c implicit function declaration warning issue

c implicit function declaration warning issue

Lets consider this c program which computes the area and perimeter of a circle.

#include<stdio.h>                                                              
                                                                                                                 
void main()                                                                     
{                                                                               
  int radius;                                                                   
  float area, perimeter;                                                        
  printf("\nEnter radius of a circle: ");                                       
  scanf("%d", &radius);                                                         
  computecircleparams(radius, &area, &perimeter);                               
  printf("\nCircle Area: %f", area);                                            
  printf("\nCircle Perimeter: %f", perimeter);                                  
}                                                                               
                                                                                
void computecircleparams(int cradius, float *carea, float *cperimeter)          
{                                                                               
  *carea = 3.14 * cradius * cradius;                                            
  *cperimeter = 2 * 3.14 * cradius;                                             
}                                                                                   

We have missed function declaration here, so getting implicit function declaration warning message during compilation.

$ cc multiple-return-values.c
multiple-return-values.c: In function ‘main’:
multiple-return-values.c:9:3: warning: implicit declaration of function ‘computecircleparams’ [-Wimplicit-function-declaration]
   computecircleparams(radius, &area, &perimeter);
   ^~~~~~~~~~~~~~~~~~~
multiple-return-values.c: At top level:
multiple-return-values.c:14:6: warning: conflicting types for ‘computecircleparams’
 void computecircleparams(int cradius, float *carea, float *cperimeter)
      ^~~~~~~~~~~~~~~~~~~
multiple-return-values.c:9:3: note: previous implicit declaration of ‘computecircleparams’ was here
   computecircleparams(radius, &area, &perimeter);

Whether c program will work due to this warning message ?

yes we can execute the c program and get the output.

Output:

$ cc multiple-return-values.c
$ ./a.out 

Enter radius of a circle: 10

Circle Area: 314.000000
Circle Perimeter: 62.799999

How to fix implicit function declaration warning message ?

If we declare a function 'computecircleparams' if we define a function after main function otherwise define before main function, we could avoid this warning message during compilation.

function declaration is in before main funtion,

#include<stdio.h>                                                              
                                                                                
void computecircleparams(int, float*, float*);                                  
void main()                                                                     
{                                                                               
  int radius;                                                                   
  float area, perimeter;                                                        
  printf("\nEnter radius of a circle: ");                                       
  scanf("%d", &radius);                                                         
  computecircleparams(radius, &area, &perimeter);                               
  printf("\nCircle Area: %f", area);                                            
  printf("\nCircle Perimeter: %f", perimeter);                                  
}                                                                               
                                                                                
void computecircleparams(int cradius, float *carea, float *cperimeter)          
{                                                                               
  *carea = 3.14 * cradius * cradius;                                            
  *cperimeter = 2 * 3.14 * cradius;                                             
}                                                                                   

function declaration is in inside main function,

#include<stdio.h>                                                                                                                                            
                                 
void main()                                                                     
{                                                                               
  int radius;                                                                   
  float area, perimeter;       
  void computecircleparams(int, float*, float*);                                                  
  printf("\nEnter radius of a circle: ");                                       
  scanf("%d", &radius);                                                         
  computecircleparams(radius, &area, &perimeter);                               
  printf("\nCircle Area: %f", area);                                            
  printf("\nCircle Perimeter: %f", perimeter);                                  
}                                                                               
                                                                                
void computecircleparams(int cradius, float *carea, float *cperimeter)          
{                                                                               
  *carea = 3.14 * cradius * cradius;                                            
  *cperimeter = 2 * 3.14 * cradius;                                             
}                                                                                   

define function before main function,

#include<stdio.h>                                                              
                                                                                
void computecircleparams(int cradius, float *carea, float *cperimeter)          
{                                                                               
  *carea = 3.14 * cradius * cradius;                                            
  *cperimeter = 2 * 3.14 * cradius;                                             
}                                                                               
                                                                                
void main()                                                                     
{                                                                               
  int radius;                                                                   
  float area, perimeter;                                                        
  printf("\nEnter radius of a circle: ");                                       
  scanf("%d", &radius);                                                         
  computecircleparams(radius, &area, &perimeter);                               
  printf("\nCircle Area: %f", area);                                            
  printf("\nCircle Perimeter: %f", perimeter);                                  
}