Preprocessor

 The VSI C preprocessor uses directives to affect the compilation of
 a source file.  For VSI 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 available:

Null_directive (#)              Conditional_Compilation         #define
#dictionary     #error          #include        #line           #module
#pragma         #undef