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diff --git a/doc/transchan.n b/doc/transchan.n new file mode 100644 index 0000000..e308e13 --- /dev/null +++ b/doc/transchan.n @@ -0,0 +1,160 @@ +'\" +'\" Copyright (c) 2008 Donal K. Fellows +'\" +'\" See the file "license.terms" for information on usage and redistribution +'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. +'\" +.so man.macros +.TH transchan n 8.6 Tcl "Tcl Built-In Commands" +.BS +'\" Note: do not modify the .SH NAME line immediately below! +.SH NAME +transchan \- command handler API of channel transforms +.SH SYNOPSIS +\fBcmdPrefix \fIoption\fR ?\fIarg arg ...\fR? +.BE +.SH DESCRIPTION +.PP +The Tcl-level handler for a channel transformation has to be a command with +subcommands (termed an \fIensemble\fR despite not implying that it must be +created with \fBnamespace ensemble create\fR; this mechanism is not tied to +\fBnamespace ensemble\fR in any way). Note that \fIcmdPrefix\fR is whatever +was specified in the call to \fBchan push\fR, and may consist of multiple +arguments; this will be expanded to multiple words in place of the prefix. +.PP +Of all the possible subcommands, the handler \fImust\fR support +\fBinitialize\fR and \fBfinalize\fR. Transformations for writable channels +must also support \fBwrite\fR, and transformations for readable channels must +also support \fBread\fR. +.PP +Note that in the descriptions below \fIcmdPrefix\fR may be more than one word, +and \fIhandle\fR is the value returned by the \fBchan push\fR call used to +create the transformation. +.SS "GENERIC SUBCOMMANDS" +.PP +The following subcommands are relevant to all types of channel. +.TP +\fIcmdPrefix \fBclear \fIhandle\fR +. +This optional subcommand is called to signify to the transformation that any +data stored in internal buffers (either incoming or outgoing) must be +cleared. It is called when a \fBchan seek\fR is performed on the channel being +transformed. +.TP +\fIcmdPrefix \fBfinalize \fIhandle\fR +. +This mandatory subcommand is called last for the given \fIhandle\fR, and then +never again, and it exists to allow for cleaning up any Tcl-level data +structures associated with the transformation. \fIWarning!\fR Any errors +thrown by this subcommand will be ignored. It is not guaranteed to be called +if the interpreter is deleted. +.TP +\fIcmdPrefix \fBinitialize \fIhandle mode\fR +. +This mandatory subcommand is called first, and then never again (for the given +\fIhandle\fR). Its responsibility is to initialize all parts of the +transformation at the Tcl level. The \fImode\fR is a list containing any of +\fBread \fRand \fBwrite\fR. +.RS +.TP +\fBwrite\fR +. +implies that the channel is writable. +.TP +\fBread\fR +. +implies that the channel is readable. +.PP +The return value of the subcommand should be a list containing the names of +all subcommands supported by this handler. Any error thrown by the subcommand +will prevent the creation of the transformation. The thrown error will appear +as error thrown by \fBchan push\fR. +.RE +.SS "READ-RELATED SUBCOMMANDS" +.PP +These subcommands are used for handling transformations applied to readable +channels; though strictly \fBread \fRis optional, it must be supported if any +of the others is or the channel will be made non-readable. +.TP +\fIcmdPrefix \fBdrain \fIhandle\fR +. +This optional subcommand is called whenever data in the transformation input +(i.e. read) buffer has to be forced upward, i.e. towards the user or script. +The result returned by the method is taken as the \fIbinary\fR data to push +upward to the level above this transformation (the reader or a higher-level +transformation). +.RS +.PP +In other words, when this method is called the transformation cannot defer the +actual transformation operation anymore and has to transform all data waiting +in its internal read buffers and return the result of that action. +.RE +.TP +\fIcmdPrefix \fBlimit? \fIhandle\fR +. +This optional subcommand is called to allow the Tcl I/O engine to determine +how far ahead it should read. If present, it should return an integer number +greater than zero which indicates how many bytes ahead should be read, or an +integer less than zero to indicate that the I/O engine may read as far ahead +as it likes. +.TP +\fIcmdPrefix \fBread \fIhandle buffer\fR +. +This subcommand, which must be present if the transformation is to work with +readable channels, is called whenever the base channel, or a transformation +below this transformation, pushes data upward. The \fIbuffer\fR contains the +binary data which has been given to us from below. It is the responsibility of +this subcommand to actually transform the data. The result returned by the +subcommand is taken as the binary data to push further upward to the +transformation above this transformation. This can also be the user or script +that originally read from the channel. +.RS +.PP +Note that the result is allowed to be empty, or even less than the data we +received; the transformation is not required to transform everything given to +it right now. It is allowed to store incoming data in internal buffers and to +defer the actual transformation until it has more data. +.RE +.SS "WRITE-RELATED SUBCOMMANDS" +.PP +These subcommands are used for handling transformations applied to writable +channels; though strictly \fBwrite\fR is optional, it must be supported if any +of the others is or the channel will be made non-writable. +.TP +\fIcmdPrefix \fBflush \fIhandle\fR +. +This optional subcommand is called whenever data in the transformation 'write' +buffer has to be forced downward, i.e. towards the base channel. The result +returned by the subcommand is taken as the binary data to write to the +transformation below the current transformation. This can be the base channel +as well. +.RS +.PP +In other words, when this subcommand is called the transformation cannot defer +the actual transformation operation anymore and has to transform all data +waiting in its internal write buffers and return the result of that action. +.RE +.TP +\fIcmdPrefix \fBwrite \fIhandle buffer\fR +. +This subcommand, which must be present if the transformation is to work with +writable channels, is called whenever the user, or a transformation above this +transformation, writes data downward. The \fIbuffer\fR contains the binary +data which has been written to us. It is the responsibility of this subcommand +to actually transform the data. +.RS +.PP +The result returned by the subcommand is taken as the binary data to write to +the transformation below this transformation. This can be the base channel as +well. Note that the result is allowed to be empty, or less than the data we +got; the transformation is not required to transform everything which was +written to it right now. It is allowed to store this data in internal buffers +and to defer the actual transformation until it has more data. +.RE +.SH "SEE ALSO" +chan(n), refchan(n) +.SH KEYWORDS +API, channel, ensemble, prefix, transformation +'\" Local Variables: +'\" mode: nroff +'\" End: |