Defect Report #012
Submission Date: 10 Dec 92
Source: X3J11/90-046 (David F. Prosser)
Bug in Standard C
I was asked a question about the validity of various expressions.
Among the list there was the following:
void *p; &*p;
After doing a quick pass through the standard, I found nothing that
disallowed such. Moreover, back in September 1987's meeting (I didn't
just recall the date ... it took a while to find when it occurred),
I distinctly remember a Committee discussion that involved the validity
of the expression above. It was as a result of this discussion and
vote that the draft was changed to allow the above.
Anyway, I wrote back that the expression was valid. This was eventually
followed by a letter from Dennis [Ritchie] pointing out the mistake
I made. As it turns out, the definition of lvalue makes at least the
unary & part of the above a constraint violation. (As Bill [Plau
ger] would say, ``I know what the standard was supposed to specify.'')
This would be just another, ``Oops, well I guess I can live with
it'' surprise in the standard, except that it turns out that unary
& of a void type is useful! What it provides is
a construction that gives C a notion of an address symbol. You are familiar with
the symbols that are created by the UNIX linker: etext, e
data, and end, which designate special addresses within the a.o
ut's address space. Of these, the last is most useful (it gives the beginning
of the dynamically allocated data space). However, the type
for these symbols was always pretty fuzzy. But, consider a declaration
of end as
extern void end;
What this gives is a name that only has an address - exactly what
these symbols do, and nothing more. They can only be used in C as
the operand of unary &, and the address must be converted to
something else (say, char *) even to do address calculation,
making the special nature of the symbol clearly evident.
What I'd like is a vote of the interpretations group that notes that
the intent of the Committee was that ``void *p; &*p;''
was supposed to be valid, even though a conforming implementation
must diagnose the expression. This means that I can continue to suggest
the ``extern void'' approach to address symbols in C.
P.S.: The following is my reply to Dennis's mail that pointed out
the error with my original interpretation. The indented parts are
from Dennis's mail.
I don't agree with Dave P's answer about ``void *vp; &*vp;.''
There is not a constraint on *, but the subclause 184.108.40.206 semantics say, ``... if it [the operand of *] points to an object,
the result is an lvalue designating the object.'' Does vp
point to an object? An object is ``a region of data storage ...
the contents of which can represent values'' (clause 3). Dicey
I took some time looking into my records of the Committee's thoughts
on this very issue. Back in '87, based on a proposal by Plauger, the
Committee voted 27 to 3 that ``*(void *)'' was not to
be an error. This was when the unary * constraint was simplified to the current form. Since void is a special instance of an incomplete object type, it can be thought of as pointing at an object
whose size we do not know, but I agree that the argument is strained.
I would still recommend that the compiler not produce a hard error
in this situation.
Moreover, the operand of & must be an lvalue, and *vp
is certainly not an lvalue (subclause 220.127.116.11): ``An lvalue
is an expression (with an object type or an incomplete type other
than void) ...''
Oops. In this case, I completely agree with Dennis: the standard does
say that unary & should not be applied to an expression with
type void since such cannot be an lvalue. Unfortunately, this
means that the standard is ``broken,'' at least according to
the Committee's decisions. One of the major arguments presented as
part of the September 1987 meeting for allowing ``*(void *)''
was that it could then be immediately used as the operand of unary
Therefore, I can state that back in 1987, the Committee's intent was
that the examples you gave were valid Standard C, but that the standard
as written does not allow the second half of the construction for
void! Nevertheless, I'd still suggest allowing the code to succe
compile, with at most a warning.
The relevant citations are subclause 18.104.22.168 (page 43, lines 36-38):
The operand of the unary & operator shall be either a
function designator or an lvalue that designates an object that is
not a bit-field and is not declared with the register
and the one supplied by you from subclause 22.214.171.124 (page 36, lines
An lvalue is an expression (with an object type or an
incomplete type other than void) that designates an object.
Given the following declaration:
the expression &*p is invalid. This is because *p is of type void and so is not an lvalue, as discussed in the quote
from subclause 126.96.36.199 above. Therefore, as discussed in the quote
from subclause 188.8.131.52 above, the operand of the & operator
in the expression &*p is invalid because it is neither a function
designator nor an lvalue.
This is a constraint violation and the translator must issue a diagnostic
The desired effect can be obtained by using the declaration
extern const void end;
(where end denotes an object of unknown size) since const
void type is not void type and thus &end
does not violate the constraint in subclause 184.108.40.206.
Footnote 6 (page 6), which is not part of the standard, provides a
suggestion for implementors who may wish to assign a meaning to the
above expression. It says ``(An implementation) may also successfully
translate an invalid program.'' Therefore, as long as a diagnostic
message is issued, a translator may assign a meaning to the expression
&*p discussed above. Conforming programs shall not use this
Previous Defect Report
< - > Next Defect Report