/*
* function.c
* basic concepts of functions
*/

#include<stdio.h>
int a;                  // global variable, visible from anywhere.   
void function1();       // function declaration or prototype, function without return value
int function2(int);     // function with int return value, has int parameter, need to pass int argument in function call

int main() {
   int x = 2;
   a = 1;                 // assign value 1 to global variable a
   function1();           // call function1
   int y = function2(x);  // call function2, pass value of x, i.e. 2 to function2, and assign returned value to variable y
   printf("a=%d, y=%d", a, y);
   return 0; 
}

// function definition/implementation, no return value
void function1() {
   int y = 2;         // y is local variable
   a += y;
}

// function definition/implementation, with return value
int function2(int x) { // parameter x is int type
   int z = x + 2;     
   function1();       // call another function
   return z;          // return value
}

/*
a=5, y=4
*/