int main()
{
    char * x = NULL;

    if (x==NULL)
        printf("is NULL\n");

    return EXIT_SUCCESS;
}

This function is correct for what it does. It assigns the address of 0 to the char pointer x. That is, it points the pointer x to the memory address 0.

Alternative:

int main()
{
    char* x = 0;

    if ( !x )
        printf(" x points to NULL\n");

    return EXIT_SUCCESS;
}