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Last update: 1997-05-20
9945-2-140
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Topic: awk - reevaluation of field varibles
Relevant Sections: 4.1.2
Defect Report:
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From: mark@mks.com (Mark Funkenhauser)
Date: Fri, 15 Sep 1995 17:28:47 -0400 (EDT)
Dear Standards Board,
I would like to an request official, binding interpretation
from WG15 concerning the following point in ISO/IEC
9945-2:1993 (POSIX.2).
In Section 4.1.2 it says:
The symbol $0 shall refer to the entire record;
setting any other field shall cause the reevaluation of $0.
and
Assigning $0 shall reset the value of all other fields and the NF
built-in variable.
The questions are:
1) how exactly is $0 to be re-evaluated?
Should it be using the current value of OFS?
Note: This is how its historically been documented on page 36
in The AWK Programming Language (1988)
by Aho, Kernighan and Weinberger.
Also, section 4.1.7.3 (line 314-317) implies this behaviour
since this is what is done when an assignment is made
to a non-existent field.
2) (a)When $0 is assigned to, how exactly should the other fields
be reset? (b)What does "reset" mean exactly?
Note: Historical documentation (The AWK Programming Language)
says that "when $0 is changed by assignment or
substitution, $1, $2, etc., and the NF will be recomputed"
(c) Is there a reason why POSIX.2 is not more specific on how
these fields should be reset?
(d) Is it reasonable to assume that the fields shall be computed
from the new $0 using FS?
(e)If so, then should this FS be the current value
(at time $0 was assignment) or should it be the value that
was current when the last record was read?
(See somewhat related 9945-2 interp. request #118 and #80)
Thank you for your attention to this matter.
Mark Funkenhauser
Interpretation response
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Q.1: The standard states on page 163, section 4.1.2 (last paragraph),
that $0 is re-evaluated if any field is set. Page 171, section 4.1.7.3 third
paragraph, indicates that assignments to non-existent
fields cause the value of $0 to be recomputed with the fields
seperated by the value of OFS
Since "recomputed" and "re-evaluated" are neither one clearly defined, then
the standard is unclear in what it means for $0 to be re-evaluated, and
no conformance distinction can be made between alternative implementations
based on this. However, it is clear that the current value of OFS
should be used whenever $0 is re-computed. When $0 is re-evaluated
it is unclear if OFS or FS is used.
Q.2: (a)The standard is unclear on this issue, it states that they
are "reset" but does not define what reset means.
(b)The standard does not specify what "reset" means.
(c)The interpretations committee believes that the authors of the standard did
not realise these ambiguities were present in the specification.
(d)Yes, although the standard is unclear on this issue,
we believe using the value of FS while resetting the values of
fields when $0 is reassigned is consistent with the standard.
(e)The standard does not speak to this issue, and as such no
conformance distinction can be made between alternative implementations
based on this.
These issues are being forwarded to the sponsor.
Rationale
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None.
Forwarded to Interpretations group: Sep 22 1995
Recirculated for 30 day review: Oct 23 1995
Finalised: Nov 29 1995