Preprocessor directives are identified by beginning with the '#' symbol, and are generally not indented beyond the first column. A trailing semicolon is NOT used.
TEXT SUBSTITUTION, or macro substitution, is performed using the '#define' directive in the following syntax, usually located at the beginning of the source file:
# define NAME replacement text /* no trailing semicolon! */
Before compilation the preprocessor will replace all occurrences of NAME in the source file with the replacement text. The name to be replaced follows the same rules as for C variable and function identifiers. By convention this is nearly always capitalized, to avoid confusion with other variable and function identifiers. There are very few grammatical restrictions on the replacement text.
One of the most common uses of macro substitution is naming constants:
