The C preprocessor processes C source files before compilation by the compiler. It performs tasks like file inclusion using #include, macro definition and expansion, and conditional compilation using #if, #ifdef, #ifndef, etc. Macros allow constants, simple textual substitutions, and basic functions to be defined, but overuse or misuse of macros can negatively impact code readability and maintainability. The printf function formats and prints output using conversion specifiers like %d for integers, %f for floats, and %s for strings, and supports options for padding, precision and justification of the output.

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