Using and dereferencing (void**)

Question

I would like to pass a "polymorphic" array of pointers to a function.

I can do the following without warnings:

foo (void* ptr);

bar()
{
  int* x;
  ...
  foo(x);
}

gcc apparently automatically casts x to a (void*), which is just dandy.

However, I get a warning when I do the following:

foo (void** ptr);

bar()
{
  int** x; // an array of pointers to int arrays
  ...
  foo(x);
}

note: expected ‘void **’ but argument is of type ‘int **’
warning: passing argument 1 of ‘foo’ from incompatible pointer type [enabled by default]

My question is: why is passing an (int*) as a (void*) argument not 'incompatible', but (int**) as a (void**) argument is?

Since all pointer types are the same size (right? it's been a while since I've used C), I can still do something like:

void mainFunc1(int** arr, int len)
{
    //goal is to apply baz to every int array
    foo(baz, arr, len);
}

void mainFunc2(double** arr, int len)
{
    //goal is to apply baz to every int array
    foo(qux, arr, len);
}

// I PROMISE that if I pass in a (int**) as ptr, then funcPtr will interpret its (void*) argument as an (int*)
void foo(funcPtr f, void** ptr, int len)
{
    for(int i = 0; i < len; i++)
    {
        f(ptr[i]);
    }
}

void baz(void* x) 
{
  int* y = (int*)x;
  ...
}

void qux(void* x)
{
  double* y = (double*)x;
  ...
}

The purpose for all the void pointers is so that I can use a function pointer applied to functions that will (down the stack) have different types of ptr arguments: some will take int arrays, some will take double arrays, etc.

Dairymaid

See this C FAQ – Genisia

WHy on earth would you want to use a void* you don't have to? – Ejective

I have a generic function (foo, above) that I'm trying to avoid duplicating, as it is somewhat more complicated than foo. It should take a list of lists and a function pointer that will be applied to each of the lists. mainFunc is just one example. – Dairymaid

Why don't templates work? Oh, nevermind, I see you didn't tag this as C++. – Ejective

Mostly because I've never used c++ before, there's no reason I couldn't, if that would make the job significantly easier. – Dairymaid

Read also "Swapping 2 string pointers with a function whose parameters are void **" very interesting answer written by H2CO3 – Carmencita

all pointer types are the same size (right? it's been a while since I've used C),, not always, There have been architectures where pointers to different types have different sizes: Read "Are there are any platforms where pointers to different types have different sizes?" – Carmencita

Also, as has been explained many times on SO, even if all pointer variables were the same size, that would be a red herring. The most fundamental reason why you have different types for, say, int *, double * is that the pointer cannot be correctly dereferenced without that type information. Same goes for pointers with multiple levels of indirection. If void ** could take anything, then you could assign a long double * to it and get really weird results when you tried to dereference it. – Prosecute

Answer 1

Note: void* is generic. but void** is not. You can assign address of any type to void* variable but void** can be assigned address of void* variable only.

void* generic;
int i;
int *ptri = &i;

generic = ptri;

or

char c;
int *ptrc = &c;

generic = ptrc;

valid but following is an error:

void**  not_generic;
int i;
int *ptri = &i;
int **ptr_to_ptr1 = &ptri;
void**  not_generic = ptr_to_ptr1;

Error: assigning int** to void**.

Yes you can do like:

void**  not_generic;
not_generic = &generic;

For generic array function simply use void* a as follows:

enum {INT, CHAR, FLOAT};
void print_array(void* a, int length, int type){
   int i = 0;
   for(i = 0; i < length; i++){
      switch(type){
         case INT: 
              printf("%d", *((int*)a + i));
              break;
         case CHAR: 
              printf("%c", *((char*)a + i));
              break;
         case FLOAT: 
              printf("%f", *((float*)a + i));
              break;
      }
   }
}

You better write this function using macros.

Call this function as:

Suppose int:

 int a[] = {1, 2, 3, 4}; 
 print_array(a, sizeof(a)/sizeof(a[0]), INT);

Suppose char:

 char a[] = {'1', '2', '3', '4'}; 
 print_array(a, sizeof(a)/sizeof(a[0]), CHAR);

Carmencita

Answer 2

Because there is no generic pointer-to-pointer type in C.

Reference: C FAQ Question 4.9

Gallagher

Answer 3

The short answer is: anything that resembles

<datatype>* <variable_name>

can be passed in where the parameter declaration is of type void* because void* is generic. However, void** is not. So automatic casting fails.

Loverly