The way transforms are handling seek requests was completely
revamped in version 2.0. This page now seeks to explain the new
behaviour to anyone trying to port an application using old-style
A short recapitulation of how it was before 2.0: Any seek
request was passed unchanged to the channel (or transformation)
below it. Therefore seeking was always relative to the notion of
the base channel about its access point.
Before actually stating the possible new behaviours of a
transformation with regard to a seek request some notes on
how it is selected.
The code now places each transformation in one of two groups:
- Either input and out are in a linear relationship,
- or they are not.
The name of attribute used to describe this is the 'seek policy'.
In the case of members of the second group its value is 'unseekable'.
For all other transformations it is a list of the two numbers
describing the ratio between input and output.
Examples from the first group are almost all conversions provided
by Trf, like 'hex', 'base64', etc. 'hex' for example converts every
byte into 2 ASCII characters during encoding, thereas 'base64' encodes
every triple of bytes into 4 ASCII characters.
Members of the other group are the (de)compression algorithms and
the 'ascii85' conversion.
What I described above is actually the 'natural seek policy' of
the transformation. The moment some transformations are stacked upon
each other the new code will consider the properties of the base
channel and the policies of all the transformations below it while
selected a 'chosen seek policy'. Under most circumstances this is
the 'used seek policy', but sometimes the user is allowed to overide
the choice with one of his own, so a difference is possible.
The rules for the selection of the 'chosen seek policy' are thus:
- An 'unseekable' base channel (= no SeekProcedure in his
driver, examples: socket, pipe) implies an 'unseekable'
transformation, regardless of the natural policy. The user
cannot overide this choice.
- An 'unseekable' transformation below (used! policy) implies
an 'unseekable' transformation, regardless of the natural policy.
The user cannot overide this choice.
- The natural policy is chosen. The user is allowed to overide
The order of the rules above implies the order of their application
and dominance. I.e. the first applicable rule determines the chosen
policy, the rules behind it are not considered anymore and have no
Remark: Whereas Trf is able to peek into the structures of
the transformations provided by itself it has no way of doing so with
transformations coming from different packages. Because of this it
goes the safe and easy way, i.e such transformations are considered as
'unseekable'. Even if they are not, Trf is unable to ascertain this.
In the case of an overidable chosen policy the option
-seekpolicy can be used during attachment of a
transformation, or later via 'fconfigure' to enforce the choice of the
user. The available values and their meanings are:
- The transformation is forced to use 'unseekable'
as its seek policy.
- The transformation is forced to be seekable, with
a ratio of 1:1. This effectively restores the old behaviour of
transformations. Beware that this is possible only if
neither the base channel nor all transformations below are
- This value restores the chosen seek policy as the
Now we can talk about the behaviour of a transformation under the
various seek policies.
- An unseekable transformation keeps a running count of the
number of bytes written to or read from it. This number is
returned upon request, i.e. a call to 'tell'.
Seekable with ratio n:m
- n:m means that n bytes in
the input are transformed into m bytes of output (for
writing to the channel).
- The transformation channel is restricted to seeking to
locations which are multiples of n. Reading and writing
is not restricted in this way.
- Seeking relative to the end of the stream is not permitted.
(I currently don't quite know how to deal with incomplete data
at the end of the channel downstream. Some transformations are
able to handle such, some are not; and the effects on
where the end of the stream actually is or should be
are not clear).
- If a non-multiple of n number of bytes is
written into the channel the incomplete tuple is stored in
the transformation for the next write. A read or seek will
discard this partial information.
- Each seek discards the information in the buffers of
the transformation, both the read-ahead buffer of complete
tuples and internal buffers containing partial information.
Actually a little optimization is made. The read-ahead
buffer is not discarded if the new location of the access
point is still inside its boundaries.
- Seeking a channel X and then stacking a transformation
upon it will cause the transformation to use the current
location in X as its base location, i.e. 0. Seeking behind
that point is not allowed.
- Forcing a transformation into 'identity', seeking it
and then restoring the chosen policy will cause the
transformation to use the new current location in the
channel downstream as its new base location.
- Seeking an unseekable transformation by forcing it into
identity will discard any state it has.
Two read-only options to 'fconfigure' are available to get at the
current configuration and state of the seek system for a particular
- The value is a list containing keywords and values
(notation suitable for 'array set') describing the current
configuration of the seek (natural and chosen policy, ability to
- The value is a list containing keywords and values
(notation suitable for 'array set') describing the current state of
the seek system (used policy, various locations).
The value is a list containing two integer numbers
describing the natural ratio of input to output for the
transformation. If either number is 0 the transformation
The value is a list as described above, but this time
containing the chosen ratio, after considering the base
channel and all transformations below.
A boolean value. Set if the user is allowed to overide
'ratioChosen' with his own preferences.
A boolean value. Set if the user forced the transformation
via -seekpolicy to use 'identity' while doing seeks.
See 'ratioNatural' and 'ratioChosen above. Describes the
ratio actually used.
A boolean value, derived from 'ratio'. Set if the
transformation is 'unseekable'.
An integer number. The location of the access point in the
stream before/above the transformation, as the transformation
sees it. This value can differ from the one returned by
[tell], the difference is the number of bytes buffered in
the generic I/O layer of Tcl for this channel.
See above, describing the range in the transformed stream
covered by the read-buffer of the transformation maintained
by Trf itself.
The location in the channel downstream which is equivalent
to 'up == 0'. See the description some paragraphs above about
a 'base location'.
The current location of the access point in the channel
downstream. Not equivalent to 'up' due to the
read-ahead done by Trf, but to 'upBufEnd'. Actually not quite
excatly equivalent as Trf may have read a non-multiple of
m number of bytes.
'down - downAhead' is the real location in downstream
equivalent to 'upBufEnd'. It basically is the number of
bytes in the last and incomplete tuple read from below.
A boolean value. Set if the transformation was forced into
identity (see above) and the user actually seeked after