CSC195, Class 47: The C Preprocessor

Overview:
* About the C Preprocessor
* What really happens with includes
* Use of #define
  + Constants
  + Macros
  + More
* Some silly tricks

Notes:
* No more homework!
* Exam coming soon, though

The C preprocessor is the part of the compiler that goes
through and does "the easy stuff".
* Might strip many of your comments.
* Lots of fun with # signs
  #include
  #define
  #ifdef

What does #include do?
* Literally includes the file that you just named
  right there in the code.
* Processes it recursively.
* Be careful about recursion.

Note that the fact that it is including the file is important.

For example, suppose you 
* put a function *definition* in a header
* include that header in two files
* compile separate
* link

Result: function **** defined twice.  Are you sure you're
  a real C programmer?
Most compilers let you get away with some things you shouldn't
  be able to get away with.

E.g.,

function declarations in headers should be declared *extern*
The same holds for global variables

--------

#define NAME val

A constant
Textual substitution

#define FRED x+y

Macros (see examples likarish.h and martinsa.c)

#ifdef NAME
code-to-use-if-defined
#endif

#ifndef NAME
code-to-use-if-not-defined
#endif

#ifdef NAME
...
#else
...
#endif

Example: Different code for different architectures

#ifdef MC68000
#include <oldstuff/foo.h>
#else
#include <newstuff/foo.h>
#endif

Where would MC68000 be defined?
(1) In an included "mysystem.h" or "config.h"
(2) Compile-time flag
  cc -DMC68000

Getting trickier:

* Building new names on the fly

  STUFF ## PARAM

Why?
* compile-time string-append
* wicked-neat polymorphism