Casting NULL to a struct pointer in C?

Question

Is there any benefit of casting NULL to a struct pointer in C ?

For example:

typedef struct List
{
....
} List;

List *listPtr = ((List *) NULL) ;

Example from PostgreSQL source:

#define NIL                     ((List *) NULL)

http://doxygen.postgresql.org/pg__list_8h_source.html

Swayne

This doesn't make any sense to me. You can assign NULL to any pointer without any cast. – Hogarth

Answer 1

In assignment example the explicit cast make no useful sense. However, it seems that you the question is really about #define NIL ((List *) NULL) macro, whose usability extends beyond assignment.

One place where it might make sense is when you pass it to a variadic function or to a function declared without a prototype. The standard NULL can be defined as0 or 0L or ((void *) 0) or in some other way, meaning that it might be interpreted differently in such type-less contexts. An explicit cast will make sure that it is interpreted correctly as a pointer.

For example, this is generally invalid (behavior is undefined)

void foo();

int main() {
  foo(NULL);
}

void foo(List *p) {
  /* whatever */
}

while replacing the call with

foo(NIL);

makes it valid.

Banerjee

Answer 2

No null is null, it's as null as you can get, you might think zero is pretty null but that's just peanuts compared to null.

Tavis

Still some HHGTTG fans in this new generation then ;-) – Tavis

oh, no, unfortunately not. sizeof(NULL) can vary from 1 to 8 depending on how NULL is defined. – Grizzle

@JensGustedt that's just the representation of NULL in our universe. – Tavis

Answer 3

Is there any benefit of casting NULL to a struct pointer in C

There's none. It should be simply:

List *listPtr = NULL;

Moreover, if the object has static storage (say, like a global variable) you don't even need to initialize it to NULL.

Swen

Answer 4

Yes, there is benefit. In this example, you can use size_of_attribute() macro to determine a size in bytes of a specific struct attribute, without actually creating an instance of said struct.

#define size_of_attribute(Struct, Attribute) sizeof(((Struct*)NULL)->Attribute)

Tobacconist