Defect Report #125
Submission Date: 03 Dec 93
Source: Ron Guilmette
ANSI/ISO C Defect Report #rfg32:
Subject: Using things declared as ``extern
May a conforming implementation fail to correctly
translate a translation unit containing the following declarations?
extern const void etext;
const void *vp = &etext;
Defect Report #012 discusses at length the issue of applying unary
& to an expression whose type is some void type. The conclusion
of that discussion seem to be that although unary & may
not be applied to an expression having the
void type (because
such expressions are not lvalues) it is permissible to apply
unary & to an expression whose type is some qualified version
of void. The text of the interpretation
for Defect Report #012 even goes so far
as to actively recommend the practice of declaring things to be extern
and to have some qualified void type (so that the address may then
The question raised herein is a different one. Tom Pennello has pointed
out the following rule from the second Semantics paragraph of subclause
If an identifier declared with external linkage is used in an
expression (other than as part of the operand of a sizeof operator),
somewhere in the entire program there shall be exactly one external
definition for the identifier; ...
Thus, as Tom has noted, applying unary & to an entity declared
to be both extern
and of some qualified void type is a ``use''
of that entity which would necessarily force you to supply a definition
of that entity, somewhere in the program. But as Tom has further noted,
there is simply no way to accomplish that (in a strictly conforming
program) because of the following rule (given in subclause 6.5):
All declarations ... that refer to the same object or function shall
specify compatible types.
Thus, if you either define or fail to define etext, it would
appear that the behavior is undefined. Is this a correct interpretation?
(Footnote: It would appear that a strictly conforming program may
contain a mere declaration of an extern
entity whose type is any qualified
or unqualified void type, but that any use of such an entity within
an expression, other than within a sizeof expression, renders
the program not strictly conforming.)
Applying & to an identifier of type
has undefined behavior.
Thus an implementation can define any semantics it wishes.
A strictly conforming program cannot contain such a construct.
Previous Defect Report
< - >
Next Defect Report