Intro To UNIX Software Development

THE C PREPROCESSOR

Tool to transform code before compilation

Define macros with:
```
#define NAME expansion
```
Subsequent occurrences of NAME will be replaced with expansion (even in preprocessor directives)
Definition must be a single line, split with \ for cosmetic reasons
Convention is to use all-caps for macro names

Macros can have arguments. Example:

	#define MIN(x,y) ((x<y)?(x):(y))
	...
	z = MIN(5,6);

For arithmetic expressions, always surround parameters in expansion and expansion with parentheses to avoid order of operations problems.

#ifdef NAME and #ifndef NAME will only insert contained code if NAME is or isn't defined (respectively). End with #endif

Example:

	#define FOO
	#ifdef FOO
	  ... this gets included ...
	#endif
	#ifndef FOO
	  ... this does NOT get included ...
	#endif

#if allows more complicated expressions with a C-like syntax (combine terms with &&, ||, and ! and also use integer operations)

The defined() function returns 1 iff a variable is defined. These two lines are identical:

	#ifdef FOO
	#if defined(FOO)

Can use #else and #elif EXPRESSION with #if EXPRESSION

Can do simple macro arithmatic, eg:

	#define X 5
	#if X == 4
	...
	#elif X == 5
	...
	#else
	...
	#endif

To omit code, use:

	#if 0
	... code to be omitted
	#endif

Rather than commenting it out. This eliminates problems with nested comments.

Uses the files foo, bar, and blah.

		/lib/cpp -P foo
		/lib/cpp -P -DINCLUDE_BAR foo
		/lib/cpp -P -DINCLUDE_BLAH foo