1. Preprocessor C program → Modified C program → Object Code
All preprocessor directives or commands begin with a #.
E.g. #include <stdio.h>
C program → Modified C program → Object Code
Can appear anywhere in the program
No “;” in the end
preprocessor
compiler

2. Preprocessor Directives
Macro definition
#define, #undef
File inclusion
#include
Conditional Compilation
#if, #ifdef, #ifndef, #elseif, #else
Others

3. #define To define constants or any macro substitution.
    #define <macro> <replacement name>  
E.g.
  #define FALSE 0
#define TRUE !FALSE
To undefined a macro.
E.g. #undef FALSE
#define D 2
printf("%d",D);
#undef D
int D=6;

4. Define Functions E.g. To get the maximum of two variables:
#define max(A,B) A > B ? A:B
In the C code:    x = max(q+r,s+t);

5. File inclusion #include directive Why need file inclusion
Include the contents of another file at the point where the directive appears.
Why need file inclusion
Use built-in functions
E.g., printf(), rand();
Reuse code
double mean(int x,int y) {
return (x+y)/2; }
double mean(int ,int );
#include “test.h”
printf("%f",mean(3,5));
test.cpp
test.h
Your code

6. File inclusion formats
#include <file>
Used for system header files
File contain function prototypes for library functions
<stdlib.h> , <math.h> , etc
#include "file"
Used for header files of your own program
looks for a file in the current directory first
then system directories if not found

7. Custom header files Steps for creating custom header files Advantage
Create a file with function prototypes
Save as a .h file. E.g.: filename.h
Load in other files with
#include "filename.h"
Advantage
Reuse functions and data structure declaration

8. Conditional compilation
Useful when
machine-dependencies
setting certain options at compile-time.
Expressions
#if expression
#ifdef expression (same as #if defined )
#ifudef
#elif and #else
#endif

9. Examples
Example: write programs that are portable to several machines or operating systems
#if defined(WINDOWS)
#elif defined(LINUX)
#elif defined(SOLARIS)
#endif
Example: Providing a default definition for a macro
#ifndef BUFFER_SIZE
#define BUFFER_SIZE 256

10. int inline func(void);
Inline functions
Ret_Type inline func_name(params);
e.g.
int inline func(void);
Inline hints to the compiler that this function should be unrolled an inlined into wherever it is called. This can avoid function call overhead.
Problems with inline:
The compiler may happily ignore this. It’s kind of like the register keyword in this manner. It is merely a hint; it is not a command.
You may (and probably will) make your program size larger.
Read the previous bullet as “cache miss” or “page fault”.