B>Windows </B>(all versions)</A>
<DD><A HREF="open.htm#M52" NAME="L910"><B>Windows NT</B></A>
<DD><A HREF="open.htm#M53" NAME="L911"><B>Windows 95</B></A>
<DD><A HREF="open.htm#M54" NAME="L912"><B>Unix</B></A>
</DL>
<DD><A HREF="open.htm#M55" NAME="L913">EXAMPLE</A>
<DD><A HREF="open.htm#M56" NAME="L914">SEE ALSO</A>
<DD><A HREF="open.htm#M57" NAME="L915">KEYWORDS</A>
</DL><HR>
<H3><A NAME="M2">NAME</A></H3>
open - Open a file-based or command pipeline channel
<H3><A NAME="M3">SYNOPSIS</A></H3>
<B>open </B><I>fileName</I><BR>
<B>open </B><I>fileName access</I><BR>
<B>open </B><I>fileName access permissions</I><BR>
<H3><A NAME="M4">DESCRIPTION</A></H3>
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
<B><A HREF="../TclCmd/read.htm">read</A></B>, <B><A HREF="../TclCmd/puts.htm">puts</A></B>, and <B><A HREF="../TclCmd/close.htm">close</A></B>.
If the first character of <I>fileName</I> is not <B>|</B> then
the command opens a file:
<I>fileName</I> gives the name of the file to open, and it must conform to the
conventions described in the <B><A HREF="../TclCmd/filename.htm">filename</A></B> manual entry.
<P>
The <I>access</I> argument, if present, indicates the way in which the file
(or command pipeline) is to be accessed.
In the first form <I>access</I> may have any of the following values:
<P>
<DL>
<DT><A NAME="M5"><B>r</B></A><DD>
Open the file for reading only; the file must already exist. This is the
default value if <I>access</I> is not specified.
<P><DT><A NAME="M6"><B>r+</B></A><DD>
Open the file for both reading and writing; the file must
already exist.
<P><DT><A NAME="M7"><B>w</B></A><DD>
Open the file for writing only.  Truncate it if it exists.  If it doesn't
exist, create a new file.
<P><DT><A NAME="M8"><B>w+</B></A><DD>
Open the file for reading and writing.  Truncate it if it exists.
If it doesn't exist, create a new file.
<P><DT><A NAME="M9"><B>a</B></A><DD>
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.
<P><DT><A NAME="M10"><B>a+</B></A><DD>
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.
<P></DL>
<P>
In the second form, <I>access</I> 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 <B>RDONLY</B>, <B>WRONLY</B> or <B>RDWR</B>.
<P>
<DL>
<DT><A NAME="M11"><B>RDONLY</B></A><DD>
Open the file for reading only.
<P><DT><A NAME="M12"><B>WRONLY</B></A><DD>
Open the file for writing only.
<P><DT><A NAME="M13"><B>RDWR</B></A><DD>
Open the file for both reading and writing.
<P><DT><A NAME="M14"><B>APPEND</B></A><DD>
Set the file pointer to the end of the file prior to each write.
<P><DT><A NAME="M15"><B>CREAT</B></A><DD>
Create the file if it doesn't already exist (without this flag it
is an error for the file not to exist).
<P><DT><A NAME="M16"><B>EXCL</B></A><DD>
If <B>CREAT</B> is also specified, an error is returned if the
file already exists.
<P><DT><A NAME="M17"><B>NOCTTY</B></A><DD>
If the file is a terminal device, this flag prevents the file from
becoming the controlling terminal of the process.
<P><DT><A NAME="M18"><B>NONBLOCK</B></A><DD>
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 <B><A HREF="../TclCmd/fconfigure.htm">fconfigure</A></B> command to put a file
in nonblocking mode).
For details refer to your system documentation on the <B>open</B> system
call's <B>O_NONBLOCK</B> flag.
<P><DT><A NAME="M19"><B>TRUNC</B></A><DD>
If the file exists it is truncated to zero length.
<P></DL>
<P>
If a new file is created as part of opening it, <I>permissions</I>
(an integer) is used to set the permissions for the new file in
conjunction with the process's file mode creation mask.
<I>Permissions</I> defaults to 0666.
<P>
Note that if you are going to be reading or writing binary data from
the channel created by this command, you should use the
<B><A HREF="../TclCmd/fconfigure.htm">fconfigure</A></B> command to change the <B>-translation</B> option of
the channel to <B><A HREF="../TclCmd/binary.htm">binary</A></B> before transferring any binary data.  This
is in contrast to the ``b'' character passed as part of the equivalent
of the <I>access</I> parameter to some versions of the C library
<I>fopen()</I> function.

<H3><A NAME="M20">COMMAND PIPELINES</A></H3>
If the first character of <I>fileName</I> is ``|'' then the
remaining characters of <I>fileName</I> are treated as a list of arguments
that describe a command pipeline to invoke, in the same style as the
arguments for <B><A HREF="../TclCmd/exec.htm">exec</A></B>.
In this case, the channel identifier returned by <B>open</B> may be used
to write to the command's input pipe or read from its output pipe,
depending on the value of <I>access</I>.
If write-only access is used (e.g. <I>access</I> is <B>w</B>), 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. <I>access</I> is <B>r</B>),
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 <B><A HREF="../TclCmd/pid.htm">pid</A></B>
command, using the channel id returned by <B>open</B> as argument.
<P>
If the command (or one of the commands) executed in the command
pipeline returns an error (according to the definition in <B><A HREF="../TclCmd/exec.htm">exec</A></B>),
a Tcl error is generated when <B><A HREF="../TclCmd/close.htm">close</A></B> is called on the channel
unless the pipeline is in non-blocking mode then no exit status is
returned (a silent <B><A HREF="../TclCmd/close.htm">close</A></B> with -blocking 0).
<P>
It is often useful to use the <B><A HREF="../TclCmd/fileevent.htm">fileevent</A></B> command with pipelines
so other processing may happen at the same time as running the command
in the background.
<H3><A NAME="M21">SERIAL COMMUNICATIONS</A></H3>
If <I>fileName</I> refers to a serial port, then the specified serial port
is opened and initialized in a platform-dependent manner.  Acceptable
values for the <I>fileName</I> to use to open a serial port are described in
the PORTABILITY ISSUES section.
<P>
The <B><A HREF="../TclCmd/fconfigure.htm">fconfigure</A></B> command can be used to query and set additional
configuration options specific to serial ports (where supported):
<P>
<DL>
<DT><A NAME="M22"><B>-mode</B> <I>baud</I><B>,</B><I>parity</I><B>,</B><I>data</I><B>,</B><I>stop</I></A><DD>
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
<I>baud</I> rate is a simple integer that specifies the connection speed.
<I>Parity</I> is one of the following letters: <B>n</B>, <B>o</B>, <B>e</B>,
<B>m</B>, <B>s</B>; respectively signifying the parity options of ``none'',
``odd'', ``even'', ``mark'', or ``space''.  <I>Data</I> is the number of
data bits and should be an integer from 5 to 8, while <I>stop</I> is the
number of stop bits and should be the integer 1 or 2.
<P><DT><A NAME="M23"><B>-handshake</B> <I>type</I></A><DD>
(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 <I>type</I> parameter is case-independent.
<P>
If <I>type</I> is <B>none</B> then any handshake is switched off.
<B>rtscts</B> activates hardware handshake. Hardware handshake signals
are described below.
For software handshake <B>xonxoff</B> the handshake characters can be redefined
with <B>-xchar</B>.
An additional hardware handshake <B>dtrdsr</B> is available only under Windows.
There is no default handshake configuration, the initial value depends
on your operating system settings.
The <B>-handshake</B> option cannot be queried.
<P><DT><A NAME="M24"><B>-queue</B></A><DD>
(Windows and Unix). The <B>-queue</B> 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.
<P><DT><A NAME="M25"><B>-timeout</B> <I>msec</I></A><DD>
(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 <B>-timeout</B> option does not affect write operations or
nonblocking reads.
This option cannot be queried.
<P><DT><A NAME="M26"><B>-ttycontrol</B> <I>{signal boolean signal boolean ...}</I></A><DD>
(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 <I>signal</I> names are case-independent.
<B>{RTS 1 DTR 0}</B> sets the RTS output to high and the DTR output to low.
The BREAK condition (see below) is enabled and disabled with <B>{BREAK 1}</B> and
<B>{BREAK 0}</B> respectively.
It's not a good idea to change the <B>RTS</B> (or <B>DTR</B>) signal
with active hardware handshake <B>rtscts</B> (or <B>dtrdsr</B>).
The result is unpredictable.
The <B>-ttycontrol</B> option cannot be queried.
<P><DT><A NAME="M27"><B>-ttystatus</B></A><DD>
(Windows and Unix). The <B>-ttystatus</B> 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. <B>{CTS 1 DSR 0 RING 1 DCD 0}</B>.
The <I>signal</I> names are returned upper case.
<P><DT><A NAME="M28"><B>-xchar</B> <I>{xonChar xoffChar}</I></A><DD>
(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.
<P><DT><A NAME="M29"><B>-pollinterval</B> <I>msec</I></A><DD>
(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).
<P><DT><A NAME="M30"><B>-sysbuffer</B> <I>inSize</I></A><DD>
<P><DT><A NAME="M31"><B>-sysbuffer</B> <I>{inSize outSize}</I></A><DD>
(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.
<P><DT><A NAME="M32"><B>-lasterror</B></A><DD>
(Windows only). This option is query only.
In case of a serial communication error, <B><A HREF="../TclCmd/read.htm">read</A></B> or <B><A HREF="../TclCmd/puts.htm">puts</A></B>
returns a general Tcl file I/O error.
<B>fconfigure -lasterror</B> can be called to get a list of error details.
See below for an explanation of the various error codes.

<P></DL>
<H3><A NAME="M33">SERIAL PORT SIGNALS</A></H3>
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 <I>workstation</I> for
your computer and <I>modem</I> 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.

<P>
<DL>
<DT><A NAME="M34"><B>TXD(output)</B></A><DD>
<B>Transmitted Data:</B> Outgoing serial data.
<P><DT><A NAME="M35"><B>RXD(input)</B></A><DD>
<B>Received Data:</B>Incoming serial data.
<P><DT><A NAME="M36"><B>RTS(output)</B></A><DD>
<B>Request To Send:</B> 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.
<P><DT><A NAME="M37"><B>CTS(input)</B></A><DD>
<B>Clear To Send:</B> The complement to RTS. Indicates that the modem is
ready to receive data.
<P><DT><A NAME="M38"><B>DTR(output)</B></A><DD>
<B>Data Terminal Ready:</B> 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.
<P><DT><A NAME="M39"><B>DSR(input)</B></A><DD>
<B>Data Set Ready:</B> The complement to DTR. Tells the workstation that the
modem is ready to establish a link.
<P><DT><A NAME="M40"><B>DCD(input)</B></A><DD>
<B>Data Carrier Detect:</B> This line becomes active when a modem detects
a &quot;Carrier&quot; signal.
<P><DT><A NAME="M41"><B>RI(input)</B></A><DD>
<B>Ring Indicator:</B> Goes active when the modem detects an incoming call.
<P><DT><A NAME="M42"><B>BREAK</B></A><DD>
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.

<P></DL>
<H3><A NAME="M43">ERROR CODES (Windows only)</A></H3>
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
<B><A HREF="../TclCmd/catch.htm">catch</A></B> serial read operations.  In cases of an error Tcl returns a
general file I/O error.  Then <B>fconfigure -lasterror</B> may help to
locate the problem.  The following error codes may be returned.

<P>
<DL>
<DT><A NAME="M44"><B>RXOVER</B></A><DD>
Windows input buffer overrun. The data comes faster than your scripts reads
it or your system is overloaded. Use <B>fconfigure -sysbuffer</B> to avoid a
temporary bottleneck and/or make your script faster.
<P><DT><A NAME="M45"><B>TXFULL</B></A><DD>
Windows output buffer overrun. Complement to RXOVER. This error should
practically not happen, because Tcl cares about the output buffer status.
<P><DT><A NAME="M46"><B>OVERRUN</B></A><DD>
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.
<P><DT><A NAME="M47"><B>RXPARITY</B></A><DD>
A parity error has been detected by your UART.
Wrong parity settings with <B>fconfigure -mode</B> or a noisy data line (RXD)
may cause this error.
<P><DT><A NAME="M48"><B>FRAME</B></A><DD>
A stop-bit error has been detected by your UART.
Wrong mode settings with <B>fconfigure -mode</B> or a noisy data line (RXD)
may cause this error.
<P><DT><A NAME="M49"><B>BREAK</B></A><DD>
A BREAK condition has been detected by your UART (see above).

<P></DL>
<H3><A NAME="M50">PORTABILITY ISSUES</A></H3>
<DL>
<DT><A NAME="M51"><B>Windows </B>(all versions)</A><DD>
Valid values for <I>fileName</I> to open a serial port are of the form
<B>com</B><I>X</I><B>:</B>, where <I>X</I> 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 <B>&#92;&#92;.&#92;comX</B>,
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.
<P><DT><A NAME="M52"><B>Windows NT</B></A><DD>
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.
<P><DT><A NAME="M53"><B>Windows 95</B></A><DD>
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.
<P>
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.
<P>
Whether or not Tcl is running interactively, if a command pipeline is opened
for reading from a 16-bit DOS application, the call to <B>open</B> 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.
<P><DT><A NAME="M54"><B>Unix</B></A><DD>
Valid values for <I>fileName</I> to open a serial port are generally of the
form <B>/dev/tty</B><I>X</I>, where <I>X</I> is <B>a</B> or <B>b</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.
<P>
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.
<P></DL>
<P>
See the PORTABILITY ISSUES section of the <B><A HREF="../TclCmd/exec.htm">exec</A></B> command for additional
information not specific to command pipelines about executing
applications on the various platforms
<H3><A NAME="M55">EXAMPLE</A></H3>
Open a command pipeline and catch any errors:
<PRE>set fl [<B>open</B> &quot;| ls this_file_does_not_exist&quot;]
set data [read $fl]
if {[catch {close $fl} err]} {
    puts &quot;ls command failed: $err&quot;
}</PRE>
<H3><A NAME="M56">SEE ALSO</A></H3>
<B><A HREF="../TclCmd/file.htm">file</A></B>, <B><A HREF="../TclCmd/close.htm">close</A></B>, <B><A HREF="../TclCmd/filename.htm">filename</A></B>, <B><A HREF="../TclCmd/fconfigure.htm">fconfigure</A></B>, <B><A HREF="../TclCmd/gets.htm">gets</A></B>, <B><A HREF="../TclCmd/read.htm">read</A></B>, <B><A HREF="../TclCmd/puts.htm">puts</A></B>, <B><A HREF="../TclCmd/exec.htm">exec</A></B>, <B><A HREF="../TclCmd/pid.htm">pid</A></B>, <B>fopen</B>
<H3><A NAME="M57">KEYWORDS</A></H3>
<A href="../Keywords/A.htm#access mode">access mode</A>, <A href="../Keywords/A.htm#append">append</A>, <A href="../Keywords/C.htm#create">create</A>, <A href="../Keywords/F.htm#file">file</A>, <A href="../Keywords/N.htm#non-blocking">non-blocking</A>, <A href="../Keywords/O.htm#open">open</A>, <A href="../Keywords/P.htm#permissions">permissions</A>, <A href="../Keywords/P.htm#pipeline">pipeline</A>, <A href="../Keywords/P.htm#process">process</A>, <A href="../Keywords/S.htm#serial">serial</A>
<HR><PRE>
<A HREF="../copyright.htm">Copyright</A> &#169; 1993 The Regents of the University of California.
<A HREF="../copyright.htm">Copyright</A> &#169; 1994-1996 Sun Microsystems, Inc.
<A HREF="../copyright.htm">Copyright</A> &#169; 1995-1997 Roger E. Critchlow Jr.</PRE>
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