typedef, structure and type compatibiliy

Question

If I have these two structs:

struct
{
   int x;
} A;

struct
{
   int x;
} B;

then making A = B; results in a compilation error because the two anonymous structs are not compatible.

However if I do:

typedef struct
{
   int x;
} S;

S A;

S B;

A = B; is a legal assignment because they are compatible.

But why? With typedef I understand that the compiler makes this when meet S A and S B:

struct { int x; } A;
struct { int x; } B;

so A and B should not be compatible...

Answer 1

Each anonymous struct declaration is a distinct type; this is why you get a type mismatch when trying to assign one to the other.

A typedef, however, declares an alias (i.e. a new name for something that already exists) for a type (it does not create a new type).

A typedef is also not a simple text replacement, like a preprocessor macro. Your statement

I understand that the compiler make this when meet S A and S B:

struct { int x; } A;
struct { int x; } B;

is where your understanding is wrong.

When you use the type alias S, as in

S A;
S B;

the types of both objects A and B are the same by definition and assigning one to the other is possible.

Answer 2

This is because C treats every untagged struct as a new kind of struct, regardless of the memory layout. However, typedef struct { } name; cannot be used if you want to use the struct in a linked list. You'll need to stick with defining a structure tag in this case, and typedef the tagged struct instead.