Provided by: tcl8.5-doc_8.5.19-1_all 
NAME
chan - Read, write and manipulate channels
SYNOPSIS
chan option ?arg arg ...?
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DESCRIPTION
This command provides several operations for reading from, writing to and otherwise manipulating open
channels (such as have been created with the open and socket commands, or the default named channels
stdin, stdout or stderr which correspond to the process's standard input, output and error streams
respectively). Option indicates what to do with the channel; any unique abbreviation for option is
acceptable. Valid options are:
chan blocked channelId
This tests whether the last input operation on the channel called channelId failed because it
would have otherwise caused the process to block, and returns 1 if that was the case. It returns 0
otherwise. Note that this only ever returns 1 when the channel has been configured to be non-
blocking; all Tcl channels have blocking turned on by default.
chan close channelId
Close and destroy the channel called channelId. Note that this deletes all existing file-events
registered on the channel.
As part of closing the channel, all buffered output is flushed to the channel's output device, any
buffered input is discarded, the underlying operating system resource is closed and channelId
becomes unavailable for future use.
If the channel is blocking, the command does not return until all output is flushed. If the
channel is nonblocking and there is unflushed output, the channel remains open and the command
returns immediately; output will be flushed in the background and the channel will be closed when
all the flushing is complete.
If channelId is a blocking channel for a command pipeline then chan close waits for the child
processes to complete.
If the channel is shared between interpreters, then chan close makes channelId unavailable in the
invoking interpreter but has no other effect until all of the sharing interpreters have closed the
channel. When the last interpreter in which the channel is registered invokes chan close (or
close), the cleanup actions described above occur. See the interp command for a description of
channel sharing.
Channels are automatically closed when an interpreter is destroyed and when the process exits.
Channels are switched to blocking mode, to ensure that all output is correctly flushed before the
process exits.
The command returns an empty string, and may generate an error if an error occurs while flushing
output. If a command in a command pipeline created with open returns an error, chan close
generates an error (similar to the exec command.)
chan configure channelId ?optionName? ?value? ?optionName value?...
Query or set the configuration options of the channel named channelId.
If no optionName or value arguments are supplied, the command returns a list containing
alternating option names and values for the channel. If optionName is supplied but no value then
the command returns the current value of the given option. If one or more pairs of optionName and
value are supplied, the command sets each of the named options to the corresponding value; in this
case the return value is an empty string.
The options described below are supported for all channels. In addition, each channel type may add
options that only it supports. See the manual entry for the command that creates each type of
channels for the options that that specific type of channel supports. For example, see the manual
entry for the socket command for its additional options.
-blocking boolean
The -blocking option determines whether I/O operations on the channel can cause the process
to block indefinitely. The value of the option must be a proper boolean value. Channels
are normally in blocking mode; if a channel is placed into nonblocking mode it will affect
the operation of the chan gets, chan read, chan puts, chan flush, and chan close commands;
see the documentation for those commands for details. For nonblocking mode to work
correctly, the application must be using the Tcl event loop (e.g. by calling Tcl_DoOneEvent
or invoking the vwait command).
-buffering newValue
If newValue is full then the I/O system will buffer output until its internal buffer is
full or until the chan flush command is invoked. If newValue is line, then the I/O system
will automatically flush output for the channel whenever a newline character is output. If
newValue is none, the I/O system will flush automatically after every output operation.
The default is for -buffering to be set to full except for channels that connect to
terminal-like devices; for these channels the initial setting is line. Additionally, stdin
and stdout are initially set to line, and stderr is set to none.
-buffersize newSize
Newvalue must be an integer; its value is used to set the size of buffers, in bytes,
subsequently allocated for this channel to store input or output. Newvalue must be a number
of no more than one million, allowing buffers of up to one million bytes in size.
-encoding name
This option is used to specify the encoding of the channel as one of the named encodings
returned by encoding names or the special value binary, so that the data can be converted
to and from Unicode for use in Tcl. For instance, in order for Tcl to read characters from
a Japanese file in shiftjis and properly process and display the contents, the encoding
would be set to shiftjis. Thereafter, when reading from the channel, the bytes in the
Japanese file would be converted to Unicode as they are read. Writing is also supported -
as Tcl strings are written to the channel they will automatically be converted to the
specified encoding on output.
If a file contains pure binary data (for instance, a JPEG image), the encoding for the
channel should be configured to be binary. Tcl will then assign no interpretation to the
data in the file and simply read or write raw bytes. The Tcl binary command can be used to
manipulate this byte-oriented data. It is usually better to set the -translation option to
binary when you want to transfer binary data, as this turns off the other automatic
interpretations of the bytes in the stream as well.
The default encoding for newly opened channels is the same platform- and locale-dependent
system encoding used for interfacing with the operating system, as returned by encoding
system.
-eofchar char
-eofchar {inChar outChar}
This option supports DOS file systems that use Control-z (\x1a) as an end of file marker.
If char is not an empty string, then this character signals end-of-file when it is
encountered during input. For output, the end-of-file character is output when the channel
is closed. If char is the empty string, then there is no special end of file character
marker. For read-write channels, a two-element list specifies the end of file marker for
input and output, respectively. As a convenience, when setting the end-of-file character
for a read-write channel you can specify a single value that will apply to both reading and
writing. When querying the end-of-file character of a read-write channel, a two-element
list will always be returned. The default value for -eofchar is the empty string in all
cases except for files under Windows. In that case the -eofchar is Control-z (\x1a) for
reading and the empty string for writing. The acceptable range for -eofchar values is \x01
- \x7f; attempting to set -eofchar to a value outside of this range will generate an error.
-translation mode
-translation {inMode outMode}
In Tcl scripts the end of a line is always represented using a single newline character
(\n). However, in actual files and devices 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 newlines are used in files, whereas carriage-return-linefeed
sequences are normally used in network connections. On input (i.e., with chan gets and
chan read) the Tcl I/O system automatically translates the external end-of-line
representation into newline characters. Upon output (i.e., with chan puts), the I/O system
translates newlines to the external end-of-line representation. The default translation
mode, auto, handles all the common cases automatically, but the -translation option
provides explicit control over the end of line translations.
The value associated with -translation is a single item for read-only and write-only
channels. The value is a two-element list for read-write channels; the read translation
mode is the first element of the list, and the write translation mode is the second
element. As a convenience, when setting the translation mode for a read-write channel you
can specify a single value that will apply to both reading and writing. When querying the
translation mode of a read-write channel, a two-element list will always be returned. The
following values are currently supported:
auto As the input translation mode, auto treats any of newline (lf), carriage return
(cr), or carriage return followed by a newline (crlf) as the end of line
representation. The end of line representation can even change from line-to-line,
and all cases are translated to a newline. As the output translation mode, auto
chooses a platform specific representation; for sockets on all platforms Tcl chooses
crlf, for all Unix flavors, it chooses lf, and for the various flavors of Windows it
chooses crlf. The default setting for -translation is auto for both input and
output.
binary No end-of-line translations are performed. This is nearly identical to lf mode,
except that in addition binary mode also sets the end-of-file character to the empty
string (which disables it) and sets the encoding to binary (which disables encoding
filtering). See the description of -eofchar and -encoding for more information.
cr The end of a line in the underlying file or device is represented by a single
carriage return character. As the input translation mode, cr mode converts carriage
returns to newline characters. As the output translation mode, cr mode translates
newline characters to carriage returns.
crlf The end of a line in the underlying file or device is represented by a carriage
return character followed by a linefeed character. As the input translation mode,
crlf mode converts carriage-return-linefeed sequences to newline characters. As the
output translation mode, crlf mode translates newline characters to carriage-return-
linefeed sequences. This mode is typically used on Windows platforms and for
network connections.
lf The end of a line in the underlying file or device is represented by a single
newline (linefeed) character. In this mode no translations occur during either
input or output. This mode is typically used on UNIX platforms.
chan copy inputChan outputChan ?-size size? ?-command callback?
Copy data from the channel inputChan, which must have been opened for reading, to the channel
outputChan, which must have been opened for writing. The chan copy command leverages the buffering
in the Tcl I/O system to avoid extra copies and to avoid buffering too much data in main memory
when copying large files to slow destinations like network sockets.
The chan copy command transfers data from inputChan until end of file or size bytes have been
transferred. If no -size argument is given, then the copy goes until end of file. All the data
read from inputChan is copied to outputChan. Without the -command option, chan copy blocks until
the copy is complete and returns the number of bytes written to outputChan.
The -command argument makes chan copy work in the background. In this case it returns immediately
and the callback is invoked later when the copy completes. The callback is called with one or two
additional arguments that indicates how many bytes were written to outputChan. If an error
occurred during the background copy, the second argument is the error string associated with the
error. With a background copy, it is not necessary to put inputChan or outputChan into non-
blocking mode; the chan copy command takes care of that automatically. However, it is necessary
to enter the event loop by using the vwait command or by using Tk.
You are not allowed to do other I/O operations with inputChan or outputChan during a background
chan copy. If either inputChan or outputChan get closed while the copy is in progress, the
current copy is stopped and the command callback is not made. If inputChan is closed, then all
data already queued for outputChan is written out.
Note that inputChan can become readable during a background copy. You should turn off any chan
event or fileevent handlers during a background copy so those handlers do not interfere with the
copy. Any I/O attempted by a chan event or fileevent handler will get a “channel busy” error.
Chan copy translates end-of-line sequences in inputChan and outputChan according to the
-translation option for these channels (see chan configure above). The translations mean that the
number of bytes read from inputChan can be different than the number of bytes written to
outputChan. Only the number of bytes written to outputChan is reported, either as the return
value of a synchronous chan copy or as the argument to the callback for an asynchronous chan copy.
Chan copy obeys the encodings and character translations configured for the channels. This means
that the incoming characters are converted internally first UTF-8 and then into the encoding of
the channel chan copy writes to (see chan configure above for details on the -encoding and
-translation options). No conversion is done if both channels are set to encoding binary and have
matching translations. If only the output channel is set to encoding binary the system will write
the internal UTF-8 representation of the incoming characters. If only the input channel is set to
encoding binary the system will assume that the incoming bytes are valid UTF-8 characters and
convert them according to the output encoding. The behaviour of the system for bytes which are not
valid UTF-8 characters is undefined in this case.
chan create mode cmdPrefix
This subcommand creates a new script level channel using the command prefix cmdPrefix as its
handler. Any such channel is called a reflected channel. The specified command prefix, cmdPrefix,
must be a non-empty list, and should provide the API described in the reflectedchan manual page.
The handle of the new channel is returned as the result of the chan create command, and the
channel is open. Use either close or chan close to remove the channel.
The argument mode specifies if the new channel is opened for reading, writing, or both. It has to
be a list containing any of the strings “read” or “write”. The list must have at least one
element, as a channel you can neither write to nor read from makes no sense. The handler command
for the new channel must support the chosen mode, or an error is thrown.
The command prefix is executed in the global namespace, at the top of call stack, following the
appending of arguments as described in the reflectedchan manual page. Command resolution happens
at the time of the call. Renaming the command, or destroying it means that the next call of a
handler method may fail, causing the channel command invoking the handler to fail as well.
Depending on the subcommand being invoked, the error message may not be able to explain the reason
for that failure.
Every channel created with this subcommand knows which interpreter it was created in, and only
ever executes its handler command in that interpreter, even if the channel was shared with and/or
was moved into a different interpreter. Each reflected channel also knows the thread it was
created in, and executes its handler command only in that thread, even if the channel was moved
into a different thread. To this end all invocations of the handler are forwarded to the original
thread by posting special events to it. This means that the original thread (i.e. the thread that
executed the chan create command) must have an active event loop, i.e. it must be able to process
such events. Otherwise the thread sending them will block indefinitely. Deadlock may occur.
Note that this permits the creation of a channel whose two endpoints live in two different
threads, providing a stream-oriented bridge between these threads. In other words, we can provide
a way for regular stream communication between threads instead of having to send commands.
When a thread or interpreter is deleted, all channels created with this subcommand and using this
thread/interpreter as their computing base are deleted as well, in all interpreters they have been
shared with or moved into, and in whatever thread they have been transfered to. While this pulls
the rug out under the other thread(s) and/or interpreter(s), this cannot be avoided. Trying to use
such a channel will cause the generation of a regular error about unknown channel handles.
This subcommand is safe and made accessible to safe interpreters. While it arranges for the
execution of arbitrary Tcl code the system also makes sure that the code is always executed within
the safe interpreter.
chan eof channelId
Test whether the last input operation on the channel called channelId failed because the end of
the data stream was reached, returning 1 if end-of-file was reached, and 0 otherwise.
chan event channelId event ?script?
Arrange for the Tcl script script to be installed as a file event handler to be called whenever
the channel called channelId enters the state described by event (which must be either readable or
writable); only one such handler may be installed per event per channel at a time. If script is
the empty string, the current handler is deleted (this also happens if the channel is closed or
the interpreter deleted). If script is omitted, the currently installed script is returned (or an
empty string if no such handler is installed). The callback is only performed if the event loop
is being serviced (e.g. via vwait or update).
A file event handler is a binding between a channel and a script, such that the script is
evaluated whenever the channel becomes readable or writable. File event handlers are most
commonly used to allow data to be received from another process on an event-driven basis, so that
the receiver can continue to interact with the user or with other channels while waiting for the
data to arrive. If an application invokes chan gets or chan read on a blocking channel when there
is no input data available, the process will block; until the input data arrives, it will not be
able to service other events, so it will appear to the user to “freeze up”. With chan event, the
process can tell when data is present and only invoke chan gets or chan read when they will not
block.
A channel is considered to be readable if there is unread data available on the underlying device.
A channel is also considered to be readable if there is unread data in an input buffer, except in
the special case where the most recent attempt to read from the channel was a chan gets call that
could not find a complete line in the input buffer. This feature allows a file to be read a line
at a time in nonblocking mode using events. A channel is also considered to be readable if an end
of file or error condition is present on the underlying file or device. It is important for
script to check for these conditions and handle them appropriately; for example, if there is no
special check for end of file, an infinite loop may occur where script reads no data, returns, and
is immediately invoked again.
A channel is considered to be writable if at least one byte of data can be written to the
underlying file or device without blocking, or if an error condition is present on the underlying
file or device. Note that client sockets opened in asynchronous mode become writable when they
become connected or if the connection fails.
Event-driven I/O works best for channels that have been placed into nonblocking mode with the chan
configure command. In blocking mode, a chan puts command may block if you give it more data than
the underlying file or device can accept, and a chan gets or chan read command will block if you
attempt to read more data than is ready; no events will be processed while the commands block. In
nonblocking mode chan puts, chan read, and chan gets never block.
The script for a file event is executed at global level (outside the context of any Tcl procedure)
in the interpreter in which the chan event command was invoked. If an error occurs while
executing the script then the command registered with interp bgerror is used to report the error.
In addition, the file event handler is deleted if it ever returns an error; this is done in order
to prevent infinite loops due to buggy handlers.
chan flush channelId
Ensures that all pending output for the channel called channelId is written.
If the channel is in blocking mode the command does not return until all the buffered output has
been flushed to the channel. If the channel is in nonblocking mode, the command may return before
all buffered output has been flushed; the remainder will be flushed in the background as fast as
the underlying file or device is able to absorb it.
chan gets channelId ?varName?
Reads the next line from the channel called channelId. If varName is not specified, the result of
the command will be the line that has been read (without a trailing newline character) or an empty
string upon end-of-file or, in non-blocking mode, if the data available is exhausted. If varName
is specified, the line that has been read will be written to the variable called varName and
result will be the number of characters that have been read or -1 if end-of-file was reached or,
in non-blocking mode, if the data available is exhausted.
If an end-of-file occurs while part way through reading a line, the partial line will be returned
(or written into varName). When varName is not specified, the end-of-file case can be
distinguished from an empty line using the chan eof command, and the partial-line-but-nonblocking
case can be distinguished with the chan blocked command.
chan names ?pattern?
Produces a list of all channel names. If pattern is specified, only those channel names that match
it (according to the rules of string match) will be returned.
chan pending mode channelId
Depending on whether mode is input or output, returns the number of bytes of input or output
(respectively) currently buffered internally for channelId (especially useful in a readable event
callback to impose application-specific limits on input line lengths to avoid a potential denial-
of-service attack where a hostile user crafts an extremely long line that exceeds the available
memory to buffer it). Returns -1 if the channel was not opened for the mode in question.
chan postevent channelId eventSpec
This subcommand is used by command handlers specified with chan create. It notifies the channel
represented by the handle channelId that the event(s) listed in the eventSpec have occurred. The
argument has to be a list containing any of the strings read and write. The list must contain at
least one element as it does not make sense to invoke the command if there are no events to post.
Note that this subcommand can only be used with channel handles that were created/opened by chan
create. All other channels will cause this subcommand to report an error.
As only the Tcl level of a channel, i.e. its command handler, should post events to it we also
restrict the usage of this command to the interpreter that created the channel. In other words,
posting events to a reflected channel from an interpreter that does not contain it's
implementation is not allowed. Attempting to post an event from any other interpreter will cause
this subcommand to report an error.
Another restriction is that it is not possible to post events that the I/O core has not registered
an interest in. Trying to do so will cause the method to throw an error. See the command handler
method watch described in reflectedchan, the document specifying the API of command handlers for
reflected channels.
This command is safe and made accessible to safe interpreters. It can trigger the execution of
chan event handlers, whether in the current interpreter or in other interpreters or other threads,
even where the event is posted from a safe interpreter and listened for by a trusted interpreter.
Chan event handlers are always executed in the interpreter that set them up.
chan puts ?-nonewline? ?channelId? string
Writes string to the channel named channelId followed by a newline character. A trailing newline
character is written unless the optional flag -nonewline is given. If channelId is omitted, the
string is written to the standard output channel, stdout.
Newline characters in the output are translated by chan puts to platform-specific end-of-line
sequences according to the currently configured value of the -translation option for the channel
(for example, on PCs newlines are normally replaced with carriage-return-linefeed sequences; see
chan configure above for details).
Tcl buffers output internally, so characters written with chan puts may not appear immediately on
the output file or device; Tcl will normally delay output until the buffer is full or the channel
is closed. You can force output to appear immediately with the chan flush command.
When the output buffer fills up, the chan puts command will normally block until all the buffered
data has been accepted for output by the operating system. If channelId is in nonblocking mode
then the chan puts command will not block even if the operating system cannot accept the data.
Instead, Tcl continues to buffer the data and writes it in the background as fast as the
underlying file or device can accept it. The application must use the Tcl event loop for
nonblocking output to work; otherwise Tcl never finds out that the file or device is ready for
more output data. It is possible for an arbitrarily large amount of data to be buffered for a
channel in nonblocking mode, which could consume a large amount of memory. To avoid wasting
memory, nonblocking I/O should normally be used in an event-driven fashion with the chan event
command (do not invoke chan puts unless you have recently been notified via a file event that the
channel is ready for more output data).
chan read channelId ?numChars?
chan read ?-nonewline? channelId
In the first form, the result will be the next numChars characters read from the channel named
channelId; if numChars is omitted, all characters up to the point when the channel would signal a
failure (whether an end-of-file, blocked or other error condition) are read. In the second form
(i.e. when numChars has been omitted) the flag -nonewline may be given to indicate that any
trailing newline in the string that has been read should be trimmed.
If channelId is in nonblocking mode, chan read may not read as many characters as requested: once
all available input has been read, the command will return the data that is available rather than
blocking for more input. If the channel is configured to use a multi-byte encoding, then there
may actually be some bytes remaining in the internal buffers that do not form a complete
character. These bytes will not be returned until a complete character is available or end-of-
file is reached. The -nonewline switch is ignored if the command returns before reaching the end
of the file.
Chan read translates end-of-line sequences in the input into newline characters according to the
-translation option for the channel (see chan configure above for a discussion on the ways in
which chan configure will alter input).
When reading from a serial port, most applications should configure the serial port channel to be
nonblocking, like this:
chan configure channelId -blocking 0.
Then chan read behaves much like described above. Note that most serial ports are comparatively
slow; it is entirely possible to get a readable event for each character read from them. Care must
be taken when using chan read on blocking serial ports:
chan read channelId numChars
In this form chan read blocks until numChars have been received from the serial port.
chan read channelId
In this form chan read blocks until the reception of the end-of-file character, see chan
configure -eofchar. If there no end-of-file character has been configured for the channel,
then chan read will block forever.
chan seek channelId offset ?origin?
Sets the current access position within the underlying data stream for the channel named channelId
to be offset bytes relative to origin. Offset must be an integer (which may be negative) and
origin must be one of the following:
start The new access position will be offset bytes from the start of the underlying file or
device.
current The new access position will be offset bytes from the current access position; a
negative offset moves the access position backwards in the underlying file or device.
end The new access position will be offset bytes from the end of the file or device. A
negative offset places the access position before the end of file, and a positive offset
places the access position after the end of file.
The origin argument defaults to start.
Chan seek flushes all buffered output for the channel before the command returns, even if the
channel is in nonblocking mode. It also discards any buffered and unread input. This command
returns an empty string. An error occurs if this command is applied to channels whose underlying
file or device does not support seeking.
Note that offset values are byte offsets, not character offsets. Both chan seek and chan tell
operate in terms of bytes, not characters, unlike chan read.
chan tell channelId
Returns a number giving the current access position within the underlying data stream for the
channel named channelId. This value returned is a byte offset that can be passed to chan seek in
order to set the channel to a particular position. Note that this value is in terms of bytes, not
characters like chan read. The value returned is -1 for channels that do not support seeking.
chan truncate channelId ?length?
Sets the byte length of the underlying data stream for the channel named channelId to be length
(or to the current byte offset within the underlying data stream if length is omitted). The
channel is flushed before truncation.
EXAMPLE
This opens a file using a known encoding (CP1252, a very common encoding on Windows), searches for a
string, rewrites that part, and truncates the file after a further two lines.
set f [open somefile.txt r+]
chan configure $f -encoding cp1252
set offset 0
# Search for string "FOOBAR" in the file
while {[chan gets $f line] >= 0} {
set idx [string first FOOBAR $line]
if {$idx > -1} {
# Found it; rewrite line
chan seek $f [expr {$offset + $idx}]
chan puts -nonewline $f BARFOO
# Skip to end of following line, and truncate
chan gets $f
chan gets $f
chan truncate $f
# Stop searching the file now
break
}
# Save offset of start of next line for later
set offset [chan tell $f]
}
chan close $f
SEE ALSO
close(3tcl), eof(3tcl), fblocked(3tcl), fconfigure(3tcl), fcopy(3tcl), file(3tcl), fileevent(3tcl),
flush(3tcl), gets(3tcl), open(3tcl), puts(3tcl), read(3tcl), seek(3tcl), socket(3tcl), tell(3tcl),
refchan(3tcl)
KEYWORDS
channel, input, output, events, offset
Tcl 8.5 chan(3tcl)