Page 71 - Programming Microcontrollers in C

56 Chapter 1 Introduction to C

The second line of code

#define abs(t) (((t)>=0) ? (t) : -(t))
is called a macro definition. In this case, the macro definition has the
appearance of a simple function. This function will calculate the ab­
solute value of the argument t. The absolute value of the argument is
a positive value. If the argument is positive, it is returned unchanged.
If it is negative, it is multiplied by –1 before it is returned. A macro
definition is a type of character expansion. Whenever the function
abs(x) is found in the code, the character string (((x)>=0) ?
(x) : -(x)) is put in its place. The argument x can be any valid
C expression. This function returns the absolute value of its argu­
ment. The macro definition

#define square(t) (t)*(t)
returns the square of t. Since these arguments can be any valid C
expression, it is necessary to be cautious when writing the macro defi­
nitions. Suppose that the parentheses were left out of the above
expression, and the macro were written
#define square(t) t*t

Also suppose that the code using this function were as follows:
x=square(y+3);

The character expansion of this expression would be

x=y+3*y+3;
The result of this calculation is 4*y+3 and not (y+3)*(y+3) as
expected. When writing macro definitions, surround all arguments
and functions created by the macro with parentheses so that all argu­
ments are evaluated prior to use in the macro definition function.
Another problem can sneak into your code through improperly
written macros. Suppose that you want a macro that doubles the value
of its argument. Such a macro could be written

#define times_two(x) (x)+(x)
This macro when expanded in the following expression
x = 7*times_two(y);