EEWeb Electronics Forum
Created By: Hussein Nosair
What is the best way to declare and define global variables and functions?
Declaration vs. Definition:
1. Declaration is a statement of a programmatic entity (ex. variable, function, union, structure) to be referenced in other parts of a program/code, which has its definition somewhere else.
2. Definition is the actually description, allocated space and initialization of a declared programmatic entity (ex. variable, function, union, structure).
Purpose:
a global variable or function is a variable or function that need to be shared or accessed across several source files, keeping in mind that the definition and declaration is consistent with each other.
Right Wrong
Definition:
int a; int a;
Declaration:
int a = 0; char a = 0;
OR
a global variable or function is a variable or function that need to be shared or accessed within a single source file, keeping in mind that the definition and declaration is consistent with each other.
*** The best way to declare and define a global variable is to place the declaration in a header file (.h) and the definition is a source file (.c).
How-to:
To define a global variable or function; you need to do the following:
1. create a header file with the declaration of variable or function.
Ex:
Common.h
#ifndef _COMMON_H_
#define _COMMON_H_
extern int i;
extern int n;
extern unsigned char Array[];
extern void func1();
extern int func2();
#endif
*** Note: The keyword extern is optional in function declarations
2. create a source file with the definition of variable or function.
Common.c
#include "common.h"
int i; // Define i and initialize
int n; // Define n and initialize
unsigned char Array[6] = {1,2,3,4,5,6}; // Define Array and initialize
// Define func1 and initialize
void func1(void)
{
printf("This is ... function 1 \n");
}
// Define func2 and initialize
int func2(void)
{
printf("This is ... function 2 \n");
return 0;
}
*** Note: The declaration source file should include (#include) the definition header file.
3. Main program
#include <stdio.h> // Include general I/O library
#include "common.h" // Include defined variables and functions
void func3(void); // Define Local function
void func4(void); // Define Local function
int main(void) // Main Program
{
printf("i = %d \n", i); // Print out initial value of i
printf("n = %d \n", n); // Print out initial value of n
i = 8; // Set a new value for i
printf("i = %d \n", i); // Print out the new value of i
n = 4; // Set a new value for n
printf("n = %d \n", n); // Print out the new value of n
func1(); // Call global function = function 1
func2(); // Call global function = function 2
for(i=0; i<6; i++) // Print out global Array
printf("%d", Array[i]);
// Print out blank lines
printf("\n");
printf("\n");
func3(); // Call local function = function 3
// Print out blank lines
printf("\n");
printf("\n");
func4(); // Call local function = function 4
return 0; // End main program
}
// Definition of local function 3
void func3(void)
{
printf("This is ... Function 3 \n");
printf("Please, Enter a new value for variable i = ");
scanf("%d", &i);
printf("The new value for variable i = %d \n", i);
}
// Definition of local function 4
void func4(void)
{
printf("This is ... Function 4 \n");
for(i=0; i<6; i++)
{
printf("Please, Enter a new value for variable Array[%d] = ", i);
scanf("%d", &Array[i]);
}
for(i=0; i<6; i++)
printf("a new value for variable Array[%d] = %d \n", i, Array[i]);
}
*** Note: Need to include (#include) the definition header file wherever the global variables and functions are needed in the program code.
Circuit Negma 