Difference between declaring and defining a function/enumerations in the C programming language

defining a function is where you actually provide a definition (what the function actually does) between { } like so:

//this is a definition
int addTwo(int a, int b) {

      return a + b

}

declaring a function is simply telling the compiler about the function. You let it know it exist by providing the name, the return type, and the number of / type of arguments.

//this is a declaration/prototype
int addTwo (int a, int b);

In C, you must either define a function before you use it, or provide a prototype declaration before you call the function and define the function somewhere else in the file. if you dont your program might not behave as you expect

Below is an example of providing a prototype, calling the function, and then defining the function below that

#include <stdio.h>

/* function declaration goes here.*/
int addTwo( int a, int b );

int main()
{
   int a = 10;
   int b = 20;

   //call the function here
   int c = addTwo(a, b)

}

//this is a definition
int addTwo(int a, int b) {

      return a + b

}

here is an example of defining the function first, then calling it:

#include <stdio.h>

//this is a definition
int addTwo(int a, int b) {

      return a + b

}

int main()
{
   int a = 10;
   int b = 20;

   //call the function here
   int c = addTwo(a, b)

}

notice we dont need the prototype declaration of the function since we defined it before we used it

most people prefer to use prototypes because its easier to organize your code that way since you dont have to worry about defining them before you use them