Programming in C

4.0 Data Types and Declarations
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Variables can be declared at the time that the union is defined, or they can be subsequently
declared using the notation
union name variableList;
provided the union was given a name when it was defined.
It is the programmer’s responsibility to ensure that the value retrieved from a union is
consistent with the last value that was stored inside the union.The first member of a
union can be initialized by enclosing the initial value, which, in the case of a global
union variable, must be a constant expression, inside a pair of curly braces:
union shared
{
long long int l;
long int w[2];
} swap = { 0xffffffff };
This declares the union variable swap and sets the l member to hexadecimal ffffffff.
A different member can be initialized instead by specifying the member name, as in
union shared swap2 = {.w[0] = 0x0, .w[1] = 0xffffffff; }
An automatic union variable can also be initialized to a union of the same type, as in
union shared swap2 = swap;
4.3.4 Pointers
The basic format for declaring a pointer variable is as follows:
type *name;
The identifier name is declared to be of type “pointer to type,” which can be a basic data
type, or a derived data type. For example,
int *ip;
declares ip to be a pointer to an int, and the declaration
struct entry *ep;
declares ep to be a pointer to an entry structure.
Pointers that point to elements in an array are declared to point to the type of element
contained in the array. For example, the previous declaration of ip can also be used
to declare a pointer into an array of integers.
More advanced forms of pointer declarations are also permitted. For example, the
declaration
char *tp[100];
declares tp to be an array of 100 character pointers, and the declaration
struct entry (*fnPtr) (int);