Compilation warning for void ** and void *

Question

I have a question regarding void* and void** and I know this is sort of an old question and has been asked (somewhat) before in stackoverflow. So the question is the following:

When I compile this code with gcc 4.4.3 under ubuntu 10.10, I get the following warning:

zz.c: In function ‘main’:
zz.c:21: warning: passing argument 1 of ‘bar’ from incompatible pointer type
zz.c:9: note: expected ‘void **’ but argument is of type ‘float **’

why is it that its ok to pass variable x as an argument of foo() but its not ok to pass variable y as an argument of bar(). I can fix this by explicitly casting both variables into void* and void** as expected.

void foo (void* a){
}

void bar(void **a){
     *a = (float *) malloc(100*sizeof(float));
}

int main (){

    float *x = (float*) malloc(100*sizeof(float));
    foo(x);
    free(x);

    float *y;
    bar(&y);
    free(y);

    return 0;
}

Answer 1

void *a means that a points to an object of unknown type. However, a itself is not an unknown type because a is known to have type void*. Only the object that a points to has an unknown type.

void **a means that a points to an object of type void*. The object that *a points to has an unknown type, but the object *a itself is a pointer with type void*.

&y is a pointer to an object of type float*. &y is not a pointer to an object of type void*.