结构A;是一个forward declaration或不完整的声明.

struct A
{
   int a;
   char b;
   float c;
};

是完整的结构声明.

Example

另请查看comp.lang.c FAQ list Question 11.5

在向前声明struct之后,您可以使用结构指针,但不能取消引用指针或使用sizeof运算符或创建结构的实例.

声明后,您还可以使用struct对象,应用sizeof运算符等.

来自6.7.2.1 C11规范中的结构和联合说明符

8 The type is incomplete until immediately after the } that terminates
the list,and complete thereafter.

并从6.7.2.3标签

If a type specifier of the form
struct-or-union identifier occurs
other than as part of one of the above forms,and no other declaration
of the identifier as a tag is visible,then it declares an incomplete
structure or union type,and declares the identifier as the tag of
that type.¹³¹)
¹³¹A similar construction with enum does not exist

这不应该与extern struct A aa相混淆; v / s struct A aa = {/ *有些值* /};这是对象aa的声明和定义.