C ifdef, elif and else directive

C ifdef directive

Also called conditional directive which is used to check whether particular fragment of code to be included or not based on conditional expression before compilation.

c ifdef directive is similar like if statement and difference is if statement is used to control program execution sequence but ifdef directive is used to control including chunk of code in current program.

Using ifdef directive, we can use different logic in the same program based on environment (software and hardware etc).

We can also control to avoid chuck of code in the current program using ifdef directive.

ifdef directive syntax,

#ifdef condition
//chunk of code
#endif

Let us see the example program for ifdef directive,

#include<stdio.h>                                                              
                                                                                
#define PI 3.14                                                               
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}     

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 314

Above program prints 0 if we comment define directive, because PI will not be defined and compute_area function will not be called.

#include<stdio.h>                                                                
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}     

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 0

C ifdef and else directive

We can also use #else directive with #ifdef directive similar like if else statements in c program.

#include<stdio.h>                                                               
                                                                                
#define PI 3.14                                                               
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #else                                                                         
  area = 200;                                                                   
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}  

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 314

if PI is not defined using #define directive, program will return area is 200 instead of actual computation value or default 0.

#include<stdio.h>                                                             
                                                                                
//#define PI 3.14                                                               
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #else                                                                         
  area = 200;                                                                   
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}  

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 200

C ifdef, elif and else directive

This elif is similar like else if statements.

ifdef directive condition fails then checks elif condition and includes chunk of code if elif condtion is true.

elif directive syntax,

#ifdef condition1
//chunk of code1
#elif conditon2
//chunk of code2
#elif condition3
//chunk of code3
#elif .....
....
#else
//chunk of code defualt
#endif

Returns actual area computation value if PI is defined.

#include<stdio.h>                                                               
                                                                                
#define PI 3.14                                                               
#define AREA 300                                                              
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #elif AREA                                                                    
  area = AREA;                                                                  
  #else                                                                         
  area =200;                                                                    
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}  

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 314

Returns area is 300 if PI is not defined and only AREA is defined.

#include<stdio.h>                                                               
                                                                                
//#define PI 3.14                                                               
#define AREA 300                                                              
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #elif AREA                                                                    
  area = AREA;                                                                  
  #else                                                                         
  area =200;                                                                    
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}  

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 300

Returns default area is 200 if PI and AREA macros are not defined.

#include<stdio.h>                                                              
                                                                                
//#define PI 3.14                                                               
//#define AREA 300                                                              
                                                                                
int compute_area(int radius)                                                    
{                                                                               
 return 3.14*radius*radius;                                                     
}                                                                               
                                                                                
void main()                                                                     
{ int area=0, radius=10;                                                        
  #ifdef PI                                                                     
  area = compute_area(radius);                                                  
  #elif AREA                                                                    
  area = AREA;                                                                  
  #else                                                                         
  area =200;                                                                    
  #endif                                                                        
  printf("\nArea of circle: %d", area);                                         
}  

Output:

$ cc ifdef_directive.c 
$ ./a.out 

Area of circle: 200