Preprocessor directives are usually placed at the beginning of your source code, but they can legally appear at any point in a program. The preprocessor detects preprocessor directives (also known as control lines) and parses the tokens embedded in them. The preprocessor supports these directives:

Any line with a leading # is taken as a preprocessing directive, unless the # is within a string literal, in a character constant, or embedded in a comment. The initial # can be preceded or followed by whitespace (excluding new lines).