Page 27 - Programming Microcontrollers in C

12 Chapter 1 Introduction to C

be able to discard accesses to that value because the program never
alters the value. In such a circumstance, if you had an analog-to-digital
converter peripheral in your system, the program would never be re­
quired to read its return value more than once. “The program did not
change the value stored in the input location subsequent to the first
read, therefore its value has not changed and it is not necessary to read
the location again.” This will always produce wrong results. The key
word volatile indicates to the program that a variable must not be
optimized. Therefore, if the input location is identified as a vola­
tile variable, it will not be optimized and the problem will go away.
As a point of interest, accessing a volatile object, modifying an
object, modifying a file, or calling a function that does any of those
operations are all defined as side effects by the standard.

Storage Classes, Linkage, and Scope

Additional modifiers are called storage classes and designate
where a variable is to be stored and how it is initialized. These stor­
age classes are auto (for automatic), static, and malloced.
The first two storage classes are described in the following sections.
The storage class malloc provides dynamic memory allocation and
is discussed in detail in Chapter 2.

Automatic variables
For local variables defined within a function, the default storage
class is auto. An automatic variable has the scope of the block in
which it is defined, and it is uninitialized when it is created. Auto­
matic variables are usually stored on the program stack, so space for
the variable is created when the function is entered. When the stack
is cleaned up prior to the return at the end of the function, all vari­
ables stored on the stack are deleted.
As we saw in our first program example, variables can be initial­
ized at the time of declaration by assigning the variable an initial value:
int rupt=17;

An automatic variable will be assigned its initial value each time
the block in which it is declared is entered. If the variable is not
initialized at declaration, it will contain the contents of uninitialized
memory, which can be any value.