Declaring a pointer to a struct, does not allocate storage for the struct and does not initialize the pointer.

typedef struct s1 {
  double x;
  int n;
  char c1;
  char c2;
} S1;

S1 *p;

p = (S1 *) malloc(sizeof(S1));

p->x = 3.5;
p->n = 0;
p->c1 = 'a';
p->c2 = 'b';

/* or you could write */

(*p).x = 3.5;
(*p).n = 0;
(*p).c1 = 'a';
(*p).c2 = 'b';