rfc242.txt

RFC 的详细文档！

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not necessarily sequential.

    Lock - During the course of processing data, it may be necessary to
lock out use of some portion of data to other users. Seqmentation of a
data set into units for locking purposes may be indicated by the
notation:

          k|[extent]|.

Whether or not the data is locked and the type of lock applied (for
example, write protect or read/write protect) is specified at the time
the data is used.

    Authorization - Authorization for a user to access data may be
governed by some access code assigned to the data.  This access code can
be specified in the description by modifying the desired elements of the
description with an indication of the code. The notation is:

          code a |[extent ]|

Control.

    Two modifiers are provided which govern the existence of items in
the definition. The first is the repetition modifier:

          factor r |[extent ]|.

This causes the following item definition or item definitions (if the
extent indicates more than one) to be repeated. Thus the description

          (3rC)

is equivalent to the description

          (C,C,C).

The other control modifier is the condition modifier:

          condition c |[extent ]|.

If the condition specified is not true, then the following item
definition is ignored. The condition is specified by a Boolean
expression.

    Since several modifiers may apply to an item definition, there is a
problem concerning the relationship among them. For example, if a
repetition modifier and a conditional modifier apply to an item, does
the condition apply to all the repeated items, or only to the first,



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assuming the extent of the condition modifier is one? The effect of
multiple modifiers is dependent on the order in which they are
evaluated. Two possible conventions come to mind. One says that
repetitions are expanded first, then properties applied, and finally
conditions applied to the resulting expanded item definitions,
independent of the order in which the modifiers were specified in the
description. Thus the description

          (A=3c [ 4 ]4rF,I)

is equivalent to

          (4rA=3c[ 4 ]F,T),

and if the condition is true, is equivalent to

          (F,F,F,F,I),

or, if the condition is not true, is equivalent to

          (T).

The other convention is that the modifiers are evaluated in the order in
which they appear in the description, perhaps.  in reverse order - the
modifier immediately preceding an item definition is evaluated first,
then the one next preceding, etc. This gives more flexibility of meaning
to the mulitple modifiers. For example, the descriptions

          (A=3c3rC)

and

          (3rA=3cC)

are not equivalent.  In the first, only the first of the three
repetitions is affected by the condition whereas in the second, ll three
repetitions are affected. Since this second convention is more flexible,
it shall be the one assumed. This convention allows, for example, the
repetition modifier to the applied to a named item as shown:

          (3rAnC).

The name A applies to the three items (in effect, the name A is applied
three times). This facility allows a name to be applied to a vertical
column in a two dimensioned array by, for example, the description:

          (3r[ 3 ]C,AnC,C)




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which given the name A to the second column of the 3x3 array.


Reference

    Named descriptions, or parts of descriptions, that have already been
defined may be inserted into a description using the notation:

          $ specification.

Is a description, the reference is used as an item definition of a
string fo item definitions. The item definitions used are those defined
by the name given. Names that apply to the named item or items as a
whole in the description in which it is defined are ignored by the
description at which is referred. However, names that apply to parts of
the named item are carried over to the description at which it is
referred. For example the description

          (An(F,F),I,$A)

is equivalent to

          (An(F,F),T,(F,F)).

Notice that the name "A" was not carried over to where the description
was referenced since it applied to the referred-to item or items as a
whole.

    Parts of a data set or description must be able to be specified for
use in a reference. This specification is in terms of the structure of
the data set or description. The specification has the form of a data
set name, or description name, followed by modifiers which particularize
to specifications, to the part desired. The four types of modifiers are
for going down a level, going up a level, going frontwards along a
level, and going backwards.

    Down - To go down a level from that previously specified, the
modifier has the following form:

         . item

or

         . (item |,extent| |,=value|).


Having gone down a level, the item indicates which particular item at
this level is the first (or only one) desired. This may be a number or a



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name. If more than one are desired, then the extent indicates how many
items. (* as extent means all remaining items at that level, !  means
the first item that meets the conditions that may he get on it or
subitems in following modifiers.) The items selected may he conditioned
by their contents. If a value is given, then only those items with the
value indicated are selected. For example,

          A.1.1

specifies the first field of the first record of data set A,

          A.(1*).1

specified the first field of all the records of A,

          A.1.(1,2)

specifies the first two fields of the first record of data

set A,

          A.(1,*).(1,="768174")

specified only the first fields of all the records of A that

have value "768174", and

          A.(1,!)-(1,="768174")

finds the first field that has value "768174".

    Up- To go up a level from that previously specified,

the modifier has the following form:

          ' item

or

          ' (item |,extent| |,=value|).

Going up a level specifies the item up one level that contains the item
previously specified. The item indicates which particular item at this
level is desired where the containing item is considered the first. For
example,

          A.(1,!).(1,=768174")'1




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specifies the first record whose first field has value "768174".

    Forward. - To go forward on the same level as that previously
specified, the modifier is as follows:

          + item

or

          + (item |,extent| |,=value|).


This modifier is useful when an item following the one which has a
certain value in a field is desired. It may also be used when the data
set name is really a pointer, into the data set, which has beet set
previously. Pointers may or may net be described in a section elsewhere.
Backward. - To go backward on the same level as that previously
specified, the modifier has the following form:

          - item

or

          - (item |,extent| |,=value|).

An example of the use of this modifier is when an item preceding the one
which has a certain value in a field is desired. This might be
specified:

          A.(1,!).(2,="768174")'1-1




       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives Gottfried Janik   9/97   ]















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