Compatible types and structures in C

Question

I have the following code:

int main(void) {     struct { int x; } a, b;     struct { int x; } c;     struct { int x; } *p;      b = a;   /* OK */     c = a;   /* Doesn't work */     p = &a;  /* Doesn't work */      return 0; } 

which fails to compile under GCC (3.4.6), with the following error:

test.c:8: error: incompatible types in assignment test.c:9: warning: assignment from incompatible pointer type 

Now, from what I understand (admittedly from the C99 standard), is that a and c should be compatible types, as they fulfill all the criteria in section 6.2.7, paragraph 1. I've tried compiling with std=c99, to no avail.

Presumably my interpretation of the standard is wrong?

Addendum

Incidentally, this question arises because I wanted to declare some template-like macros to wrap various datatypes without the overhead of having to declare named types/typedefs everywhere, e.g. a trivial example:

#define LINKED_LIST(T)   \     struct {             \         T    *pHead;     \         T    *pTail;     \     }  ...  LINKED_LIST(foo) list1; LINKED_LIST(foo) list2;  ...  LINKED_LIST(foo) *pList = &list1;  /* Doesn't work */ 

Answer 1

struct { int x; } is a anonymous structure tag, two anonymous structures cannot have "the same name", which is a necessary condition for type compatibility. You can declare types that are compatible with a non-anonymous structure using typedef.

struct tmp { int x; }; // declare structure tag typedef struct tmp type1; typedef struct tmp type2; // declare 3 types compatible with struct tmp typedef struct tmp type3; // and with each other  type1 a, b; type2 c; type3 *p; b = a; c = a; p = &a; 

Answer 2

Looking at the draft specification I'm guessing you're relying on the conditions that come after the statement:

Moreover, two structure, union, or enumerated types declared in separate translation units are compatible if their tags and members satisfy the following requirements ...

I think that the fact that these are all decared in the same C file means that they are in a single translation unit.

At a guess it would seem that this guarantees that when two C files include a header that declares a type then instances of that type will be compatible.