What's the difference between `typedef struct X { }` and `typedef struct { } X`?

Question

What's the difference between these two declarations in C:

typedef struct square{

   //Some fields

};

and

typedef struct{  

           //Some fields

} square;

Answer 1

The first declaration:

typedef struct square {
    // Some fields
};

defines a type named struct square. The typedef keyword is redundant (thanks to HolyBlackCat for pointing that out). It's equivalent to:

struct square {
   //Some fields
};

(The fact that you can use the typedef keyword in a declaration without defining a type name is a glitch in C's syntax.)

This first declaration probably should have been:

typedef struct square {
    // Some fields
} square;

The second declaration:

typedef struct {
    // Some fields
} square;

defines an anonymous struct type and then gives it the alias square.

Remember that typedef by itself doesn't define a new type, only a new name for an existing type. In this case the typedef and the (anonymous) struct definition happen to be combined into a single declaration.