Defect Report #106
Submission Date: 03 Dec 93
Source: Ron Guilmette
ANSI/ISO C Defect report #rfg13:
Subclause 18.104.22.168 says:
The (nonexistent) value of a void expression (an expression
that has type void) shall not be used in any way, ...
There are two separate (but related) problems with this rule.
First, it is not entirely clear what constitutes a ``use'' of
a value (or of an expression). In which lines of the following code
is a type void value actually ``used?''
void example(void *pv, int i)
&*pv; /* ? */
*pv; /* ? */
i ? *pv : *pv; /* ? */
*pv, *pv; /* ? */
(The answer to this question will determine which of the above lines
cause undefined behavior, and which cause well defined behavior.)
If one or more of the (questionable) lines from the above example
are judged by the Committee to result in well defined behavior, then
a second (separate) issue arises. This second issue requires some
Subclause 22.214.171.124 contains the following rules:
An lvalue is an expression (with an object type
or an incomplete type other than void) ...
Note that the final rule (specifying a condition under which undefined
behavior arises) seems, based upon the context, to only apply to those
cases in which ``...an lvalue that does not have an array type
is converted to the value...'' More specifically, it appears that
undefined behavior is not necessarily produced for non-lvalue
expressions (appearing in the indicated contexts).
Except when it is the operand of the sizeof operator,
the unary & operator, the ++ operator, the -- operator,
or the left operand of the . operator or an assignment operator,
an lvalue that does not have array type is converted to the value
stored in the designated object (and is no longer an lvalue)... If
the lvalue has an incomplete type and does not have array type, the
behavior is undefined.
Furthermore, it should be noted that the definition of an lvalue (quoted
above) does not include all void types. Rather, it only includes
the void type.
The result is that the indicated lines in following example would
seem to yield well defined behavior (or at least they will
yield well defined behavior if the Committee decides that their unqualified
counterparts do), however I suspect that this may not have been what
the Committee intended.
void example(const void *pcv, volatile void *pvv, int i)
&*pcv; /* ? */
*pcv; /* ? */
i ? *pcv : *pcv; /* ? */
*pcv, *pcv; /* ? */
&*pvv; /* ? */
*pvv; /* ? */
i ? *pvv : *pvv; /* ? */
*pvv, *pvv; /* ? */
In summary, I would ask that the Committee comment upon and/or clarify
the behavior produced by each of the examples shown herein. Separately,
I would request that the Committee make changes in the existing C
Standard in order to make the rules applicable to such cases more
In the first function called example, the expression
statement &*pv is dealt with in Defect Report #012.
The remaining three statements are well formed. See the last
sentence of the cited reference and also subclause 6.6.3.
In the second function called example, the expression
statements &*pcv and &*pvv are
dealt with in Defect Report #012. The remaining six statements
are well formed. The restrictions given in subclause 6.5.3 apply
to object types, not incomplete types.
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