void *vp;  double *dp;  int *ip;

  vp = dp;  dp = vp; // Ok in C (in C++ requires a cast)

  vp = ip;  ip = vp;

  dp = ip; // warning, type mismatch

  dp = (double *) ip; // no warning, but probably unsafe to use dp


typedef double (*Func)(double);

typedef int (*Gfunc)(void);

  Func f;   // same as:  double (*f)(double);

  Gfunc g;  // same as:  int (*g)(void);

  vp = f; // NO, can not store function pointer in void*

  vp = (void *) f; // NO, can not use cast with function pointer

  g = f; // NO, type mismatch

  g = (Gfunc) f; // no warning, but unsafe to use g

---

C99 standard:

6.3.2.3 Pointers

 A pointer to a function of one type may be converted to a pointer to a
 function of another type and back again; the result shall compare equal to
 the original pointer. If a converted pointer is used to call a function whose
 type is not compatible with the pointed-to type, the behavior is undefined.

---

6.5.4 Cast operators - nothing specifically about functions

---

J.5 Common extensions

 The following extensions are widely used in many systems, but are not portable to all
 implementations. ...

J.5.7 Function pointer casts

 A pointer to an object or to void may be cast to a pointer to a function,
 allowing data to be invoked as a function (6.5.4).

 A pointer to a function may be cast to a pointer to an object or to void,
 allowing a function to be inspected or modified (for example, by a debugger) (6.5.4).