chan(n) Tcl Built-In Commands chan(n) ______________________________________________________________________________
NAME
chan - Read, write and manipulate channels
SYNOPSIS
chan option ?arg arg ...?
______________________________________________________________________________
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 ?direction?
Close and destroy the channel called channelId. Note that this
deletes all existing file-events registered on the channel. If |
the direction argument (which must be read or write or any |
unique abbreviation of them) is present, the channel will only |
be half-closed, so that it can go from being read-write to |
write-only or read-only respectively. If a read-only channel is |
closed for reading, it is the same as if the channel is fully |
closed, and respectively similar for write-only channels. |
Without the direction argument, the channel is closed for both |
reading and writing (but only if those directions are currently |
open). It is an error to close a read-only channel for writing, |
or a write-only channel for reading.
As part of closing the channel, all buffered output is flushed
to the channel's output device (only if the channel is ceasing
to be writable), any buffered input is discarded (only if the
channel is ceasing to be readable), the underlying operating
system resource is closed and channelId becomes unavailable for
future use (both only if the channel is being completely
closed).
If the channel is blocking and the channel is ceasing to be
writable, the command does not return until all output is
flushed. If the channel is non-blocking 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. With half-closing, the half-close
of the channel only applies to the current interpreter's view of
the channel until all channels have closed it in that direction
(or completely). See the interp command for a description of
channel sharing.
Channels are automatically fully 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.)
Note that half-closes of sockets and command pipelines can have |
important side effects because they result in a shutdown() or |
close() of the underlying system resource, which can change how |
other processes or systems respond to the Tcl program.
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 channel for the options supported by that specific type
of channel. For example, see the manual entry for the socket
command for additional options for sockets, and the open command
for additional options for serial devices.
-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 non-
blocking 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 non-
blocking 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
newSize 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. newSize 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 translation
-translation {inTranslation outTranslation}
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 Like lf, no end-of-line translation is performed, but in
addition, sets -eofchar to the empty string to disable
it, and sets -encoding to iso8859-1. With this one
setting, a 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.
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 or characters have been transferred; size is
in bytes if the two channels are using the same encoding, and is
in characters otherwise. 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 or characters (using the same rules as for the
-size option) 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 refchan 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 refchan 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 transferred 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 non-blocking 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 non-blocking 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 non-blocking 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 non-blocking 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-non-blocking 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 pipe
Creates a standalone pipe whose read- and write-side channels |
are returned as a 2-element list, the first element being the |
read side and the second the write side. Can be useful e.g. to |
redirect separately stderr and stdout from a subprocess. To do |
this, spawn with "2>@" or ">@" redirection operators onto the |
write side of a pipe, and then immediately close it in the |
parent. This is necessary to get an EOF on the read side once |
the child has exited or otherwise closed its output. |
Note that the pipe buffering semantics can vary at the operating |
system level substantially; it is not safe to assume that a |
write performed on the output side of the pipe will appear |
instantly to the input side. This is a fundamental difference |
and Tcl cannot conceal it. The overall stream semantics are |
compatible, so blocking reads and writes will not see most of |
the differences, but the details of what exactly gets written |
when are not. This is most likely to show up when using |
pipelines for testing; care should be taken to ensure that |
deadlocks do not occur and that potential short reads are |
allowed for.
chan pop channelId
Removes the topmost transformation from the channel channelId, |
if there is any. If there are no transformations added to |
channelId, this is equivalent to chan close of that channel. The |
result is normally the empty string, but can be an error in some |
situations (i.e. where the underlying system stream is closed |
and that results in an error).
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 refchan, 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 push channelId cmdPrefix
Adds a new transformation on top of the channel channelId. The |
cmdPrefix argument describes a list of one or more words which |
represent a handler that will be used to implement the |
transformation. The command prefix must provide the API |
described in the transchan manual page. The result of this |
subcommand is a handle to the transformation. Note that it is |
important to make sure that the transformation is capable of |
supporting the channel mode that it is used with or this can |
make the channel neither readable nor writable.
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 non-blocking
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 non-blocking 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 non-blocking
mode, which could consume a large amount of memory. To avoid
wasting memory, non-blocking 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 non-blocking 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 non-blocking, 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 non-blocking 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.
EXAMPLES
SIMPLE CHANNEL OPERATION EXAMPLES
Instruct Tcl to always send output to stdout immediately, whether or
not it is to a terminal:
fconfigure stdout -buffering none
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.
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 >= 0} {
# 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
A network server that echoes its input line-by-line without preventing
servicing of other connections at the same time:
# This is a very simple logger...
proc log {message} {
chan puts 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"
chan configure $chan -blocking 0 -buffering line
chan event $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} {
chan gets $chan line
if {[chan eof $chan]} {
log "finishing connection from $clientName"
chan close $chan
} elseif {![chan blocked $chan]} {
# Didn't block waiting for end-of-line
log "$clientName - $line"
chan puts $chan $line
}
}
# Create the server socket and enter the event-loop to wait
# for incoming connections...
socket -server connect 12345
vwait forever
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),
Tcl_StandardChannels(3)
KEYWORDS
blocking, channel, end of file, events, input, non-blocking, offset,
output, readable, seek, stdio, tell, writable
Tcl 8.5 chan(n)
tcl 8.6.15 - Generated Tue Dec 3 16:48:48 CST 2024