Are extra semicolons allowed inside struct declarations? - c

Question

This question is by mere curiosity. It is not about the empty struct.
I just stumbled over an interesting typo of the kind
struct {
int member1; /*comment*/ ; /* <-- note the ';' */
int member2;
} variable[] = { /* initializers */ };
which the compiler (xc32, derived from gcc) accepted without any
complaints. Of course, I corrected this but the software was running
smoothly before and after the correction and the additional ; seemingly
causes no problems. I then tried various lengthes of ;;; in he struct definition an they seem to make no difference neither to functionality
nor to sizeof. So in a struct any sequence of ;;;; seems to be
equivalent to a single ;.
I couldn't find anything about such "empty members" of a struct/union
in the specification, neither that they are allowed nor that they are
disallowed. To me it seems as if the grammar rejected them. This contrasts
to "empty declarations" ; at the top level of a compilation unit, which the
standard clearly forbids and the "null" statement ; in functions which is a
clearly allowed language feature.
Does anyone know about this behaviour? Is it compiler specific or does the
C specification somehow tolerate such empty struct members?

Answer 1

The syntax is specified in C11 6.7.2.1
struct-declaration:
specifier-qualifier-list struct-declarator-listopt ;
static_assert-declaration
There is 1 semicolon at the end, so that's the only allowed syntax. You cannot skip the semicolon, you can't add extra ones. And that's that.
(You can however have a static assert inside a struct declaration, from C11.)

Answer 2

The standard doesn't talk about that, it's just a gcc tolerance. See 6.7.2.1:
struct-or-union-specifier:
struct-or-union identifieropt { struct-declaration-list }
struct-or-union identifier
struct-or-union:
struct
union
struct-declaration-list:
struct-declaration
struct-declaration-list struct-declaration
struct-declaration:
specifier-qualifier-list struct-declarator-listopt ;
static_assert-declaration
specifier-qualifier-list:
type-specifier specifier-qualifier-listopt
type-qualifier specifier-qualifier-listopt
(type-specifier and type-qualifier can't be empty, see the related sections in the standard for details.)
Some compilers, like gcc, tolerate extra semi-colons, but -Wpedantic option reveals that it's only a tolerance:
struct foo {
int a;
;;;
};
int main() {
;;;
}
With -pedantic option gcc complains, not on the main empty statements, but on the extra semicolons of the structure declaration.
<source>:3:5: warning: extra semicolon in struct or union specified [-Wpedantic]
;;;
^
<source>:3:6: warning: extra semicolon in struct or union specified [-Wpedantic]
;;;
^
<source>:3:7: warning: extra semicolon in struct or union specified [-Wpedantic]
;;;
Other compilers may not be that friendly, so the typo must be fixed, since it doesn't bring anything useful.