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-'\"
-'\" Copyright (c) 2005-2006 Donal K. Fellows
-'\"
-'\" See the file "license.terms" for information on usage and redistribution
-'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
-.TH chan n 8.5 Tcl "Tcl Built-In Commands"
-.so man.macros
-.BS
-'\" Note:  do not modify the .SH NAME line immediately below!
-.SH NAME
-chan \- Read, write and manipulate channels
-.SH SYNOPSIS
-\fBchan \fIoption\fR ?\fIarg arg ...\fR?
-.BE
-.SH DESCRIPTION
-.PP
-This command provides several operations for reading from, writing to
-and otherwise manipulating open channels (such as have been created
-with the \fBopen\fR and \fBsocket\fR commands, or the default named
-channels \fBstdin\fR, \fBstdout\fR or \fBstderr\fR which correspond to
-the process's standard input, output and error streams respectively).
-\fIOption\fR indicates what to do with the channel; any unique
-abbreviation for \fIoption\fR is acceptable. Valid options are:
-.TP
-\fBchan blocked \fIchannelId\fR
-.
-This tests whether the last input operation on the channel called
-\fIchannelId\fR 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.
-.TP
-\fBchan close \fIchannelId\fR ?\fIdirection\fR?
-.
-Close and destroy the channel called \fIchannelId\fR. Note that this
-deletes all existing file-events registered on the channel.
-.VS 8.6
-If the \fIdirection\fR argument (which must be \fBread\fR or \fBwrite\fR 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 \fIdirection\fR 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.
-.VE 8.6
-.RS
-.PP
-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 \fIchannelId\fR becomes
-unavailable for future use (both only if the channel is being completely
-closed).
-.PP
-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.
-.PP
-If \fIchannelId\fR is a blocking channel for a command pipeline then
-\fBchan close\fR waits for the child processes to complete.
-.PP
-If the channel is shared between interpreters, then \fBchan close\fR
-makes \fIchannelId\fR 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 \fBchan close\fR (or \fBclose\fR), 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 \fBinterp\fR command for a description of channel sharing.
-.PP
-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.
-.PP
-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 \fBopen\fR returns an error, \fBchan close\fR
-generates an error (similar to the \fBexec\fR command.)
-.PP
-.VS 8.6
-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.
-.VE 8.6
-.RE
-.TP
-\fBchan configure \fIchannelId\fR ?\fIoptionName\fR? ?\fIvalue\fR? ?\fIoptionName value\fR?...
-.
-Query or set the configuration options of the channel named
-\fIchannelId\fR.
-.RS
-.PP
-If no \fIoptionName\fR or \fIvalue\fR arguments are supplied, the
-command returns a list containing alternating option names and values
-for the channel.  If \fIoptionName\fR is supplied but no \fIvalue\fR
-then the command returns the current value of the given option.  If
-one or more pairs of \fIoptionName\fR and \fIvalue\fR are supplied,
-the command sets each of the named options to the corresponding
-\fIvalue\fR; in this case the return value is an empty string.
-.PP
-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 \fBsocket\fR command for additional
-options for sockets, and the \fBopen\fR command for additional options for
-serial devices.
-.TP
-\fB\-blocking\fR \fIboolean\fR
-.
-The \fB\-blocking\fR 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 \fBchan gets\fR, \fBchan read\fR, \fBchan
-puts\fR, \fBchan flush\fR, and \fBchan close\fR 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 \fBTcl_DoOneEvent\fR or invoking the \fBvwait\fR
-command).
-.TP
-\fB\-buffering\fR \fInewValue\fR
-.
-If \fInewValue\fR is \fBfull\fR then the I/O system will buffer output
-until its internal buffer is full or until the \fBchan flush\fR
-command is invoked. If \fInewValue\fR is \fBline\fR, then the I/O
-system will automatically flush output for the channel whenever a
-newline character is output. If \fInewValue\fR is \fBnone\fR, the I/O
-system will flush automatically after every output operation.  The
-default is for \fB\-buffering\fR to be set to \fBfull\fR except for
-channels that connect to terminal-like devices; for these channels the
-initial setting is \fBline\fR.  Additionally, \fBstdin\fR and
-\fBstdout\fR are initially set to \fBline\fR, and \fBstderr\fR is set
-to \fBnone\fR.
-.TP
-\fB\-buffersize\fR \fInewSize\fR
-.
-\fINewvalue\fR 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. \fINewvalue\fR must be a number of no more than one
-million, allowing buffers of up to one million bytes in size.
-.TP
-\fB\-encoding\fR \fIname\fR
-.
-This option is used to specify the encoding of the channel as one of
-the named encodings returned by \fBencoding names\fR or the special
-value \fBbinary\fR, 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 \fBshiftjis\fR and properly process
-and display the contents, the encoding would be set to \fBshiftjis\fR.
-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.
-.RS
-.PP
-If a file contains pure binary data (for instance, a JPEG image), the
-encoding for the channel should be configured to be \fBbinary\fR.  Tcl
-will then assign no interpretation to the data in the file and simply
-read or write raw bytes.  The Tcl \fBbinary\fR command can be used to
-manipulate this byte-oriented data.  It is usually better to set the
-\fB\-translation\fR option to \fBbinary\fR when you want to transfer
-binary data, as this turns off the other automatic interpretations of
-the bytes in the stream as well.
-.PP
-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 \fBencoding system\fR.
-.RE
-.TP
-\fB\-eofchar\fR \fIchar\fR
-.TP
-\fB\-eofchar\fR \fB{\fIinChar outChar\fB}\fR
-.
-This option supports DOS file systems that use Control-z (\ex1a) as an
-end of file marker.  If \fIchar\fR 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 \fIchar\fR 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 \fB\-eofchar\fR is the empty string
-in all cases except for files under Windows.  In that case the
-\fB\-eofchar\fR is Control-z (\ex1a) for reading and the empty string
-for writing.
-The acceptable range for \fB\-eofchar\fR values is \ex01 - \ex7f;
-attempting to set \fB\-eofchar\fR to a value outside of this range will
-generate an error.
-.TP
-\fB\-translation\fR \fImode\fR
-.TP
-\fB\-translation\fR \fB{\fIinMode outMode\fB}\fR
-.
-In Tcl scripts the end of a line is always represented using a single
-newline character (\en).  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 \fBchan gets\fR and \fBchan read\fR) the Tcl I/O system
-automatically translates the external end-of-line representation into
-newline characters.  Upon output (i.e., with \fBchan puts\fR), the I/O
-system translates newlines to the external end-of-line representation.
-The default translation mode, \fBauto\fR, handles all the common cases
-automatically, but the \fB\-translation\fR option provides explicit
-control over the end of line translations.
-.RS
-.PP
-The value associated with \fB\-translation\fR 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:
-.TP
-\fBauto\fR
-.
-As the input translation mode, \fBauto\fR treats any of newline
-(\fBlf\fR), carriage return (\fBcr\fR), or carriage return followed by
-a newline (\fBcrlf\fR) 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,
-\fBauto\fR chooses a platform specific representation; for sockets on
-all platforms Tcl chooses \fBcrlf\fR, for all Unix flavors, it chooses
-\fBlf\fR, and for the various flavors of Windows it chooses
-\fBcrlf\fR.  The default setting for \fB\-translation\fR is \fBauto\fR
-for both input and output.
-.TP
-\fBbinary\fR
-.
-No end-of-line translations are performed.  This is nearly identical
-to \fBlf\fR mode, except that in addition \fBbinary\fR mode also sets
-the end-of-file character to the empty string (which disables it) and
-sets the encoding to \fBbinary\fR (which disables encoding filtering).
-See the description of \fB\-eofchar\fR and \fB\-encoding\fR for more
-information.
-.TP
-\fBcr\fR
-.
-The end of a line in the underlying file or device is represented by a
-single carriage return character.  As the input translation mode,
-\fBcr\fR mode converts carriage returns to newline characters.  As the
-output translation mode, \fBcr\fR mode translates newline characters
-to carriage returns.
-.TP
-\fBcrlf\fR
-.
-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, \fBcrlf\fR mode converts
-carriage-return-linefeed sequences to newline characters.  As the
-output translation mode, \fBcrlf\fR mode translates newline characters
-to carriage-return-linefeed sequences.  This mode is typically used on
-Windows platforms and for network connections.
-.TP
-\fBlf\fR
-.
-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.
-.RE
-.RE
-.TP
-\fBchan copy \fIinputChan outputChan\fR ?\fB\-size \fIsize\fR? ?\fB\-command \fIcallback\fR?
-.
-Copy data from the channel \fIinputChan\fR, which must have been
-opened for reading, to the channel \fIoutputChan\fR, which must have
-been opened for writing. The \fBchan copy\fR 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.
-.RS
-.PP
-The \fBchan copy\fR command transfers data from \fIinputChan\fR until
-end of file or \fIsize\fR bytes or characters have been transferred;
-\fIsize\fR is in bytes if the two channels are using the same encoding,
-and is in characters otherwise.  If no \fB\-size\fR argument is given,
-then the copy goes until end of file. All the data read from
-\fIinputChan\fR is copied to \fIoutputChan\fR.  Without the
-\fB\-command\fR option, \fBchan copy\fR blocks until the copy is
-complete and returns the number of bytes or characters (using the same
-rules as for the \fB\-size\fR option) written to \fIoutputChan\fR.
-.PP
-The \fB\-command\fR argument makes \fBchan copy\fR work in the
-background.  In this case it returns immediately and the
-\fIcallback\fR is invoked later when the copy completes.  The
-\fIcallback\fR is called with one or two additional arguments that
-indicates how many bytes were written to \fIoutputChan\fR.  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 \fIinputChan\fR or \fIoutputChan\fR into
-non-blocking mode; the \fBchan copy\fR command takes care of that
-automatically.  However, it is necessary to enter the event loop by
-using the \fBvwait\fR command or by using Tk.
-.PP
-You are not allowed to do other I/O operations with \fIinputChan\fR or
-\fIoutputChan\fR during a background \fBchan copy\fR.  If either
-\fIinputChan\fR or \fIoutputChan\fR get closed while the copy is in
-progress, the current copy is stopped and the command callback is
-\fInot\fR made.  If \fIinputChan\fR is closed, then all data already
-queued for \fIoutputChan\fR is written out.
-.PP
-Note that \fIinputChan\fR can become readable during a background
-copy.  You should turn off any \fBchan event\fR or \fBfileevent\fR
-handlers during a background copy so those handlers do not interfere
-with the copy.  Any I/O attempted by a \fBchan event\fR or
-\fBfileevent\fR handler will get a
-.QW "channel busy"
-error.
-.PP
-\fBChan copy\fR translates end-of-line sequences in \fIinputChan\fR
-and \fIoutputChan\fR according to the \fB\-translation\fR option for
-these channels (see \fBchan configure\fR above).  The translations
-mean that the number of bytes read from \fIinputChan\fR can be
-different than the number of bytes written to \fIoutputChan\fR.  Only
-the number of bytes written to \fIoutputChan\fR is reported, either as
-the return value of a synchronous \fBchan copy\fR or as the argument
-to the callback for an asynchronous \fBchan copy\fR.
-.PP
-\fBChan copy\fR 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 \fBchan copy\fR writes to (see \fBchan configure\fR above for
-details on the \fB\-encoding\fR and \fB\-translation\fR options). No
-conversion is done if both channels are set to encoding \fBbinary\fR
-and have matching translations. If only the output channel is set to
-encoding \fBbinary\fR the system will write the internal UTF-8
-representation of the incoming characters. If only the input channel
-is set to encoding \fBbinary\fR 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.
-.RE
-.TP
-\fBchan create \fImode cmdPrefix\fR
-.
-This subcommand creates a new script level channel using the command
-prefix \fIcmdPrefix\fR as its handler. Any such channel is called a
-\fBreflected\fR channel. The specified command prefix, \fBcmdPrefix\fR,
-must be a non-empty list, and should provide the API described in the
-\fBrefchan\fR manual page. The handle of the new channel is
-returned as the result of the \fBchan create\fR command, and the
-channel is open. Use either \fBclose\fR or \fBchan close\fR to remove
-the channel.
-.RS
-.PP
-The argument \fImode\fR specifies if the new channel is opened for
-reading, writing, or both. It has to be a list containing any of the
-strings
-.QW \fBread\fR
-or
-.QW \fBwrite\fR .
-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.
-.PP
-The command prefix is executed in the global namespace, at the top of
-call stack, following the appending of arguments as described in the
-\fBrefchan\fR 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.
-.PP
-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 \fBchan create\fR command)
-must have an active event loop, i.e. it must be able to process such
-events. Otherwise the thread sending them will \fIblock
-indefinitely\fR. Deadlock may occur.
-.PP
-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.
-.PP
-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.
-.PP
-This subcommand is \fBsafe\fR 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.
-.RE
-.TP
-\fBchan eof \fIchannelId\fR
-.
-Test whether the last input operation on the channel called
-\fIchannelId\fR failed because the end of the data stream was reached,
-returning 1 if end-of-file was reached, and 0 otherwise.
-.TP
-\fBchan event \fIchannelId event\fR ?\fIscript\fR?
-.
-Arrange for the Tcl script \fIscript\fR to be installed as a \fIfile
-event handler\fR to be called whenever the channel called
-\fIchannelId\fR enters the state described by \fIevent\fR (which must
-be either \fBreadable\fR or \fBwritable\fR); only one such handler may
-be installed per event per channel at a time.  If \fIscript\fR is the
-empty string, the current handler is deleted (this also happens if the
-channel is closed or the interpreter deleted).  If \fIscript\fR 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 \fBvwait\fR or
-\fBupdate\fR).
-.RS
-.PP
-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
-\fBchan gets\fR or \fBchan read\fR 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
-.QW "freeze up" .
-With \fBchan event\fR, the
-process can tell when data is present and only invoke \fBchan gets\fR
-or \fBchan read\fR when they will not block.
-.PP
-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 \fBchan gets\fR 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 \fIscript\fR 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 \fIscript\fR reads no data, returns, and is immediately invoked
-again.
-.PP
-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.
-.PP
-Event-driven I/O works best for channels that have been placed into
-non-blocking mode with the \fBchan configure\fR command.  In blocking
-mode, a \fBchan puts\fR command may block if you give it more data
-than the underlying file or device can accept, and a \fBchan gets\fR
-or \fBchan read\fR 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 \fBchan puts\fR, \fBchan read\fR, and \fBchan
-gets\fR never block.
-.PP
-The script for a file event is executed at global level (outside the
-context of any Tcl procedure) in the interpreter in which the \fBchan
-event\fR command was invoked.  If an error occurs while executing the
-script then the command registered with \fBinterp bgerror\fR 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.
-.RE
-.TP
-\fBchan flush \fIchannelId\fR
-.
-Ensures that all pending output for the channel called \fIchannelId\fR
-is written.
-.RS
-.PP
-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.
-.RE
-.TP
-\fBchan gets \fIchannelId\fR ?\fIvarName\fR?
-.
-Reads the next line from the channel called \fIchannelId\fR. If
-\fIvarName\fR 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 \fIvarName\fR is specified, the line that
-has been read will be written to the variable called \fIvarName\fR 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.
-.RS
-.PP
-If an end-of-file occurs while part way through reading a line, the
-partial line will be returned (or written into \fIvarName\fR). When
-\fIvarName\fR is not specified, the end-of-file case can be
-distinguished from an empty line using the \fBchan eof\fR command, and
-the partial-line-but-non-blocking case can be distinguished with the
-\fBchan blocked\fR command.
-.RE
-.TP
-\fBchan names\fR ?\fIpattern\fR?
-.
-Produces a list of all channel names. If \fIpattern\fR is specified,
-only those channel names that match it (according to the rules of
-\fBstring match\fR) will be returned.
-.TP
-\fBchan pending \fImode channelId\fR
-.
-Depending on whether \fImode\fR is \fBinput\fR or \fBoutput\fR,
-returns the number of
-bytes of input or output (respectively) currently buffered
-internally for \fIchannelId\fR (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.
-.TP
-\fBchan pipe\fR
-.VS 8.6
-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 \fBstderr\fR and \fBstdout\fR 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.
-.RS
-.PP
-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
-\fIare\fR 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.
-.RE
-.VE 8.6
-.TP
-\fBchan pop \fIchannelId\fR
-.VS 8.6
-Removes the topmost transformation from the channel \fIchannelId\fR, if there
-is any. If there are no transformations added to \fIchannelId\fR, this is
-equivalent to \fBchan close\fR 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).
-.VE 8.6
-.TP
-\fBchan postevent \fIchannelId eventSpec\fR
-.
-This subcommand is used by command handlers specified with \fBchan
-create\fR. It notifies the channel represented by the handle
-\fIchannelId\fR that the event(s) listed in the \fIeventSpec\fR have
-occurred. The argument has to be a list containing any of the strings
-\fBread\fR and \fBwrite\fR. 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.
-.RS
-.PP
-Note that this subcommand can only be used with channel handles that
-were created/opened by \fBchan create\fR. All other channels will
-cause this subcommand to report an error.
-.PP
-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.
-.PP
-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
-\fBwatch\fR described in \fBrefchan\fR, the document specifying
-the API of command handlers for reflected channels.
-.PP
-This command is \fBsafe\fR and made accessible to safe interpreters.
-It can trigger the execution of \fBchan event\fR 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. \fBChan event\fR handlers are \fIalways\fR
-executed in the interpreter that set them up.
-.RE
-.TP
-\fBchan push \fIchannelId cmdPrefix\fR
-.VS 8.6
-Adds a new transformation on top of the channel \fIchannelId\fR. The
-\fIcmdPrefix\fR 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 \fBtranschan\fR 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.
-.VE 8.6
-.TP
-\fBchan puts\fR ?\fB\-nonewline\fR? ?\fIchannelId\fR? \fIstring\fR
-.
-Writes \fIstring\fR to the channel named \fIchannelId\fR followed by a
-newline character. A trailing newline character is written unless the
-optional flag \fB\-nonewline\fR is given. If \fIchannelId\fR is
-omitted, the string is written to the standard output channel,
-\fBstdout\fR.
-.RS
-.PP
-Newline characters in the output are translated by \fBchan puts\fR to
-platform-specific end-of-line sequences according to the currently
-configured value of the \fB\-translation\fR option for the channel
-(for example, on PCs newlines are normally replaced with
-carriage-return-linefeed sequences; see \fBchan configure\fR above for
-details).
-.PP
-Tcl buffers output internally, so characters written with \fBchan
-puts\fR 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 \fBchan
-flush\fR command.
-.PP
-When the output buffer fills up, the \fBchan puts\fR command will
-normally block until all the buffered data has been accepted for
-output by the operating system.  If \fIchannelId\fR is in non-blocking
-mode then the \fBchan puts\fR 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 \fBchan event\fR command
-(do not invoke \fBchan puts\fR unless you have recently been notified
-via a file event that the channel is ready for more output data).
-.RE
-.TP
-\fBchan read \fIchannelId\fR ?\fInumChars\fR?
-.TP
-\fBchan read \fR?\fB\-nonewline\fR? \fIchannelId\fR
-.
-In the first form, the result will be the next \fInumChars\fR
-characters read from the channel named \fIchannelId\fR; if
-\fInumChars\fR 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
-\fInumChars\fR has been omitted) the flag \fB\-nonewline\fR may be
-given to indicate that any trailing newline in the string that has
-been read should be trimmed.
-.RS
-.PP
-If \fIchannelId\fR is in non-blocking mode, \fBchan read\fR 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 \fB\-nonewline\fR switch is ignored if
-the command returns before reaching the end of the file.
-.PP
-\fBChan read\fR translates end-of-line sequences in the input into
-newline characters according to the \fB\-translation\fR option for the
-channel (see \fBchan configure\fR above for a discussion on the ways
-in which \fBchan configure\fR will alter input).
-.PP
-When reading from a serial port, most applications should configure
-the serial port channel to be non-blocking, like this:
-.PP
-.CS
-\fBchan configure \fIchannelId \fB\-blocking \fI0\fR.
-.CE
-.PP
-Then \fBchan read\fR behaves much like described above.  Note that
-most serial ports are comparatively slow; it is entirely possible to
-get a \fBreadable\fR event for each character read from them. Care
-must be taken when using \fBchan read\fR on blocking serial ports:
-.TP
-\fBchan read \fIchannelId numChars\fR
-.
-In this form \fBchan read\fR blocks until \fInumChars\fR have been
-received from the serial port.
-.TP
-\fBchan read \fIchannelId\fR
-.
-In this form \fBchan read\fR blocks until the reception of the
-end-of-file character, see \fBchan configure -eofchar\fR. If there no
-end-of-file character has been configured for the channel, then
-\fBchan read\fR will block forever.
-.RE
-.TP
-\fBchan seek \fIchannelId offset\fR ?\fIorigin\fR?
-.
-Sets the current access position within the underlying data stream for
-the channel named \fIchannelId\fR to be \fIoffset\fR bytes relative to
-\fIorigin\fR. \fIOffset\fR must be an integer (which may be negative)
-and \fIorigin\fR must be one of the following:
-.RS
-.TP 10
-\fBstart\fR
-.
-The new access position will be \fIoffset\fR bytes from the start
-of the underlying file or device.
-.TP 10
-\fBcurrent\fR
-.
-The new access position will be \fIoffset\fR bytes from the current
-access position; a negative \fIoffset\fR moves the access position
-backwards in the underlying file or device.
-.TP 10
-\fBend\fR
-.
-The new access position will be \fIoffset\fR bytes from the end of the
-file or device.  A negative \fIoffset\fR places the access position
-before the end of file, and a positive \fIoffset\fR places the access
-position after the end of file.
-.PP
-The \fIorigin\fR argument defaults to \fBstart\fR.
-.PP
-\fBChan seek\fR 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.
-.PP
-Note that \fIoffset\fR values are byte offsets, not character offsets.
-Both \fBchan seek\fR and \fBchan tell\fR operate in terms of bytes,
-not characters, unlike \fBchan read\fR.
-.RE
-.TP
-\fBchan tell \fIchannelId\fR
-.
-Returns a number giving the current access position within the
-underlying data stream for the channel named \fIchannelId\fR. This
-value returned is a byte offset that can be passed to \fBchan seek\fR
-in order to set the channel to a particular position.  Note that this
-value is in terms of bytes, not characters like \fBchan read\fR.  The
-value returned is -1 for channels that do not support seeking.
-.TP
-\fBchan truncate \fIchannelId\fR ?\fIlength\fR?
-.
-Sets the byte length of the underlying data stream for the channel
-named \fIchannelId\fR to be \fIlength\fR (or to the current byte
-offset within the underlying data stream if \fIlength\fR is
-omitted). The channel is flushed before truncation.
-.
-.SH EXAMPLES
-.PP
-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.
-.PP
-.CS
-set f [open somefile.txt r+]
-\fBchan configure\fR $f -encoding cp1252
-set offset 0
-
-\fI# Search for string "FOOBAR" in the file\fR
-while {[\fBchan gets\fR $f line] >= 0} {
-    set idx [string first FOOBAR $line]
-    if {$idx > -1} {
-        \fI# Found it; rewrite line\fR
-
-        \fBchan seek\fR $f [expr {$offset + $idx}]
-        \fBchan puts\fR -nonewline $f BARFOO
-
-        \fI# Skip to end of following line, and truncate\fR
-        \fBchan gets\fR $f
-        \fBchan gets\fR $f
-        \fBchan truncate\fR $f
-
-        \fI# Stop searching the file now\fR
-        break
-    }
-
-    \fI# Save offset of start of next line for later\fR
-    set offset [\fBchan tell\fR $f]
-}
-\fBchan close\fR $f
-.CE
-.PP
-A network server that does echoing of its input line-by-line without
-preventing servicing of other connections at the same time.
-.PP
-.CS
-# This is a very simple logger...
-proc log {message} {
-    \fBchan puts\fR stdout $message
-}
-
-# This is called whenever a new client connects to the server
-proc connect {chan host port} {
-    set clientName [format <%s:%d> $host $port]
-    log "connection from $clientName"
-    \fBchan configure\fR $chan -blocking 0 -buffering line
-    \fBchan event\fR $chan readable [list echoLine $chan $clientName]
-}
-
-# This is called whenever either at least one byte of input
-# data is available, or the channel was closed by the client.
-proc echoLine {chan clientName} {
-    \fBchan gets\fR $chan line
-    if {[\fBchan eof\fR $chan]} {
-        log "finishing connection from $clientName"
-        \fBchan close\fR $chan
-    } elseif {![\fBchan blocked\fR $chan]} {
-        # Didn't block waiting for end-of-line
-        log "$clientName - $line"
-        \fBchan puts\fR $chan $line
-    }
-}
-
-# Create the server socket and enter the event-loop to wait
-# for incoming connections...
-socket -server connect 12345
-vwait forever
-.CE
-.SH "SEE ALSO"
-close(n), eof(n), fblocked(n), fconfigure(n), fcopy(n), file(n),
-fileevent(n), flush(n), gets(n), open(n), puts(n), read(n), seek(n),
-socket(n), tell(n), refchan(n), transchan(n)
-.SH KEYWORDS
-channel, input, output, events, offset
-'\" Local Variables:
-'\" mode: nroff
-'\" End: