What is the meaning of `struct X typedef` vs. `typedef struct X`?

Question

I have the following (working) code in an existing code base, used in include file that is shared between C and C++, compiling on MSVC (2010) and Windows DDK:

struct X {     USHORT x; } typedef X, *PX; 

And:

enum MY_ENUM {     enum_item_1,     enum_item_2  } typedef MY_ENUM; 

As far as I know, correct definition should look like this:

typedef struct {     USHORT x; } X, *PX; 

Is there any purpose for having the form below? Am I missing something?

Answer 1

The fact that both typedef <type> <alias> and <type> typedef <alias> are valid simply comes from the language grammar definition.

typedef is classified as a storage-class specfifier (just like static, auto), and the type itself is known as the type-specifier. From the syntax definitions in section 6.7 of the standard, you'll see that these are free to be interchanged:

declaration:     declaration-specifiers init-declarator-list ;  declaration-specifiers:     storage-class-specifier declaration-specifiers     type-specifier declaration-specifiers     type-qualifier declaration-specifiers     function-specifier declaration-specifiers  init-declarator-list:     init-declarator     init-declarator-list , init-declarator  init-declarator:     declarator     declarator = initializer 

(Note, of course, that this is equally true for structs and for non-structs, meaning that double typedef trouble; is also valid.)

Answer 2

As others said, typedef is a storage-class specifier and as with other storage-class specifiers you are also allowed to put the specifier between the type and the declarator.

While this is valid and it is also a form that should be avoided as C marked it as an obsolescent feature:

(C11, 6.11.5p1) "The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature."