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adler(n)							      adler(n)

______________________________________________________________________________

NAME
       adler - Message digest "adler"

SYNOPSIS
       package require Tcl  ?8.2?

       package require Trf  ?2.1.4?

       adler ?options...? ?data?

_________________________________________________________________

DESCRIPTION
       The  command  adler  is	one of several message digests provided by the
       package trf. See trf-intro for an overview of the whole package.

       adler ?options...? ?data?
	      The options listed below are understood by  the  digest  if  and
	      only  if the digest is attached to a channel.  See section IMME‐
	      DIATE versus ATTACHED for an explanation of the term attached.

	      -mode absorb|write|transparent
		     This option has to be  present.  The  specified  argument
		     determines the behaviour of the digest in attached mode.

		     Beyond the argument values listed above all unique abbre‐
		     viations are recognized too. Their meaning	 is  explained
		     below:

		     absorb All	 data written to the channel is used to calcu‐
			    late the value of  the  message  digest  and  then
			    passed unchanged to the next level in the stack of
			    transformations for	 the  channel  the  digest  is
			    attached  to.  When the channel is closed the com‐
			    pleted digest  is  written	out  too,  essentially
			    attaching  the  vlaue  of  the  diggest  after the
			    information actually written to the channel.

			    When reading from the  channel  a  value  for  the
			    digest  is	computed too, and after closing of the
			    channel compared to the digest which was attached,
			    i.e.  came	behind	the  actual  data.  The option
			    -matchflag has to be specified so that the	digest
			    knows  where  to store the result of said compari‐
			    son. This result is a string and either  "ok",  or
			    "failed".

		     write  All	 data  read from or written to the channel the
			    digest is attached to is ignored and thrown	 away.
			    Only  a  value  for the digest of the data is com‐
			    puted.  When the channel is	 closed	 the  computed
			    values  are	 stored as ordered through the options
			    -write-destination,	 -write-type,	-read-destina‐
			    tion, and -read-type.

		     transparent
			    This  mode	is  a mixture of both absorb and write
			    modes. As for absorb all data,  read  or  written,
			    passes through the digest unchanged. The generated
			    values for the digest however are handled  in  the
			    same way as for write.

	      -matchflag varname
		     This  option can be used if and only if the option "-mode
		     absorb" is present. In that situation the argument is the
		     name  of a global or namespaced variable. The digest will
		     write the result of comparing two digest values into this
		     variable.	The  option  will be ignored if the channel is
		     write-only, because in that case there will be no compar‐
		     ison of digest values.

	      -write-type variable|channel
		     This  option  can	be  used  for digests in mode write or
		     transparent. Beyond the values  listed  above  all	 their
		     unique abbreviations are also allowed as argument values.
		     The option determines the type of the argument to	option
		     -write-destination. It defaults to variable.

	      -read-type variable|channel
		     Like  option  -write-type,	 but for option -read-destina‐
		     tion.

	      -write-destination data
		     This option can be used for  digests  in  mode  write  or
		     transparent.   The	 value	data  is  either the name of a
		     global (or	 namespaced)  variable	or  the	 handle	 of  a
		     writable  channel,	 dependent  on	the  value  of	option
		     -write-type. The message digest computed for data written
		     to	 the  attached	channel	 is  written into it after the
		     attached channel was closed.  The option  is  ignored  if
		     the channel is read-only.

		     Note  that	 using	a variable may yield incorrect results
		     under tcl 7.6, due to embedded \0's.

	      -read-destination data
		     This option can be used for  digests  in  mode  write  or
		     transparent.   The	 value	data  is  either the name of a
		     global (or	 namespaced)  variable	or  the	 handle	 of  a
		     writable channel, dependent on the value of option -read-
		     type. The message digest computed for data read from  the
		     attached  channel	is  written into it after the attached
		     channel was closed.  The option is ignored if the channel
		     is write-only.

		     Note  that	 using	a variable may yield incorrect results
		     under tcl 7.6, due to embedded \0's.

       The options listed below are always understood by the digest,  attached
       versus immediate does not matter. See section IMMEDIATE versus ATTACHED
       for explanations of these two terms.

	      -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.

NOTES
       This command implements the ADLER digest used by the  zlib  compression
       library (http://www.gzip.org/zlib/).

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
       adler, crc, crc-zlib, haval, md2, md5, md5_otp, ripemd-128, ripemd-160,
       sha, sha1, sha1_otp, trf-intro

KEYWORDS
       adler, authentication, hash, hashing, mac, message digest, zip, zlib

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

Trf transformer commands	     2.1.4			      adler(n)
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