chan manual page - Tcl Built-In Commands

## Tcl/Tk Documentation > Tcl Commands, version 8.6.18 > chan

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  NAME
    chan — Read, write and manipulate channels
  SYNOPSIS
  DESCRIPTION
    chan blocked channelId
    chan close channelId ?direction?
    chan configure channelId ?optionName? ?value? ?optionName value?...
    -blocking boolean
    -buffering newValue
    -buffersize newSize
    -encoding name
    -eofchar char
    -eofchar {inChar outChar}
    -translation translation
    -translation {inTranslation outTranslation}
      auto
      binary
      cr
      crlf
      lf
    chan copy inputChan outputChan ?-size size? ?-command callback?
    chan create mode cmdPrefix
    chan eof channelId
    chan event channelId event ?script?
    chan flush channelId
    chan gets channelId ?varName?
    chan names ?pattern?
    chan pending mode channelId
    chan pipe
    chan pop channelId
    chan postevent channelId eventSpec
    chan push channelId cmdPrefix
    chan puts ?-nonewline? ?channelId? string
    chan read channelId ?numChars?
    chan read ?-nonewline? channelId
      chan read channelId numChars
      chan read channelId
    chan seek channelId offset ?origin?
      start
      current
      end
    chan tell channelId
    chan truncate channelId ?length?
  EXAMPLES
  SIMPLE CHANNEL OPERATION EXAMPLES
  SEE ALSO
  KEYWORDS

### 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:

    chan configure 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, eof, fblocked, fconfigure, fcopy, file, fileevent, flush, gets, open, puts, read, seek, socket, tell, refchan, transchan, Tcl_StandardChannels

### KEYWORDS

blocking, channel, end of file, events, input, non-blocking, offset, output, readable, seek, stdio, tell, writable
Copyright © 2005-2006 Donal K. Fellows