Question

Possible Duplicate:
Purpose of struct, typedef struct, in C++
typedef struct vs struct definitions

I am aware that there are two ways of declaring structs in C

struct point{
    int x, y;
};

and:

typedef struct{
    int x, y;
} point;

but what is the difference between these two methods and when would I use the typedef method and not the other ?

Answer 1

There is only one way to declare a struct in C, using the struct keyword, optionally followed by the name of the structure, then followed by the member fields list in braces. So you could have:

  struct point_st {
     int x, y;
  };

Here point_st is the name (or tag) of your structure. Notice that structure names have a different namespace in C than types (this is different in C++). So I have the habit of suffixing structure names with _st as shown above.

You can (independently) define a type name using typedef, e.g.

  typedef struct point_st Point;

(you can use typedef on any C type, BTW).

For exemple Gtk and Glib has a lot of opaque types which are such opaque structures; only the implementation knows and cares about the structure members.

Of course the compiler needs to know the field of a structure to allocate it; but if you only use pointers to an opaque structure, you don’t need to declare the structure (i.e. its fields in braces).