path: root/doc/chan.n

'\"
'\" Copyright (c) 2005-2006 Donal K. Fellows
'\" Copyright (c) 2021 Nathan Coulter
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
.TH chan n 8.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
chan \- Reads, writes and manipulates channels.
.SH SYNOPSIS
\fBchan \fIoperation\fR ?\fIarg arg ...\fR?
.BE
.SH DESCRIPTION
.PP
\fBchan\fR provides several operations for reading from, writing to, and
otherwise manipulating channels, e.g. those created by \fBopen\fR and
\fBsocket\fR, or the default channels \fBstdin\fR, \fBstdout\fR or \fBstderr\fR
which correspond respectively to the standard input, output, and error streams
of the process.  Any unique abbreviation for \fIoperation\fR is acceptable.
Available operations are:
.TP
\fBchan blocked \fIchannelName\fR
.
Returns 1 when the channel is in non-blocking mode and the last input operation
on the channel failed because it would have otherwise caused the process to
block, and 0 otherwise.  Each Tcl channel is in blocking mode unless configured
otherwise.
.TP
\fBchan close \fIchannelName\fR ?\fIdirection\fR?
.
Closes and destroys the named channel, deleting any existing event handlers
established for the channel, and returns the empty string.  If \fIdirection\fR is
given, it is
.QW\fBread\fR
or
.QW\fBwrite\fR
or any unique abbreviation of those words, and only that side of the channel is
closed. I.e. a read-write channel may become read-only or write-only.
Closing a read-only channel for reading, or closing a write-only channel for
writing is the same as simply closing the channel.  It is an error to close a
read-only channel for writing or to close a write-only channel for reading.
.RS
.PP
When a channel is closed for writing, any buffered output on the channel is
flushed. When a channel is closed for reading, any buffered input is discarded.
When a channel is destroyed the underlying resource is closed and the channel
is thereafter unavailable.
.PP
\fBchan close\fR fully flushes any output before closing the write side of a
channel unless it is non-blocking mode, where it returns immediately and the
channel is flushed in the background before finally being closed.
.PP
\fBchan close\fR may return an error if an error occurs while flushing
output.  If a process in a command pipeline created by \fBopen\fR returns an
error (either by returning a non-zero exit code or writing to its standard
error file descriptor), \fBchan close\fR generates an error in the same
manner as \fBexec\fR.
.PP
Closing one side of a socket or command pipeline may lead to the shutdown() or
close() of the underlying system resource, leading to a reaction from whatever
is on the other side of the pipeline or socket.
.PP
If the channel for a command pipeline is in blocking mode, \fBchan close\fR
waits for the connected processes to complete.
.PP
\fBchan close\fR only affects the current interpreter.  If the channel is open
in any other interpreter, its state is unchanged there.  See \fBinterp\fR for a
description of channel sharing.
.PP
When the last interpreter sharing a channel is destroyed, the channel is
switched to blocking mode and fully flushed and then closed.
.PP
Channels are automatically closed when an interpreter is destroyed and
when the process exits.
From 8.6 on (TIP#398), nonblocking channels are no longer switched to
blocking mode when exiting; this guarantees a timely exit even when the
peer or a communication channel is stalled. To ensure proper flushing of
stalled nonblocking channels on exit, one must now either (a) actively
switch them back to blocking or (b) use the environment variable
\fBTCL_FLUSH_NONBLOCKING_ON_EXIT\fR, which when set and not equal to
.QW \fB0\fR
restores the previous behavior.
.RE
.TP
\fBchan configure \fIchannelName\fR ?\fIoptionName\fR? ?\fIvalue\fR? ?\fIoptionName value\fR?...
.
Configures or reports the configuration of \fIchannelName\fR.
.RS
.PP
If no \fIoptionName\fR or \fIvalue\fR arguments are given,
\fBchan configure\fR returns a dictionary of option names and
values for the channel.  If \fIoptionName\fR is supplied without a \fIvalue\fR,
\fBchan configure\fR returns the current value of the named option.  If one or
more pairs of \fIoptionName\fR and \fIvalue\fR are supplied,
\fBchan configure\fR sets each of the named options to the corresponding
\fIvalue\fR and returns the empty string.
.PP
The options described below are supported for all channels. Each type of
channel may provide additional options. Those options are described in the
relevant documentation. For example, additional options are documented for
\fBsocket\fR, and also for serial devices at \fBopen\fR.
.TP
\fB\-blocking\fR \fIboolean\fR
.
If \fB\-blocking\fR is set to \fBtrue\fR, which is the default, reading from or
writing to the channel may cause the process to block indefinitely.  Otherwise,
operations such as \fBchan gets\fR, \fBchan read\fR, \fBchan puts\fR, \fBchan
flush\fR, and \fBchan close\fR take care not to block.  Non-blocking mode in
generally requires that the event loop is entered, e.g. by calling
\fBTcl_DoOneEvent\fR or \fBvwait\fR or by using Tk, to give Tcl a chance to
process events on the channel.
.TP
\fB\-buffering\fR \fInewValue\fR
.
If \fInewValue\fR is \fBfull\fR, which is the default, output is buffered
until the internal buffer is full or until \fBchan flush\fR is called. If
\fInewValue\fR is \fBline\fR, output is flushed each time a end-of-line
character is written. If \fInewValue\fR is \fBnone\fR, output is flushed after
every output operation.  For \fBstdin\fR, \fBstdout\fR, and channels that
connect to terminal-like devices, the default value is \fBline\fR.  For
\fBstderr\fR the default value is \fBnone\fR.
.TP
\fB\-buffersize\fR \fInewSize\fR
.
\fInewSize\fR, an integer no greater than one million, is the size in bytes of
any input or output buffers subsequently allocated for this channel.
.TP
\fB\-encoding\fR ?\fIname\fR?
.
Sets the encoding of the channel.  \fIname\fR is either one of the names
returned by \fBencoding names\fR, or
.QW \fBbinary\fR
\&. Input is converted from the encoding into Unicode, and output is converted
from Unicode to the encoding.
.RS
.PP
\fBbinary\fR is an alias for \fBiso8859-1\fR.  This alone is not sufficient for
working with binary data.  Use \fB\-translation binary\fR instead.
.PP
The encoding of a new channel is the value of \fBencoding system\fR,
which returns the platform- and locale-dependent system encoding used to
interface with the operating system,
.RE
.TP
\fB\-eofchar\fR \fIchar\fR
.
\fIchar\fR signals the end of the data when it is encountered in the input.
If \fIchar\fR is the empty string, there is no special character that marks
the end of the data.

The default value is the empty string.  The acceptable range is \ex01 -
\ex7F.  A value outside this range results in an error.
.VS "TCL8.7 TIP656"
.TP
\fB\-profile\fR \fIprofile\fR
.
Specifies the encoding profile to be used on the channel. The encoding
transforms in use for the channel's input and output will then be subject to the
rules of that profile. Any failures will result in a channel error. See
\fBPROFILES\fR in the \fBencoding(n)\fR documentation for details about encoding
profiles.
.VE "TCL8.7 TIP656"
.TP
\fB\-translation\fR \fItranslation\fR
.TP
\fB\-translation\fR \fB{\fIinTranslation outTranslation\fB}\fR
.
In Tcl a single line feed (\en) represents the end of a line.  However,
at the destination the end of a line may be represented differently on
different platforms, or even for different devices on the same platform.  For
example, under UNIX line feed is used in files and a
carriage-return-linefeed sequence is normally used in network connections.
Therefore, on input, e.g. with \fBchan gets\fR and \fBchan read\fR, each
external end-of-line character is translated into a line feed.  On
output, e.g. with \fBchan puts\fR, each line feed is translated to the external
end-of-line character.  The default translation, \fBauto\fR, handles all the common
cases, and \fB\-translation\fR provides explicit control over the end-of-line
character.
.RS
.PP
Returns the input translation for a read-only channel, the output translation
for a write-only channel, and both the input translation and the the output
translation for a  read-write channel.  When two translations are given, they
are the input and output translation, respectively.  When only one translation
is given for a read-write channel, it is the translation for both input and
output.  The following values are currently supported:
.TP
\fBauto\fR
.
The default.  For input each occurrence of a line feed (\fBlf\fR), carriage
return (\fBcr\fR), or carriage return followed by a line feed (\fBcrlf\fR) is
translated into a line feed.  For output, each line feed is translated into a
platform-specific representation:  For all Unix variants it is \fBlf\fR, and
for all Windows variants it is \fBcrlf\fR, except that for sockets on all
platforms it is \fBcrlf\fR for both input and output.
.TP
\fBbinary\fR
.
Like \fBlf\fR, no end-of-line translation is performed, but in addition, sets
\fB\-eofchar\fR to the empty string to disable it, sets \fB\-encoding\fR to
\fBiso8859-1\fR, and sets \fB-profile\fR to \fBstrict\fR so the the channel is
fully configured for binary input and output:  Each byte read from the channel
becomes the Unicode character having the same value as that byte, and each
character written to the channel becomes a single byte in the output.  This
makes it possible to work seamlessly with binary data as long as each character
in the data remains in the range of 0 to 255 so that there is no distinction
between binary data and text.  For example, A JPEG image can be read from a
such a channel, manipulated, and then written back to such a channel.

.TP
\fBcr\fR
.
The end of a line is represented in the external data by a single carriage
return character.  For input, each carriage return is translated to a line
feed, and for output each line feed character is translated to a carriage
return.
.TP
\fBcrlf\fR
.
The end of a line is represented in the external data by a carriage return
character followed by a line feed.  For input, each carriage-return-linefeed
sequence is translated to a line feed.  For output, each line feed is
translated to a carriage-return-linefeed sequence.  This translation is
typically used for network connections, and also on Windows systems.
.TP
\fBlf\fR
.
The end of a line in the external data is represented by a line feed so no
translations occur during either input or output.  This translation is
typically used on UNIX platforms,
.RE
.RE
.TP
\fBchan copy \fIinputChan outputChan\fR ?\fB\-size \fIsize\fR? ?\fB\-command \fIcallback\fR?
.
Reads characters from \fIinputChan\fR and writes them to \fIoutputChan\fR until
all characters are copied, blocking until the copy is complete and returning
the number of characters copied.  Leverages internal buffers to avoid extra
copies and to avoid buffering too much data in main memory when copying large
files to slow destinations like network sockets.
.RS
.PP
\fB\-size\fR limits the number of characters copied.
.PP
If \fB\-command\fR is given, \fBchan copy\fR returns immediately, works in the
background, and calls \fIcallback\fR when the copy completes, providing as an
additional argument the number of characters written to \fIoutputChan\fR.  If
an error occurs during the background copy, another argument provides message
for the error.  \fIinputChan\fR and \fIoutputChan\fR are automatically
configured for non-blocking mode if needed.  Background copying only works
correctly if events are being processed, e.g. via \fBvwait\fR or Tk.
.PP
During a background copy no other read operation may be performed on
\fIinputChan\fR, and no write operation may be performed on
\fIoutputChan\fR.  However, write operations may by performed on
\fIinputChan\fR and read operations may be performed on \fIoutputChan\fR, as
exhibited by the bidirectional copy example below.
.PP
If either \fIinputChan\fR or \fIoutputChan\fR is closed while the copy is in
progress, copying ceases and \fBno\fR callback is made.  If \fIinputChan\fR is
closed all data already queued is written to \fIoutputChan\fR.
.PP
There should be no event handler established for \fIinputChan\fR  because it
may become readable during a background copy.  An attempt to read or write from
within an event handler results result in the error,  "channel busy".  Any
wrong-sided I/O attempted (by a \fBfileevent\fR handler or otherwise) results
in a
.QW "channel busy"
error.
.PP
.PP
.IP \fBEXAMPLES\fR
.PP
The first example transfers the contents of one channel exactly to
another. Note that when copying one file to another, it is better to
use \fBfile copy\fR which also copies file metadata (e.g. the file
access permissions) where possible.
.PP
.CS
fconfigure $in -translation binary
fconfigure $out -translation binary
\fBfcopy\fR $in $out
.CE
.PP
This second example shows how the callback gets
passed the number of bytes transferred.
It also uses vwait to put the application into the event loop.
Of course, this simplified example could be done without the command
callback.
.PP
.CS
proc Cleanup {in out bytes {error {}}} {
    global total
    set total $bytes
    close $in
    close $out
    if {[string length $error] != 0} {
        # error occurred during the copy
    }
}
set in [open $file1]
set out [socket $server $port]
\fBfcopy\fR $in $out -command [list Cleanup $in $out]
vwait total
.CE
.PP
The third example copies in chunks and tests for end of file
in the command callback.
.PP
.CS
proc CopyMore {in out chunk bytes {error {}}} {
    global total done
    incr total $bytes
    if {([string length $error] != 0) || [eof $in]} {
        set done $total
        close $in
        close $out
    } else {
        \fBfcopy\fR $in $out -size $chunk \e
            -command [list CopyMore $in $out $chunk]
    }
}
set in [open $file1]
set out [socket $server $port]
set chunk 1024
set total 0
\fBfcopy\fR $in $out -size $chunk \e
    -command [list CopyMore $in $out $chunk]
vwait done
.CE
.PP
The fourth example starts an asynchronous, bidirectional fcopy between
two sockets. Those could also be pipes from two [open "|hal 9000" r+]
(though their conversation would remain secret to the script, since
all four fileevent slots are busy).
.PP
.CS
set flows 2
proc Done {dir args} {
     global flows done
     puts "$dir is over."
     incr flows -1
     if {$flows<=0} {set done 1}
}
\fBfcopy\fR $sok1 $sok2 -command [list Done UP]
\fBfcopy\fR $sok2 $sok1 -command [list Done DOWN]
vwait done
.CE
.RE
.TP
\fBchan create \fImode cmdPrefix\fR
.
Creates a new channel, called a \fBreflected\fR channel, with \fIcmdPrefix\fR
as its handler, and returns the name of the channel.  \fBcmdPrefix\fR is the
first words of a command that provides the interface for a \fBrefchan\fR.
.RS
.PP
\fBImode\fR is a list of one or more of the strings
.QW \fBread\fR
or
.QW \fBwrite\fR ,
indicating whether the channel is a read channel, a write channel, or both.
It is an error if the handler does not support the chosen mode.
.PP
The handler is called as needed from the global namespace at the top level, and
command resolution happens there at the time of the call.  If the handler is
renamed or deleted any subsequent attempt to call it is an error, which may
not be able to describe the failure.
.PP
The handler is always called in the interpreter and thread it was created in,
even if the channel was shared with or moved into a different interpreter in a
different thread.  This is achieved through event dispatch, so if the event
loop is not entered, e.g. by calling \fBTcl_DoOneEvent\fR or \fBvwait\fR or
using Tk, the thread performing the channel operation \fIblocks
indefinitely\fR, resulting in deadlock.
.PP
One side of a channel may be in one thread while the other side is in a
different thread, providing a stream-oriented bridge between the threads. This
provides a method for regular stream communication between threads as an
alternative to sending commands.
.PP
When the interpreter the handler is in is deleted each channel associated with
the handler is deleted as well, regardless of which interpreter or thread it
is currently in or shared with.
.PP
\fBchan create\fR is \fBsafe\fR and is accessible to safe interpreters.  The
handler is always called in the safe interpreter it was created in.
.RE
.TP
\fBchan eof \fIchannelName\fR
.
Returns 1 if the last read on the channel failed because the end of the data
was already reached, and 0 otherwise.
.TP
\fBchan event \fIchannelName event\fR ?\fIscript\fR?
.
Arranges for the given script, called a \fBchannel event hndler\fR, to be
called whenever the given event, one of
.QW \fBreadable\fR
or
.QW \fBwritable\fR
occurs on the given channel, replacing any script that was previously set.  If
\fIscript\fR is the empty string the current handler is deleted.  It is also
deleted when the channel is closed.  If \fIscript\fR is omitted, either the
existing script or the empty string is returned.  The event loop must be
entered, e.g. via \fBvwait\fR or \fBupdate\fR, or by using Tk, for handlers to
be evaluated.

.RS
.PP
\fIscript\fR is evaluated at the global level in the interpreter it was
established in.  Any resulting error is handled in the background, i.e. via
\fBinterp bgerror\fR.  In order to prevent an endless loop due to a buggy
handler, the handler is deleted if \fIscript\fR returns an error so that it is
not evaluated again.

.PP
Without an event handler, \fBchan gets\fR or \fBchan read\fR on a channel in
blocking mode may block until data becomes available, become during which the
thread is unable to perform other work or respond to events on other channels.
This could cause the application to appear to
.QW "freeze up"
\&.
Channel event handlers allow events on the channel to direct channel handling
so that the reader or writer can continue to perform other processing while
waiting for a channel to become available and then handle channel operations
when the channel is ready for the operation.
.PP
A
.QW readable
event occurs when there is data that can be read from the channel and also when
there is an error on the channel.  The handler must check for these conditions
and handle them appropriately.  For example, a handler that does not check
whether the end of the data has been reached may be repeatedly evaluated in a
busy loop until the channel is closed.
.PP
A
.QW writable
event occurs when at least one byte of data can be written, or if there is an
error on the channel.  A client socket opened in non-blocking mode becomes
writable when it becomes connected or if the connection fails.
.PP
Event-driven channel handling works best for channels in non-blocking mode.  A
channel in blocking mode blocks when \fBchan puts\fR writes more data than the
channel can accept at the moment, and when \fBchan gets\fR or \fBchan read\fR
requests more data than is currently available.  When a channel blocks, the
thread can not do any other processing or service any other events.  A channel
in non-blocking mode allows a thread to carry on with other work and get back
to the channel at the right time.
.RE
.TP
\fBchan flush \fIchannelName\fR
.
For a channel in blocking mode, flushes all buffered output to the destination,
and then returns.  For a channel in non-blocking mode, returns immediately
while all buffered output is flushed in the background as soon as possible.
.TP
\fBchan gets \fIchannelName\fR ?\fIvarName\fR?
.
Returns the next line from the channel, removing the trailing line feed, or if
\fIvarName\fR is given, assigns the line to that variable and returns the
number of characters read.
the line that was read, removing the trailing line feed, or returns the
empty string if there is no data to return and the end of the file has been
reached, or in non-blocking mode, if no complete line is currently available.
If \fIvarName\fR is given, assigns the line that was read to variable named
\fIvarName\fR  and returns the number of characters that were read, or -1 if
there no data available and the end of the channel was reached or the channel
is in non-blocking mode.
.RS
.PP
If the end of the channel is reached the data read so far is returned or
assigned to \fIvarName\fR. When \fIvarName\fR is not given, \fBchan eof\fR may
indicate that the empty string means that the end of the data has been reached,
and \fBchan blocked\fR may indicate that that the empty string means there
isn't currently enough data do return the next line.
.RE
.TP
\fBchan names\fR ?\fIpattern\fR?
.
Returns a list of all channel names, or if \fIpattern\fR is given, only those
names that match according to the rules of \fBstring match\fR.
.TP
\fBchan pending \fImode channelName\fR
.
Returns the number of bytes of input
when \fImode\fR is
.QW\fBinput\fR
, or output when \fImode\fR is
.QW\fBoutput\fR
, that are currently internally buffered for the channel.  Useful in a readable
event callback to impose limits on input line length to avoid a potential
denial-of-service attack where an extremely long line exceeds the available
memory to buffer it.  Returns -1 if the channel was not opened for the mode in
question.
.TP
\fBchan pipe\fR
Creates a pipe, i.e. a readable channel and a writable channel, and returns the
names of the readable channel and the writable channel.  Data written to the
writable channel can be read from the readable channel.  Because the pipe is a
real system-level pipe, it can be connected to other processes using
redirection.  For example, to redirect \fBstderr\fR from a subprocess into one
channel, and \fBstdout\fR into another, \fBexec\fR with "2>@" and ">@", each
onto the writable side of a pipe, closing the writable side immediately
thereafter so that EOF is signaled on the read side once the subprocess has
closed its output, typically on exit.
.RS
.PP
Due to buffering, data written to one side of a pipe might not immediately
become available on the other side.  Tcl's own buffers can be configured via
\fBchan configure -buffering\fR, but overall behaviour still depends on
operating system buffers outside of Tcl's control. Once the write side of the
channel is closed, any data remaining in the buffers is flushed through to the
read side.  It may be useful to arrange for the connected process to flush at
some point after writing to the channel or to have it use some system-provided
mechanism to configure buffering.  When two pipes are connected to the same
process, one to send data to the process, and one to read data from the
process, a deadlock may occur if the channels are in blocking mode:  If
reading, the channel may block waiting for data that can never come because
buffers are only flushed on subsequent writes, and if writing, the channel may
block while waiting for the buffers to become free, which can never happen
because the reader can not read while the writer is blocking.  To avoid this
issue, either put the channels into non-blocking mode and use event handlers,
or place the read channel and the write channel in separate interpreters in
separate threads.
.RE
.TP
\fBchan pop \fIchannelName\fR
Removes the topmost transformation handler from the channel if there is one,
and closes the channel otherwise. The result is normally the empty string, but
may be an error in some situations, e.g. when closing the underlying resource
results in an error.
.TP
\fBchan postevent \fIchannelName eventSpec\fR
.
For use by handlers established with \fBchan create\fR.  Notifies Tcl that
that one or more event(s) listed in \fIeventSpec\fR, each of which is either
.QW\fBread\fR
or
.QW\fBwrite\fR.
, have occurred.
.RS
.PP
For use only by handlers for a channel created by \fBchan create\fR.  It is an
error to post an event for any other channel.
.PP
Since only the handler for a reflected channel channel should post events it is
an error to post an event from any interpreter other than the interpreter that
created the channel.
.PP
It is an error to post an event that the channel has no interest in.  See
\fBwatch\fR in the \fBrefchan\fR documentation for more information
.PP
\fBchan postevent\fR is available in safe interpreters, as any handler for a
reflected channel would have been created, and will be evaluated in that
interpreter as well.
.RE
.TP
\fBchan push \fIchannelName cmdPrefix\fR
Adds a new transformation handler on top of the channel and returns a handle
for the transformation.  \fIcmdPrefix\fR is the first words of a command that
provides the interface documented for \fBtranschan\fR, and transforms data on
the channel, It is an error if handler does not support the mode(s) the channel
is in.
.TP
\fBchan puts\fR ?\fB\-nonewline\fR? ?\fIchannelName\fR? \fIstring\fR
.
Writes \fIstring\fR and a line feed to the channel.  If \fB\-nonewline\fR is
given, the trailing line feed is not written. The default channel is
\fBstdout\fR.
.RS
.PP
Each line feed in the output is translated according to the configuration of
\fB\-translation\fR.
.PP
Because Tcl internally buffers output, characters written to a channel may not
immediately be available at the destination.  Tcl normally delays output until
the buffer is full or the channel is closed. \fBchan flush\fR forces output in
the direction of the destination.
.PP
When the output for a channel in blocking mode fills up, \fBchan puts\fR blocks
until space in the buffer is available again, but for a channel in non-blocking
mode, it returns immediately and the data is written in the background as fast
possible, constrained by the speed at which as the destination accepts it.
Output to a channel in non-blocking mode only works properly when the
application enters the event loop, giving Tcl a chance to find out that the
destination is ready to accept more data.  When a channel is in non-blocking
mode, Tcl's internal buffers can hold an arbitrary amount of data, possibly
consuming a large amount of memory.  To avoid wasting memory, channels in
non-blocking mode should normally be handled using \fBchan event\fR, where the
application only invokes \fBchan puts\fR after being recently notified through
a file event handler that the channel is ready for more output data.
.RE
.TP
\fBchan read \fIchannelName\fR ?\fInumChars\fR?
.TP
\fBchan read \fR?\fB\-nonewline\fR? \fIchannelName\fR
.
Reads and returns the next \fInumChars\fR characters from the channel. If
\fInumChars\fR is omitted, all available characters up to the end of the file
are read, or if the channel is in non-blocking mode, all currently-available
characters are read.  If there is an error on the channel, reading ceases and
an error is returned.  If \fInumChars\fR is not given, \fB\-nonewline\fR
may be given, causing any any trailing line feed to be trimmed.
.RS
.PP
If the channel is in non-blocking mode, fewer characters than requested may be
returned.  If the channel is configured to use a multi-byte encoding, bytes
that do not form a complete character are retained in the buffers until enough
bytes to complete the character accumulate, or the end of the data is reached.
\fB\-nonewline\fR is ignored if characters are returned before reaching the end
of the file.
.PP
Each end-of-line sequence according to the value of \fB\-translation\fR is
translated into a line feed.
.PP
When reading from a serial port, most applications should configure the serial
port channel to be in non-blocking mode, but not necessarily use an event
handler since most serial ports are comparatively slow.  It is entirely
possible to get a \fBreadable\fR event for each individual character.  In
blocking mode, \fBchan read\fR blocks forever when reading to the end of the
data if there is no \fBchan configure -eofchar\fR configured for the channel.
.RE
.TP
\fBchan seek \fIchannelName offset\fR ?\fIorigin\fR?
.
Sets the current position for the data in the channel to integer \fIoffset\fR
bytes relative to \fIorigin\fR.  A negative offset moves the current position
backwards from the origin.  \fIorigin\fR is one of the
following:
.RS
.PP
.TP 10
\fBstart\fR
.
The origin is the start of the data.  This is the default.
.TP 10
\fBcurrent\fR
.
The origin is the current position.
.TP 10
\fBend\fR
.
The origin is the end of the data.
.PP
\fBChan seek\fR flushes all buffered output even if the channel is in
non-blocking mode, discards any buffered and unread input, and returns the
empty string or an error if the channel does not support seeking.
.PP
\fIoffset\fR values are byte offsets, not character offsets.  Unlike \fBchan
read\fR, both \fBchan seek\fR and \fBchan tell\fR operate in terms of bytes,
not characters,
.RE
.TP
\fBchan tell \fIchannelName\fR
.
Returns the offset in bytes of the current position in the underlying data, or
-1 if the channel does not suport seeking. The value can be passed to \fBchan
seek\fR to set current position to that offset.
.TP
\fBchan truncate \fIchannelName\fR ?\fIlength\fR?
.
Flushes the channel and truncates the data in the channel to \fIlength\fR
bytes, or to the current position in bytes if \fIlength\fR is omitted.
.
.SH EXAMPLES
.PP
In the following example a file is opened using the encoding CP1252, which is
common on Windows, searches for a string, rewrites that part, and truncates the
file two lines later.
.PP
.CS
set f [open somefile.txt r+]
\fBchan configure\fR $f -encoding cp1252
set offset 0

\fI# Search for string "FOOBAR" in the file\fR
while {[\fBchan gets\fR $f line] >= 0} {
    set idx [string first FOOBAR $line]
    if {$idx >= 0} {
        \fI# Found it; rewrite line\fR

        \fBchan seek\fR $f [expr {$offset + $idx}]
        \fBchan puts\fR -nonewline $f BARFOO

        \fI# Skip to end of following line, and truncate\fR
        \fBchan gets\fR $f
        \fBchan gets\fR $f
        \fBchan truncate\fR $f

        \fI# Stop searching the file now\fR
        break
    }

    \fI# Save offset of start of next line for later\fR
    set offset [\fBchan tell\fR $f]
}
\fBchan close\fR $f
.CE
.PP
A network server that echoes its input line-by-line without
preventing servicing of other connections at the same time:
.PP
.CS
# This is a very simple logger...
proc log message {
    \fBchan puts\fR stdout $message
}

# This is called whenever a new client connects to the server
proc connect {chan host port} {
    set clientName [format <%s:%d> $host $port]
    log "connection from $clientName"
    \fBchan configure\fR $chan -blocking 0 -buffering line
    \fBchan event\fR $chan readable [list echoLine $chan $clientName]
}

# This is called whenever either at least one byte of input
# data is available, or the channel was closed by the client.
proc echoLine {chan clientName} {
    \fBchan gets\fR $chan line
    if {[\fBchan eof\fR $chan]} {
        log "finishing connection from $clientName"
        \fBchan close\fR $chan
    } elseif {![\fBchan blocked\fR $chan]} {
        # Didn't block waiting for end-of-line
        log "$clientName - $line"
        \fBchan puts\fR $chan $line
    }
}

# Create the server socket and enter the event-loop to wait
# for incoming connections...
socket -server connect 12345
vwait forever
.CE
.SH "SEE ALSO"
close(n), eof(n), fblocked(n), fconfigure(n), fcopy(n), file(n),
fileevent(n), flush(n), gets(n), open(n), puts(n), read(n), seek(n),
socket(n), tell(n), refchan(n), transchan(n)
.SH KEYWORDS
channel, input, output, events, offset
'\" Local Variables:
'\" mode: nroff
'\" End: