JTC1/SC22/WG14 N913

Document Number: WG14 N913
Docuemnt Date:   13-Apr-2000

1.  Title

Object access model for the programming language C.


2.  Scope

To define an unambiguous model for the evaluation status of
a full expression in the C language (as defined by
ISO/IEC 9899:1999).


3.  Purpose and Justification

The C Standard puts certain restrictions on expressions based
on the concept of a "sequence point". Clause 6.5p2:

   Between the previous and next sequence point an  object
   shall  have  its  stored  value modified at most once by the
   evaluation of an expression.  Furthermore, the  prior  value
   shall  be  accessed  only  to  determine  the  value  to  be
   stored.

The problem with this and other wording is that, while its
meaning seems obvious at first glance, it is open to different
interpretations by different people. For example, in (with x
having an integer type):

   (x += 0) + (x *= 1);

neither assignment to x alters the stored value, and some
readers would argued that neither modifies it and the behaviour
is defined; while others would argue that modification can
involve storing an unchanged value back into an object, the
behaviour is undefined in this case.

Optimizers want the freedom to rewrite expressions to generate
efficient code; but the standard constrains the rewriting that
is allowed.  Developers want the expressions they write to
always evaluate to the same value; but the standard does not
always guarantee such an evaluation.

The output from the proposed model is a YES/NO answer to the
question: is the result of the full expression input to the model
unambiguously defined?  The model will take into account the full
state of the abstract machine during the evaluation of the
expression.  The model will follow existing practice.  The intent
is to clarify the current behaviour and help different readers to
arrive at the same result.

It is expected that a number of issues involving the evaluation
of expressions will be found over time.  It is hoped that as
experience with the final, published model grows, it might
eventually be considered for inclusion in a future revision of
the C standard.


4.  Program of Work

A Technical Report Type 2 will be developed for this project.


5.  Relevant documents to be considered

ISO/IEC 9899:1999 -- Programming Languages C

WG14 N908 A formal model of sequence points and related issues


6.  Cooperation and liaison

All ISO/IEC JTC1/SC22 Working groups that have an interest
in having an unambiguous interpretation of C expressions.
In particular ISO/IEC JTC1/SC22 WG21 (C++).


7.  Preparatory work offered with target date(s)

A PDTR document will be ready for registration 24 months
after the approval of the project by JTC1.