Source : Free On-Line Dictionary of Computing

C preprocessor
     
         (cpp) The standard {Unix}
        {macro}-expansion utility run as the first phase of the {C}
        compiler, {cc}.  Cpp interprets lines beginning with "#" such
        as
     
        	#define BUFFER_SIZE 256
     
        as a textual {assignment} giving the symbol BUFFER_SIZE a
        value "256".  Symbols defined with cpp are traditionally given
        upper case names to distinguish them from C identifiers.  This
        symbol can be used later in the input, as in
     
        	char input_buffer[BUFFER_SIZE];
     
        This use of cpp to name constants, rather than writing these
        {magic numbers} inline, makes a program easier to read and
        maintain, especially if there is more than one occurrence of
        BUFFER_SIZE all of which must all have the same value.
     
        Cpp macros can have parameters:
     
        	#define BIT(n) (1<<(n))
     
        This can be used with any appropriate actual argument:
     
        	msb = BIT(nbits-1);
     
        Note the parentheses around the "n" in the definition of BIT.
        Without these, operator precedence might mean that the
        expression substituted in place of n might not be interpreted
        correctly (though the example above would be OK).
     
        Cpp also supports conditional compilation with the use of
     
        	#ifdef SYMBOL
        	...
        	#else
        	...
        	#endif
        and
        	#if EXPR
        	...
        	#else
        	...
        	#endif
     
        constructs, where SYMBOL is a Cpp symbol which may or may not
        be defined and EXPR is an arithmetic expression involving only
        Cpp symbols, constants and C operators which Cpp can evaluate
        to a constant at {compile time}.
     
        {Decus cpp} is a free implementation for {VMS}.
     
        The most widely used C preprocessor today is the {GNU} CPP,
        distributed as part of {GCC}.
     
        (2001-12-31)