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zip(n)                                                                  zip(n)



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NAME

       zip - Data compression "zip"


SYNOPSIS

       package require Tcl  ?8.2?

       package require Trf  ?2.1.4?

       zip ?options...? ?data?

_________________________________________________________________


DESCRIPTION

       The  command  zip  is  one of several data compressions provided by the
       package trf. See trf-intro for an overview of the whole package.

       The command is based on the deflate compression algorithm as  specified
       in  RFC  1951 (http://www.rfc-editor.org/rfc/rfc1951.txt) and as imple-
       mented by the  zlib  compression  library  (http://www.gzip.org/zlib/).
       See also RFC 1950 (http://www.rfc-editor.org/rfc/rfc1950.txt)


       zip ?options...? ?data?

              -mode compress|decompress
                     This option has to be present and is always understood by
                     the compression.

                     For immediate mode the argument value specifies the oper-
                     ation  to use.  For an attached compress it specifies the
                     operation to use for writing. Reading will  automatically
                     use  the reverse operation.  See section IMMEDIATE versus
                     ATTACHED for explanations of these two terms.

                     Beyond the argument values listed above all unique abbre-
                     viations are recognized too.

                     Compress causes the compression of arbitrary (most likely
                     binary) data. Decompression does the reverse .

              -level integer
                     Specifies the compression level.  Is  either  the  string
                     default or an integer number in the range 1 (minimal com-
                     pression) to 9 (maximal compression).

              -nowrap boolean
                     If set to true the command will not create the  zip  spe-
                     cific  header  (See RFC 1950) normally written before the
                     compressed data. The options defaults to false.   It  has
                     to  be  used when writing a gzip emulation in Tcl as gzip
                     creates a different header.

              -attach channel
                     The presence/absence of this option determines  the  main
                     operation mode of the transformation.

                     If  present  the  transformation will be stacked onto the
                     channel whose handle was given to the option and  run  in
                     attached  mode. More about this in section IMMEDIATE ver-
                     sus ATTACHED.

                     If the option is absent the  transformation  is  used  in
                     immediate  mode  and  the options listed below are recog-
                     nized.  More  about  this  in  section  IMMEDIATE  versus
                     ATTACHED.

              -in channel
                     This  options  is legal if and only if the transformation
                     is used in immediate mode. It provides the handle of  the
                     channel the data to transform has to be read from.

                     If  the  transformation  is  in  immediate  mode and this
                     option is absent the data to transform is expected as the
                     last argument to the transformation.

              -out channel
                     This  options  is legal if and only if the transformation
                     is used in immediate mode. It provides the handle of  the
                     channel  the  generated  transformation result is written
                     to.

                     If the transformation  is  in  immediate  mode  and  this
                     option  is  absent  the generated data is returned as the
                     result of the command itself.



IMMEDIATE VERSUS ATTACHED

       The transformation distinguishes between two main  ways  of  using  it.
       These are the immediate and attached operation modes.

       For  the  attached  mode  the  option  -attach is used to associate the
       transformation with an existing channel. During the  execution  of  the
       command  no transformation is performed, instead the channel is changed
       in such a way, that from then on all data written to or  read  from  it
       passes  through  the  transformation and is modified by it according to
       the definition above.  This attachment can be revoked by executing  the
       command unstack for the chosen channel. This is the only way to do this
       at the Tcl level.

       In the second mode, which can be detected  by  the  absence  of  option
       -attach, the transformation immediately takes data from either its com-
       mandline or a channel, transforms it, and returns the result either  as
       result  of the command, or writes it into a channel.  The mode is named
       after the immediate nature of its execution.

       Where the data is taken from, and delivered  to,  is  governed  by  the
       presence  and  absence of the options -in and -out.  It should be noted
       that this ability to immediately read from and/or write to a channel is
       an  historic  artifact  which  was introduced at the beginning of Trf's
       life when Tcl version 7.6 was current as this and earlier versions have
       trouble  to  deal with \0 characters embedded into either input or out-
       put.


SEE ALSO

       bz2(n), trf-intro(n), zip(n)


KEYWORDS

       compression, data compression, decompression, rfc 1950, rfc  1951,  rfc
       1952, zip


COPYRIGHT

       Copyright (c) 1996-2003, Andreas Kupries <andreas_kupries@users.sourceforge.net>




Trf transformer commands             2.1.4                              zip(n)

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