Argument conversion: (normal) pointer to void pointer, cast needed?

Question

When assigning to or from a void-pointer no cast is needed (C99 §6.3.2.2 sub 1 / §6.5.16.1 sub 1). Is this also true when passing a (for example int-)pointer to a function that expects a void-pointer?

For example:

void foo(void * p){
    // Do something
}

int main(){
    int i = 12;
    foo(&i); // int * to void *: no cast needed?
}

When I compile this with GCC (4.8.1, MinGW-32) I get neither errors nor warnings (with -Wall & -pedantic).

In contrast in this answer it is suggested that a cast is needed for this call (to eliminate -Wformat warnings):

int main(){
    int i = 12;
    printf("%p\n", &i);
}

But in my case GCC doesn't complain.

So: are casts needed when passing a non-void-pointer to a function that expects a void-pointer?

Answer 1

The difference is printf is a variadic function and variadic functions follow different conversion rules on their trailing arguments.

foo(&i); 

no cast is needed here as foo is a prototyped function. C says &i is converted to the type of p as if by assignment and in C there is an implicit between all object pointer types to void *.

The case with printf is different as variadic functions like printf have default argument promotions on their remaining arguments and no conversion occur on the argument of pointer types.

C on prototyped functions:

(C99, 6.5.2.2p7) "If the expression that denotes the called function has a type that does include a prototype, the arguments are implicitly converted, as if by assignment, to the types of the corresponding parameters, taking the type of each parameter to be the unqualified version of its declared type."

C on variadic functions:

(C99, 6.5.2.2p7) "(C99, 6.5.2.2p7) "The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments."

So: are casts needed when passing a non-void-pointer to a function that expects a void-pointer?

For printf, p conversion specifier requires a void * argument. If the argument is of a different type, the function call invokes undefined behavior. So if the argument of p is an object pointer type, the (void *) cast is required.

(C99, 7.19.6.1p8) "p The argument shall be a pointer to void."