The C Preprocessor Versus the Compiler

One important thing you need to remember is that the C preprocessor is not part of the C compiler.
The C preprocessor uses a different syntax. All directives in the C preprocessor begin with a pound sign (#). In other
words, the pound sign denotes the beginning of a preprocessor directive, and it must be the first nonspace character
on the line.
The C preprocessor is line oriented. Each macro statement ends with a newline character, not a semicolon. (Only C
statements end with semicolons.) One of the most common mistakes made by the programmer is to place a semicolon
at the end of a macro statement. Fortunately, many C compilers can catch such errors.

TIP
Macro names, especially those that will be substituted with constants, are normally represented with
uppercase letters so that they can be distinguished from other variable names in the program.