struct pMG;
typedef struct pMG pMG;
struct pMG {
   ....
};

##</code><code>##

struct pMG {
    ....
};
typedef struct pMG pMG;

##</code><code>##

typedef struct pMG {
    ....
} pMG;

##</code><code>##

typedef struct {
    ....
} pMG;