*.h files

It is traditional and good programming practice to put all the type definitions, function prototypes, and extern variable definitions needed to use a file foobar.c in file called foobar.h. Then to use the stuff in foobar.c, all another file has to do is have the line
```
#include <foobar.h>
```
"Dot-h" files also sometimes contain useful #define constants and macros. A #define macro looks like this:
```
#define ADD(a,b)  (a+b)
```
and is used like this:
```
x = ADD(y,5) * 7;
```
[Do you see why the paranthesis around a+b in the macro definition is a good idea?] Just like #define constants, #define macros get expanded textually before compilation.
#define macros are often used in conjection with the ? operator, which has the following synatx:
```
(boolean-expression) ? (expression-1) : (expression-2)
```
If the boolean-expression is true, then the result of the entire thing is expression-1. Otherwise, it is expression-2. The single two most popular uses of this are
```
#define MAX(a,b)  ( a > b ? a : b )
#define MIN(a,b)  ( a < b ? a : b )
```
Another preprocessor command often observed in .h files is #if. If a program has the lines
```
#if defined(DEBUG)
   printf( "x = %d\n", x );
#endif
```
then the printf statement is included in the file at compile time only if there is a preprocessor macro called DEBUG defined at that point. You often see, for example, the lines
```
#if defined(FOOBAR.H)
#define FOOBAR.H

...

#endif
```
surrounding the rest of the file foobar.h. This prevents bad things from happening if foobar.h is included more than once in another program (which can easily happen if you include both stuff1.h and stuff2.h, say, and stuff2.h includes stuff1.h.)
Some useful things to know about #ifdef:
- You can include boolean operators on the #if line, like
```
#if defined(DEBUG) || defined(VERBOSE)
```
  or
```
#if !defined(MAC) && defined(MSDOS)
```
- #ifdef SYMBOL is short for #if defined(SYMBOL) and #ifndef SYMBOL is short for #if !defined(SYMBOL).
- You can define symbols on the command line by the -D option:
```
gcc foobar.c -c -DDEBUG -DFALSE=0
```
- You can undefine symbols with #undefine SYMBOL.