Directives

1. The C Preprocessor
Yongjoon Lee
April 22, 2005

2. What is it?
• Things with #
– #include <stdio.h>
• Processes the C source files
BEFORE handing it to compiler.
– `Pre`-process
– gcc –E
– cpp

3. Why care?
• File Inclusion
• Basic Macros
• Parameterized Macros
• Conditional Compilation
• Style and (mis)use of Macros

4. File Inclusion
• Recall : #include <filename>
– #include <foo.h>
• System directories
– #include “foo.h”
• Current directories
– gcc -I

5. Macros
• We can #define constants
#define HALF_PI 1.5708
#define BAD_PI HALF_PI+HALF_PI
• Solution?
((((((((((The parentheses!))))))))))
#define PI ((HALF_PI)+(HALF_PI))

6. Parameterized Macros
• Macros with param :
#define BAAAD_DOUBLE(x) (x*2)
#define BAD_DOUBLE(x) ((x)+(x))
#define OKAY_DOUBLE(x) ((x)*2)
• WHAT TO TAKE HOME:
– Don’t use 2+ parameters.
– No assignments within macro

7. Parameterized Macros cont’d
• Want to wrap malloc() calls.
– Can we?
• YES!
#define NEW_T(t,sz)
((t*) malloc((sz)*sizeof(t))

8. Appropriate use of Macros
• Do
– Use parentheses
– Handle header file inclusion
– Capitalize macro names
• Don’t
– Use more than 2 parameters.
– Replace function calls with macros for
‘performance’
– Make the code harder to read
#define LOOP(s) while(1) {s};

9. Conditional Inclusion
• #if
• #ifdef
• #ifndef
• #elif
• #else
• #endif

10. Conditional Compilation
• Debugging
#define DEBUG_MODE
#ifdef DEBUG_MODE
/* … Debug code … */
#endif
• OS/Platform
#ifdef _WIN32
#include <windows.h>
#elif
/* …… */

11. File Header
• File header
– #ifndef FOO_H
#define FOO_H
/* … the whole file … *
#endif
• Why???
– Global variables
– Cycles of includes
– Inefficient

12. The Printf
Yongjoon Lee
April 22, 2005

13. Basics
• printf(“format string”,vars);
• Format string?
– “This year is %dn”
– “Your score is %dn”
• Conversion by %
– %d : int
– %f : float, double
– %c : char
– %s : char *, string
– %e : float, double in scientific form

14. Examples %d conversion
• Padding ( width )
%12d
%012d
• Precision
%12.4d
%12.6d
• Left/right justification
%12d
%-12d
• Similar in %f, %e, … conversions

15. Example %c, %s conversion
• %c has only precision 1
• Precision in %s means string length
starting from the beginning
• Padding and justification are same