C：将 function 指针从通用输入 arg (void*) 类型转换为特定 (char*/int *) arg 和/或不同的返回类型，void，int

Question

I have specific data types:

typedef struct _A_s { ... } A_t;
typedef struct _B_s { ... } B_t;

I also have specific functions to operate on these (eg A_t * init_A(); free_A(A_t *); print(A_t *); etc.

A 3rd function takes in any of those data struct types via a void * and also function pointers in order to free and print its input. I want to keep this function generic, hence converting A_t* to void* . But I am not sure how that will work with function pointers.

Here is a rough (sorry:) sketch:

typedef (void )(*free_generic_t)(void *);
typedef (void )(*print_generic_t)(void *);
void myfunc(
  void *object,
  free_generic_t free_fn,
  print_generic_t print_fn
){
  ...
  print_fn(object);
  free_fn(object);
}

My aim is to do this:

A_t *a = init_A(...);
B_t *b = init_B(...);
myfunc((void *)a, free_A, print_A);
myfunc((void *)b, free_B, print_B);

but gcc complains because of -Werror=incompatible-pointer-types , which I want to keep on. Ignoring this warning, the program works fine.

When I did this typecastI got a SIGSEGV it worked fine and passed the valgrind test:

myfunc(
  (void *)a,
  (free_generic_t )free_A,
  (free_generic_t )print_A
);

Question 1: How far can I go in typecasting functions to function pointers in the above scenario? Assume that the number of input parameters is fixed. That is, all print_A() 's take just 1 argument in, which will be a pointer.

Question 2: The problem becomes more complex when some of those specific functions return int and some nothing ( void ). Can I adjust the generic function pointers to return int in order to accommodate both flavours of specific functions even if some of them return void ?

Update : I don't get a SIGSEGV when I typecast the function pointers, it was a mistake of mine. I have edited my post in place.

Answer 1

Give those structs

typedef struct _A_s { ... } A_t;
typedef struct _B_s { ... } B_t;

an pointer to a deallocator function and populate it in the A_t * init_A(); respectively B_t * init_B(); .

Then you may write a generic deallocator for both of them which indirectly calls the deallocator via the stored pointer.

// Anything, which is shared between structures
typedef struct _X_s { void (*deallocator)(void *); ... } X_t;

typedef struct _A_s { X_t x; ... } A_t;
typedef struct _B_s { X_t x; ... } B_t;

Then

void deallocate(X_t *x){
    x -> deallocator(x);
}

int main(void){
    A_t *a = init_A();
    B_t *b = init_B();

    deallocate((X_t *)a);
    deallocate((X_t *)b);
}

Please be aware that this is only a concept of how you can do it. You must work out the details by yourself and add validity checks and such.