Why can't structs be compared for NULLness in C?

Question

I have the following struct:

typedef struct School
{
    int numOfStudents;
} School;

For example if I have a struct:

School s;

I want to check whether the struct is null like this:

if(s) {
    printf("null");
}

This will not compile and error message is as follows:

error: used struct type value where scalar is required

Why can't I check structs for NULLness inside if statement in C?

Answer 1

In the case of:

School s;

s is definitely not NULL, because it is not a pointer. It is however, an uninitialized instance of a struct, and the value of s.numOfStudents is undefined.