Improve the [chan] documentationmsofer_wcodes_20050611

1 files changed, 513 insertions, 44 deletions

diff --git a/doc/chan.n b/doc/chan.n
index 19753cd..21484f6 100644
--- a/doc/chan.n
+++ b/doc/chan.n
@@ -4,7 +4,7 @@
 '\" See the file "license.terms" for information on usage and redistribution
 '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 '\"
-'\" RCS: @(#) $Id: chan.n,v 1.1 2005/06/06 23:45:41 dkf Exp $
+'\" RCS: @(#) $Id: chan.n,v 1.2 2005/06/10 15:22:43 dkf Exp $
 .so man.macros
 .TH chan n 8.5 Tcl "Tcl Built-In Commands"
 .BS
@@ -19,73 +19,463 @@ chan \- Read, write and manipulate channels
 .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). \fIOption\fR indicates
-what to do with the channel; any unique abbreviation for \fIoption\fR
-is acceptable. Valid options are:
+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
-Test whether the last input operation on the channel called
+.
+This tests whether the last input operation on the channel called
 \fIchannelId\fR failed because it would have otherwise caused the
-process to block.
+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
+.
 Close and destroy the channel called \fIchannelId\fR. Note that this
 deletes all existing file-events registered on the channel.
+.RS
+.PP
+As part of closing the channel, all buffered output is flushed to the
+channel's outpuot device, any buffered input is discarded, the
+underlying operating system resource is closed and \fIchannelId\fR
+becomes unavailable for future use.
+.PP
+If the channel is blocking, the command does not return until all
+output is flushed.  If the channel is nonblocking and there is
+unflushed output, the channel remains open and the command returns
+immediately; output will be flushed in the background and the channel
+will be closed when all the flushing is complete.
+.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. See the \fBinterp\fR command for a description
+of channel sharing.
+.PP
+Channels are automatically closed when an interpreter is destroyed and
+when the process exits.  Channels are switched to blocking mode, to
+ensure that all output is correctly flushed before the process exits.
+.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.)
+.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 (see \fBfconfigure\fR for details).
-'\" FIXME: describe common channel options here and refer to creator
-'\" commands for special option descriptions.
-.TP
-\fBchan copy \fIinput output\fR ?\fB\-size \fIsize\fR? ?\fB\-command \fIcallback\fR?
-Copy the contents of the channel \fIinput\fR, which must have been
-opened for reading, to the channel \fIoutput\fR, which must have been
-opened for writing. If \fIsize\fR is specified, only that many bytes
-will be transferred. If \fIcallback\fR is specified, this command
-returns immediately and arranges for \fIcallback\fR to be called when
-the requested transfer has completed or an error occurs (assuming that
-the event loop is running) with extra arguments appended to
-\fIcallback\fR to indicate what happened.
+\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 channels
+for the options that that specific type of channel supports. For
+example, see the manual entry for the \fBsocket\fR command for its
+additional options.
+.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 nonblocking 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 nonblocking 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.
+.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\fP and \fBchan read\fP) the Tcl I/O system
+automatically translates the external end-of-line representation into
+newline characters.  Upon output (i.e., with \fBchan puts\fP), the I/O
+system translates newlines to the external end-of-line representation.
+The default translation mode, \fBauto\fP, 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\fP), carriage return (\fBcr\fP), or carriage return followed by
+a newline (\fBcrlf\fP) 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\fP mode, except that in addition \fBbinary\fP 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\fP mode converts carriage returns to newline characters.  As the
+output translation mode, \fBcr\fP 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\fP mode converts
+carriage-return-linefeed sequences to newline characters.  As the
+output translation mode, \fBcrlf\fP 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\fP command transfers data from \fIinputChan\fR until
+end of file or \fIsize\fP bytes have been transferred. If no
+\fB\-size\fP 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\fP option, \fBchan copy\fP blocks until the
+copy is complete and returns the number of bytes written to
+\fIoutputChan\fR.
+.PP
+The \fB\-command\fP argument makes \fBchan copy\fP work in the
+background.  In this case it returns immediately and the
+\fIcallback\fP is invoked later when the copy completes.  The
+\fIcallback\fP 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\fP command takes care of that
+automatically.  However, it is necessary to enter the event loop by
+using the \fBvwait\fP 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\fP 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\fP 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\fP handler will get a "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\fP or as the argument
+to the callback for an asynchronous \fBchan copy\fP.
+.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 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.
+\fIchannelId\fR failed because the end of the data stream was reached,
+returning 1 if end-fo-file was reached, and 0 otherwise.
 .TP
-\fBchan event \fIchannelId mode\fR ?\fIscript\fR?
-Arrange for the Tcl script \fIscript\fR to be called whenever the
-channel called \fIchannelId\fR enters the state described by
-\fImode\fR (which must be either \fBreadable\fR or \fBwritable\fR). If
-\fIscript\fR is omitted, the currently installed script is returned.
-The callback is only performed if the event loop is being serviced.
+\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 ``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 won't 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 nonblocking mode using events.  A channel is also considered to be
+readable if an end of file or error condition is present on the
+underlying file or device.  It is important for \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
+nonblocking 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 nonblocking 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 nonblocking mode, the command may return before all
+buffered output has been flushed; the remainder will be flushed in the
+background as fast as the underlying file or device is able to absorb
+it.
+.RE
 .TP
 \fBchan gets \fIchannelId\fR ?\fIvarName\fR?
-Reads a 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 error. 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 an error
-occurred.
+.
+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-nonblocking 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 puts\fR ?\fB\-nonewline\fR? ?\fIchannelId\fR? \fIstring\fR
-Writes \fIstring\fR to the channel named \fIchannelId\fR (by default,
-to the standard output stream) followed by a newline character. If the
-optional flag \fB\-nonewline\fR is given, no trailing newline
-character is written.
+.
+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 nonblocking
+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 nonblocking output to work; otherwise Tcl never
+finds out that the file or device is ready for more output data.  It
+is possible for an arbitrarily large amount of data to be buffered for
+a channel in nonblocking mode, which could consume a large amount of
+memory.  To avoid wasting memory, nonblocking I/O should normally be
+used in an event-driven fashion with the \fBchan event\fR command
+(don't 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
@@ -94,21 +484,100 @@ 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 nonblocking 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 nonblocking, like this:
+.CS
+\fBchan configure \fIchannelId \fB\-blocking \fI0\fR.
+.CE
+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 position within the underlying data stream for the
-channel named \fIchannelId\fR to be \fIoffset\fR bytes relative to
-\fIorigin\fR. \fIOrigin\fR should be one of \fBstart\fR (the default
-origin), \fBcurrent\fR or \fBend\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 nonblocking mode.  It also
+discards any buffered and unread input.  This command returns an empty
+string.  An error occurs if this command is applied to channels whose
+underlying file or device does not support seeking.
+.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
-Reports the current byte offset within the underlying data stream for
-the channel named \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).
+offset within the underlying data stream if \fIlength\fR is
+omitted). The channel is flushed before truncation.
 
 .SH "SEE ALSO"
 close(n), eof(n), fblocked(n), fconfigure(n), fcopy(n), file(n),