CHAPTER 8 - The Preprocessor
============================

Basic directives: define and include
------------------------------------

#define         identifier      string
substitutes identifier with string, but not inside "..."

#undef	identifier
undefines a macro

#define 	PI	3.14159
#define 	C	299792.458	/* speed of light */

#define		EOF	(-1)
#define		MAXINT	2147483647

can continue to the next line by ending the current line with a \

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#include <math.h>
#include "mydef.h"

=============================================================

Macros with parameters
----------------------

#define identifier( identifier,...,identifier ) string

Example: 

#define 	SQ(x)	((x) * (x))

SQ(7 + w)	expands to ((7 + w) * (7 + w))

SQ(SQ(*p))	expands to ((((*p) * (*p))) * (((*p) * (*p))))

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

bad declarations
----------------

#define SQ(x)	x * x

SQ(a + b) expands to a + b * a + b

#define SQ(x) (x) * (x)

4 / SQ (2) expands to 4 / (2) * (2)

#define SQ (x) ((x) * (x))

SQ(7)  expands to (x) ((x) * (x)) (7)

#define SQ(x)   ((x) * (x));  /* error */

if (x == 2) 
   x = SQ(y);
else
   ++x;

=============================================================

Macro nesting
-------------

#define 	min(x, y)	(((x) < (y)) ? (x) : (y))

m = min(u, v) expands to m = (((u) < (v)) ? (u) : (v))

#define   	min4(a, b, c, d)   min(min(a,b), min(c,d))

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#define	SQ(x) 		((x) * (x))
#define CUBE(x) 	(SQ(x) * (x))
#define F_POW(x) 	sqrt(sqrt(CUBE(x)))) /* fractional power: 3/4 */

=============================================================

operating the preprocessor only
-------------------------------

gcc -E file.c

=============================================================

conditional compilation
-----------------------

#if	constant_integral_expression
#ifdef  identifier
#ifndef identifier

if expr is true will compile the lines till:
#endif

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#if defined(HP9000) || defined(SUN4) && !defined(VAX)
   ...... /* machine_dependent code */
#endif


#if defined(identifier)
 is equivalent to
#ifdef identifier

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#define DEBUG	

#ifdef DEBUG
	printf("debug: a = %d\n", a);		
#endif

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#include "everything.h"

#undef   PIE
#define   PIE "I like apple."
.......

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

commenting out a code fragment
------------------------------

statement
#if 0
more statements
#endif
and still more statements

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#elif constant_interal_expression

#else

#endif

=============================================================

A sample of ready-made macros, functions and typedefs
------------------------------------------------------

in stddef.h
-----------
(possible implementation:)
typedef unsigned size_t;
#define NULL ((void *) 0)

in stdio.h
----------
getchar(), putchar(c)

in ctype.h
----------
isalpha(c), isupper(c), isdigit(c): all get int and return int

predefined (no need to include a special file)
----------------------------------------------
always there, cannot be undefined
__DATE__    string 
__FILE__    string
__LINE__    int
__TIME__    string 
__STDC__    non-zero integer if the implementation follows ANSI C

/* assert.h */

#include <stdio.h>
#include <stdlib.h>                      /* for abort() */

#if defined(NDEBUG)
   #define   assert(ignore)   ((void) 0) /* ignore it */
#else
   #define   assert(expr)                  \
      if (!(expr)) {                       \
         printf("\n%s%s\n%s%s\n%s%d\n\n",  \
            "Assertion failed: ", #expr,   \
            "in file ", __FILE__,          \
            "at line ", __LINE__);         \
         abort();                          \
      }
#endif