Chapter 22: Pointers to Pointers:
One side point about pointers to pointers and memory allocation: although the void * type, as returned by malloc, is a "generic pointer," suitable for assigning to or from pointers of any type, the hypothetical type void ** is not a "generic pointer to pointer".
So, only void * is a generic pointer. void ** is not generic pointer. Therefore the argument passed to your function must be of type void **.
Also see C-FAQ:
There is no generic pointer-to-pointer type in C. void * acts as a generic pointer only because conversions (if necessary) are applied automatically when other pointer types are assigned to and from void *'s; these conversions cannot be performed if an attempt is made to indirect upon a void ** value which points at a pointer type other than void *. When you make use of a void ** pointer value (for instance, when you use the * operator to access the void * value to which the void ** points), the compiler has no way of knowing whether that void * value was once converted from some other pointer type. It must assume that it is nothing more than a void *; it cannot perform any implicit conversions.
In other words, any void ** value you play with must be the address of an actual void * value somewhere; casts like (void **)&dp, though they may shut the compiler up, are nonportable (and may not even do what you want; see also question 13.9). If the pointer that the void ** points to is not a void *, and if it has a different size or representation than a void *, then the compiler isn't going to be able to access it correctly.
