What is the size of an empty struct in C?

Question

According to me, it is zero but there seems to be bit confusion here

I have tested it with gcc compiler and it gives me zero as output. I know that in C++, size of an

Answer 1

A struct cannot be empty in C because the syntax forbids it. Furthermore, there is a semantic constraint that makes behavior undefined if a struct has no named member:

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-list ;

/* type-specifier or qualifier required here! */
specifier-qualifier-list:
  type-specifier specifier-qualifier-listopt
  type-qualifier specifier-qualifier-listopt

struct-declarator-list:
  struct-declarator
  struct-declarator-list , struct-declarator

struct-declarator:
  declarator
  declaratoropt : constant-expression

If you write

struct identifier { };

It will give you a diagnostic message, because you violate syntactic rules. If you write

struct identifier { int : 0; };

Then you have a non-empty struct with no named members, thus making behavior undefined, and not requiring a diagnostic:

If the struct-declaration-list contains no named members, the behavior is undefined.

Notice that the following is disallowed because a flexible array member cannot be the first member:

struct identifier { type ident[]; };

Answer 2

In C99: "If the struct-declaration-list contains no named members, the behavior is undefined."

The syntax doesn't really allow it anyway, though I don't see anything that says a diagnostic is required, which puts it pretty much back in the "undefined behavior" camp.

Answer 3

on VC 8 It gives error if we try to get the sizeof empty struct, on the other way round on linux with gcc it gives size 1 because it uses gcc extention instead of c language specification which says this is undefined behaviour.

struct node
{
// empty struct.
};

int main()
{
printf("%d", sizeof(struct node));
return 0;
}

on windows vc 2005 It gives compilation error on linux with gcc it gives size 1 because gcc extension http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Empty-Structures.html#Empty-Structures (As Pointed out by Michael Burr)

Answer 4

The C grammar doesn't allow the contents of a struct to be empty - there has to be at least an unnamed bitfield or a named member (as far as the grammar is concerned - I'm not sure if a struct that contains only an unnamed bitfield is otherwise valid).

Support for empty structs in C are an extension in GCC.

In C++ and empty struct/class member-specification is explicitly permitted, but the size is defined to be 1 - unless as part of the empty base optimization the compiler is allowed to make an empty base class take no space in the derived class.