Page 253 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

230 Chapter 8 Programming in C and C++

The procedure fun( ) in §8.2 illustrated the accessing of elements of vectors in
expressions. The expression c = d[a + b], accessed the a + b th element of the 8-
bit 10-element vector d. When reading the assembly code generated by C, be wary of
the implicit multiplication of the vector's precision (in bytes) when calculating offset
addresses of elements of the vector. Because C does not check that indexes are within a
vector, a C program must be able to implicitly or explicitly assure this to avoid nasty
bugs, as when a vector's data is inadvertently stored outside memory allocated to a
vector.
The C structure mechanism can store different-sized elements. The mechanism is
implemented by a declaration that begins with the word struct and has a definition of the
structure within angle brackets and a list of variables of that structure type after the
brackets, as in
struct { char 11; int 12; char 13;} list;
A globally defined list can be initialized as we did with vectors, as in

struct { char 11; int 12; char 13;} list = {5,7,9};
The data in a list are identified by "dot" notation, where a dot (.) means "element." For
instance, list. 11 is the 11 element of the list list. If P is a pointer to a struct,
then arrow notation, such as P->11, can access the element 11 of the list. The
typedef statement, though it can be used to create a new data type in terms of existing
data types, is often used with structs. If typedef a struct { char 11; int 12;
char 13;} list; is written, then list is a data type, like int or char, and can be
used in declarations such as list b; that declare b to be an instance of type list.
We will find the typedef statement to be quite useful when a struct has to be
declared many times and pointers to it need to be declared as well. A structure can have
bit fields, which are unsigned integer elements having less than 16 bits. Such a
structure as
struct {unsigned a:l, b:2, c:3;}l;
has a one-bit field 1. a, two-bit field 1. b, and three-bit field 1. c. A linked list
structure, a list in which some elements are addresses of (the first word in) other lists, is
flexible and powerful and is widely used in advanced software.
We normally think of an array as a two-dimensional pattern, as in
1 2 3
4 5 6
7 8 9
10 11 12
An array is considered a vector whose elements are themselves vectors, and C syntax
reflects this philosophy. For instance, the global declaration

int arl[4][3]={{l,2,3},{4,5,6},{7,8,9},{10,ll,12}>;
allocates and initializes a row major ordered array (rows occupy consecutive memory
words) ar 1, and a = ar 1 [ i ] [ j ]; puts the row-z column-,/ element of ar 1 into a.
A table is a vector of identically formatted structs. Tables often store characters,
where either a single character or a collection of n consecutive characters is considered an