The HP C preprocessor uses directives to affect the compilation of
  a source file.  For HP C on OpenVMS systems, these directives are
  processed by an early phase of the compiler, not by a separate
  program.

  The preprocessor directives begin with a number sign (#) and do not
  end with a semicolon.  The number sign must appear in the first
  column of the source line.

  Additional Information (explode) :

• Null directive (#)
• Conditional Compilation
• #define
• #dictionary
• #error
• #include
• #line
• #module
• #pragma
• #undef