/* Function type. */
typedef double (func_t) (double);
/* Data type for links in the chain of symbols. */
struct symrec
{
  char *name;  /* name of symbol */
  int type;    /* type of symbol: either VAR or FUN */
  union
  {
    double var;    /* value of a VAR */
    func_t *fun;   /* value of a FUN */
  } value;
  struct symrec *next;  /* link field */
};
typedef struct symrec symrec;

/* The symbol table: a chain of 'struct symrec'. */
extern symrec *sym_table;

symrec *putsym (char const *name, int sym_type);
symrec *getsym (char const *name);
struct init
{
  char const *name;
  func_t *fun;
};
struct init const funs[] =
{
  { "atan", atan },
  { "cos",  cos  },
  { "exp",  exp  },
  { "ln",   log  },
  { "sin",  sin  },
  { "sqrt", sqrt },
  { 0, 0 },
};
/* The symbol table: a chain of 'struct symrec'. */
symrec *sym_table;
/* Put functions in table. */
static void
init_table (void)
{
  for (int i = 0; funs[i].name; i++)
    {
      symrec *ptr = putsym (funs[i].name, FUN);
      ptr->value.fun = funs[i].fun;
    }
}