1. Preprocessor Directives and Macros Chapter 21
CSCI 130
Preprocessor Directives and Macros
Chapter 21

2. Preprocessor Part of all C compiler packages
First component that processes source code
Source code changed based on directives
all preprocessor directives begin with #
Output - modified source code file
used in next step of compilation
deleted automatically by system

3. #include Imports other files into source code
maybe library functions (stdio.h)
use < >
may be user defined functions
use “ ”
may have more than one function in file

4. Advantages of #include
Structure
Portability
Conventions:
similar functions grouped in one file
file given descriptive name

5. Example - math library file
Contents of user created mathfn.h file:
int areaOfSquare(int length) {
return length * length; }
int areaOfRectangle(int width, int height) {
return width * height;

6. main.c main.c is sometimes called the driver
‘drives’ the flow of logic
often does not contain any function definitions
contains function prototypes - includes functions with #include preprocessor directive
#include “mathfn.h”

7. #define Used for substitution macros Used for function macros
substituting values for variables
Used for function macros
defining a function ‘on the fly’

8. #define - substitution macro
Creates substitution macro
#define PI 3.14
area = radius * radius * PI
circumference = 2 * radius * PI
Changes in source code after precompiling
area = radius * radius * 3.14
circumference = 2 * radius * 3.14
Space after constant indicates substitution macro

9. #define - function macro
Shorthand for a more complicated operation
Arguments not type sensitive
Ex:
#define HALFOF(value) ((value)/2)
printf(“%f”, HALFOF(x + y));
Changes in source code after precompiling
printf(“%f”, ((x+y)/2));
No space after function name indicates function macro

10. Other function macro examples
#define AVG3(a, b, c) (((a) + (b) + (c)) / 5)
#define SMALLER(x, y) ((x) < (y) ? (x) : (y))
#define SUM (x, y, z) ((x) + (y) + (z))

11. Common Errors-function macros
Spaces after function macro name
#define SUM (x, y, z) ((x) + (y) + (z))
Forgetting parenthesis
#define AREA(x, y) x*y
All parameters must be used
#define SUM(x, y, z) ((x) + (y))

12. Macros vs. Functions Macros can be used for simple functions
Size of program
Functions exist as a single copy
Macro expanded in code every time it is called
Execution efficiency
no overhead to use a macro
overhead required for functions

13. #if and #endif
Preprocessor directives controlling conditional compilation
if statement determines if statements executed
#if statement determines if statements compiled
#elif, #else work as else if, else

14. Where would #if be used For purposes of CSCI 130: When including files
If file included more than once, code is imported for each time
out of memory
Use #if and ‘defined’ keywords to conditionally include statements for compilation

15. Example #if #if defined mathfn_h #else
#define mathfn_h //Note: no periods can be used here
int areaOfSquare(int length) {
return length * length; }
int areaOfRectangle(int width, int height) {
return width * height;
#endif

16. Not (!) is allowed #if !defined mathfn_h #define mathfn_h
int areaOfSquare(int length) {
return length * length; }
int areaOfRectangle(int width, int height) {
return width * height;
#endif

17. #ifndef directive #ifndef mathfn_h #define mathfn_h
int areaOfSquare(int length) {
return length * length; }
int areaOfRectangle(int width, int height) {
return width * height;
#endif

18. #undef
Opposite effect of #define
#define PI 3.14
...
#undef PI