Table 1 Preprocessor directives

#include  Includes text of header or source file.

#define   Enters a symbol into the symbol table for the
          current compilation unit, with an optional
          value.
#undef    Removes a symbol from the symbol table.

#if       Control flow directives for conditional
#elif     compilation.
#else
#endif

#ifdef    Symbol table query directives.
#ifndef   (ALso used for conditional compilation).

#line     Renumbers the current source line. UtiLities
          like code generators use this to synchronize
          generated lines with original source lines in
          error messages.

#pragma   Compiler-dependent actions.