341 lines
13 KiB
Plaintext
341 lines
13 KiB
Plaintext
'\"
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'\" Copyright (c) 2006 Andreas Kupries <andreas_kupries@users.sourceforge.net>
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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.TH refchan n 8.5 Tcl "Tcl Built-In Commands"
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.so man.macros
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.BS
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.\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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refchan \- Command handler API of reflected channels, version 1
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.SH SYNOPSIS
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\fBcmdPrefix \fIoption\fR ?\fIarg arg ...\fR?
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.BE
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.SH DESCRIPTION
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.PP
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The Tcl-level handler for a reflected channel has to be a command with
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subcommands (termed an \fIensemble\fR, as it is a command such as that
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created by \fBnamespace ensemble create\fR, though the implementation
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of handlers for reflected channel \fIis not\fR tied to \fBnamespace
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ensemble\fRs in any way). Note that \fIcmdPrefix\fR is whatever was
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specified in the call to \fBchan create\fR, and may consist of
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multiple arguments; this will be expanded to multiple words in place
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of the prefix.
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.PP
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Of all the possible subcommands, the handler \fImust\fR support
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\fBinitialize\fR, \fBfinalize\fR, and \fBwatch\fR. Support for the
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other subcommands is optional.
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.SS "MANDATORY SUBCOMMANDS"
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.TP
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\fIcmdPrefix \fBinitialize \fIchannelId mode\fR
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.
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An invocation of this subcommand will be the first call the
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\fIcmdPrefix\fR will receive for the specified new \fIchannelId\fR. It
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is the responsibility of this subcommand to set up any internal data
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structures required to keep track of the channel and its state.
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.RS
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.PP
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The return value of the method has to be a list containing the names
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of all subcommands supported by the \fIcmdPrefix\fR. This also tells
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the Tcl core which version of the API for reflected channels is used by
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this command handler.
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.PP
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Any error thrown by the method will abort the creation of the channel
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and no channel will be created. The thrown error will appear as error
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thrown by \fBchan create\fR. Any exception other than an \fBerror\fR
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(e.g. \fBbreak\fR, etc.) is treated as (and converted to) an error.
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.PP
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\fBNote:\fR If the creation of the channel was aborted due to failures
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here, then the \fBfinalize\fR subcommand will not be called.
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.PP
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The \fImode\fR argument tells the handler whether the channel was
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opened for reading, writing, or both. It is a list containing any of
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the strings \fBread\fR or \fBwrite\fR. The list will always
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contain at least one element.
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.PP
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The subcommand must throw an error if the chosen mode is not
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supported by the \fIcmdPrefix\fR.
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.RE
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.TP
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\fIcmdPrefix \fBfinalize \fIchannelId\fR
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.
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An invocation of this subcommand will be the last call the
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\fIcmdPrefix\fR will receive for the specified \fIchannelId\fR. It will
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be generated just before the destruction of the data structures of the
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channel held by the Tcl core. The command handler \fImust not\fR
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access the \fIchannelId\fR anymore in no way. Upon this subcommand being
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called, any internal resources allocated to this channel must be
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cleaned up.
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.RS
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.PP
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The return value of this subcommand is ignored.
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.PP
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If the subcommand throws an error the command which caused its
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invocation (usually \fBclose\fR) will appear to have thrown this
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error. Any exception beyond \fIerror\fR (e.g. \fIbreak\fR, etc.) is
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treated as (and converted to) an error.
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.PP
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This subcommand is not invoked if the creation of the channel was
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aborted during \fBinitialize\fR (See above).
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.RE
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.TP
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\fIcmdPrefix \fBwatch \fIchannelId eventspec\fR
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.
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This subcommand notifies the \fIcmdPrefix\fR that the specified
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\fIchannelId\fR is interested in the events listed in the
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\fIeventspec\fR. This argument is a list containing any of \fBread\fR
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and \fBwrite\fR. The list may be empty, which signals that the
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channel does not wish to be notified of any events. In that situation,
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the handler should disable event generation completely.
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.RS
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.PP
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\fBWarning:\fR Any return value of the subcommand is ignored. This
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includes all errors thrown by the subcommand, break, continue, and
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custom return codes.
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.PP
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This subcommand interacts with \fBchan postevent\fR. Trying to post an
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event which was not listed in the last call to \fBwatch\fR will cause
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\fBchan postevent\fR to throw an error.
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.RE
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.SS "OPTIONAL SUBCOMMANDS"
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.TP
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\fIcmdPrefix \fBread \fIchannelId count\fR
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.
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This \fIoptional\fR subcommand is called when the user requests data from the
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channel \fIchannelId\fR. \fIcount\fR specifies how many \fBbytes\fR have been
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requested. If the subcommand is not supported then it is not possible to read
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from the channel handled by the command.
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.RS
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.PP
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The return value of this subcommand is taken as the requested data
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\fIbytes\fR. If the returned data contains more bytes than requested,
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an error will be signaled and later thrown by the command which
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performed the read (usually \fBgets\fR or \fBread\fR). However,
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returning fewer bytes than requested is acceptable.
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.PP
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Note that returning nothing (0 bytes) is a signal to the higher layers
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that \fBEOF\fR has been reached on the channel. To signal that the
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channel is out of data right now, but has not yet reached \fBEOF\fR,
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it is necessary to throw the error "EAGAIN", i.e. to either
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.PP
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.CS
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return -code error EAGAIN
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.CE
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or
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.CS
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error EAGAIN
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.CE
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.PP
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For extensibility any error whose value is a negative integer number
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will cause the higher layers to set the C-level variable "\fBerrno\fR"
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to the absolute value of this number, signaling a system error. This
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means that both
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.PP
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.CS
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return -code error -11
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.CE
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and
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.CS
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error -11
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.CE
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.PP
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are equivalent to the examples above, using the more readable string "EAGAIN".
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No other error value has such a mapping to a symbolic string.
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.PP
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If the subcommand throws any other error, the command which caused its
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invocation (usually \fBgets\fR, or \fBread\fR) will appear to have
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thrown this error. Any exception beyond \fIerror\fR, (e.g.
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\fIbreak\fR, etc.) is treated as and converted to an error.
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.RE
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.TP
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\fIcmdPrefix \fBwrite \fIchannelId data\fR
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.
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This \fIoptional\fR subcommand is called when the user writes data to
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the channel \fIchannelId\fR. The \fIdata\fR argument contains \fIbytes\fR, not
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characters. Any type of transformation (EOL, encoding) configured for
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the channel has already been applied at this point. If this subcommand
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is not supported then it is not possible to write to the channel
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handled by the command.
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.RS
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.PP
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The return value of the subcommand is taken as the number of bytes
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written by the channel. Anything non-numeric will cause an error to be
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signaled and later thrown by the command which performed the write. A
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negative value implies that the write failed. Returning a value
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greater than the number of bytes given to the handler, or zero, is
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forbidden and will cause the Tcl core to throw an error.
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.PP
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To signal that the channel is not able to accept data for writing
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right now, it is necessary to throw the error "EAGAIN", i.e. to either
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.PP
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.CS
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return -code error EAGAIN
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.CE
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or
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.CS
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error EAGAIN
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.CE
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.PP
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For extensibility any error whose value is a negative integer number
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will cause the higher layers to set the C-level variable "\fBerrno\fR"
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to the absolute value of this number, signaling a system error.
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However, note that the exact mapping between these error numbers and
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their meanings is operating system dependent.
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.PP
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For example, while on Linux both
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.PP
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.CS
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return -code error -11
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.CE
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and
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.CS
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error -11
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.CE
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.PP
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are equivalent to the examples above, using the more readable string "EAGAIN",
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this is not true for BSD, where the equivalent number is -35.
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.PP
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The symbolic string however is the same across systems, and internally
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translated to the correct number. No other error value has such a mapping
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to a symbolic string.
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.PP
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If the subcommand throws any other error the command which caused its
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invocation (usually \fBputs\fR) will appear to have thrown this error.
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Any exception beyond \fIerror\fR (e.g.\ \fIbreak\fR, etc.) is treated
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as and converted to an error.
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.RE
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.TP
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\fIcmdPrefix \fBseek \fIchannelId offset base\fR
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.
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This \fIoptional\fR subcommand is responsible for the handling of
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\fBseek\fR and \fBtell\fR requests on the channel \fIchannelId\fR. If it is not
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supported then seeking will not be possible for the channel.
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.RS
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.PP
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The \fIbase\fR argument is one of
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.TP 10
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\fBstart\fR
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.
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Seeking is relative to the beginning of the channel.
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.TP 10
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\fBcurrent\fR
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.
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Seeking is relative to the current seek position.
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.TP 10
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\fBend\fR
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.
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Seeking is relative to the end of the channel.
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.PP
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The \fIbase\fR argument of the builtin \fBchan seek\fR command takes
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the same names.
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.PP
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The \fIoffset\fR is an integer number specifying the amount of
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\fBbytes\fR to seek forward or backward. A positive number should seek
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forward, and a negative number should seek backward.
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.PP
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A channel may provide only limited seeking. For example sockets can
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seek forward, but not backward.
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.PP
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The return value of the subcommand is taken as the (new) location of
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the channel, counted from the start. This has to be an integer number
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greater than or equal to zero.
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.PP
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If the subcommand throws an error the command which caused its
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invocation (usually \fBseek\fR, or \fBtell\fR) will appear to have
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thrown this error. Any exception beyond \fIerror\fR (e.g. \fIbreak\fR,
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etc.) is treated as and converted to an error.
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.PP
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The offset/base combination of 0/\fBcurrent\fR signals a \fBtell\fR
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request, i.e. seek nothing relative to the current location, making
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the new location identical to the current one, which is then returned.
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.RE
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.TP
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\fIcmdPrefix \fBconfigure \fIchannelId option value\fR
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.
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This \fIoptional\fR subcommand is for setting the type-specific options of
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channel \fIchannelId\fR. The \fIoption\fR argument indicates the option to be
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written, and the \fIvalue\fR argument indicates the value to set the option to.
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.RS
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.PP
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This subcommand will never try to update more than one option at a
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time; that is behavior implemented in the Tcl channel core.
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.PP
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The return value of the subcommand is ignored.
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.PP
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If the subcommand throws an error the command which performed the
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(re)configuration or query (usually \fBfconfigure\fR or \fBchan
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configure\fR) will appear to have thrown this error. Any exception
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beyond \fIerror\fR (e.g. \fIbreak\fR, etc.) is treated as and
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converted to an error.
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.RE
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.TP
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\fIcmdPrefix \fBcget \fIchannelId option\fR
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.
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This \fIoptional\fR subcommand is used when reading a single type-specific
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option of channel \fIchannelId\fR. If this subcommand is supported then the
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subcommand \fBcgetall\fR must be supported as well.
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.RS
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.PP
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The subcommand should return the value of the specified \fIoption\fR.
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.PP
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If the subcommand throws an error, the command which performed the
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(re)configuration or query (usually \fBfconfigure\fR) will appear to
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have thrown this error. Any exception beyond \fIerror\fR (e.g.
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\fIbreak\fR, etc.) is treated as and converted to an error.
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.RE
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.TP
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\fIcmdPrefix \fBcgetall \fIchannelId\fR
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.
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This \fIoptional\fR subcommand is used for reading all type-specific options
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of channel \fIchannelId\fR. If this subcommand is supported then the
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subcommand \fBcget\fR has to be supported as well.
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.RS
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.PP
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The subcommand should return a list of all options and their values.
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This list must have an even number of elements.
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.PP
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If the subcommand throws an error the command which performed the
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(re)configuration or query (usually \fBfconfigure\fR) will appear to
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have thrown this error. Any exception beyond \fIerror\fR (e.g.
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\fIbreak\fR, etc.) is treated as and converted to an error.
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.RE
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.TP
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\fIcmdPrefix \fBblocking \fIchannelId mode\fR
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.
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This \fIoptional\fR subcommand handles changes to the blocking mode of the
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channel \fIchannelId\fR. The \fImode\fR is a boolean flag. A true value means
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that the channel has to be set to blocking, and a false value means that the
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channel should be non-blocking.
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.RS
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.PP
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The return value of the subcommand is ignored.
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.PP
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If the subcommand throws an error the command which caused its
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invocation (usually \fBfconfigure\fR) will appear to have thrown this
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error. Any exception beyond \fIerror\fR (e.g. \fIbreak\fR, etc.) is
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treated as and converted to an error.
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.RE
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.SH NOTES
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Some of the functions supported in channels defined in Tcl's C
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interface are not available to channels reflected to the Tcl level.
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.PP
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The function \fBTcl_DriverGetHandleProc\fR is not supported; i.e.
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reflected channels do not have OS specific handles.
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.PP
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The function \fBTcl_DriverHandlerProc\fR is not supported. This driver
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function is relevant only for stacked channels, i.e. transformations.
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Reflected channels are always base channels, not transformations.
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.PP
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The function \fBTcl_DriverFlushProc\fR is not supported. This is
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because the current generic I/O layer of Tcl does not use this
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function anywhere at all. Therefore support at the Tcl level makes no
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sense either. This may be altered in the future (through extending the
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API defined here and changing its version number) should the function
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be used at some time in the future.
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.SH "SEE ALSO"
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chan(n)
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.SH KEYWORDS
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channel, reflection
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