Page 28 - Programming Microcontrollers in C

Storage Classes, Linkage, and Scope 13

Another class of variable is register. A register class variable
is automatic, i.e., it comes into being at the beginning of the block in
which it is defined and it goes out of scope at the end of the block. If
a register is available in the computer, a register variable
will be stored in a register. To define a register variable, you should
use the form

register int roger=10;
These variables can be long, short, int, or char.
When a register is not available, a register variable will be
stored just like any other automatic variable. A programmer might
consider the use of register variables in code that contains “tight
loops” to save the time of memory accesses while executing the loop.
A bit of advice. Compilers have improved continuously over the past
years. With today’s compilers, the optimizers are so efficient that the
compiler can probably do a better job of assigning register vari­
ables than the programmer. Therefore, it makes little sense to specify a
lot of register variables just to improve the efficiency of your code.

Static variables

Sometimes you might want to assign a value to a variable and
have it retain that value for later function calls. Such a variable can
be created by calling it static at its definition. There are two groups
of static variables: Local static variables which have a scope
of the function in which they are defined, and global or external
static class variables. Unless otherwise declared, all static class
variables are initialized to 0 when they are created.
There are two groups of external static variables. Any exter­
nal variable is a static class variable. It is automatically initialized
to the value 0 when the program is loaded unless the value is other­
wise declared in the definition statement. An external variable that is
declared as static in its definition statement like

static int redoubt;
will have file scope. Remember normal external variables can be
accessed from any module in the entire program. A static exter­
nal variable can be accessed only from within the file in which it is
defined. Note that static variables are not stored on the stack, but
rather stored in a static data memory area.