Storage Classes in C(Extern)

External Variables (or) Global Variables

• The scope of a external variables remains from the point of the definition through the remainder of the program.
• External variables are recognized globally, that can be accessed from any function 
• Using external variables we can transfer information in to a function without using arguments.
• The key word for external variable is extern.
  

External Variable definition:

• External variable definition is written in the same manner as an ordinary variable declaration.
• It must appear outside of ,and usually before the functions that access the external variables
• Example:

  int x;     //External variable definitions fun1(){} fun2(){}

• An external variable definition will automatically allocate the required storage space for the external variables with in the computer memory.
• The assignment of initial values can be included.
  Example:

  int x=10;     //initializing external variable in definition fun1(){} fun2(){}

• The keyword extern is not required in external variable definition.
• If a function requires an external variable, that has been defined earlier in the program, then the function may access the external variable freely.
  Example:

  #include<stdio.h> int x=10; fun1 { } fun2 {     int a,b=10;     a=b+x; //no need to declare x here }

External variable Declaration:

• If the program is a multi file program  then the external variable must be declared where ever its needed.
• An external variable declaration begins with the keyword extern.
• The name and the datatype of the external variable must agree with the corresponding external variable definition that appear outside of the function.
• Example:

  file1

  #include<stdio.h> int x=10; //external var   defined here fun1() { } fun2() { }

  file2

  #include<stdio.h> fun3() {     extern int x; //declaration                }

  In file 2,x should be of type integer because in file 1, x is defined as integer.
• The storage space for external variables will not be allocated because the value is already defined.
• External variable declaration cannot include the assignment of initial value. 

Initialization of External variable:

• External variables can be assigned initial value during variable definition.
• The initial values will be assigned only once, at the beginning of the program.
• The initial values must be expressed as constants not expressions or statements
  Example:

  #include<stdio.h> #define z 10 /*definition of external variable x and Y int x=z; int y=z+x;       //error fun1() { } fun2() {     int a,b=10;     a=b+x;     printf("x=%d y=%d\n",x,y); } int main() { fun1(); fun2(); return; }

  while compiling this program It shows an error   "error: initializer element is not constant"
            
• The external variables will then retain these values, unless they are altered during the execution of the program.
• If the value of the external variable is changed with in a function, then the changed value will be carried over in to other parts of the program.
  Example

  #include<stdio.h> #define z 10 int x=z; fun1() {    x=30;    printf("In fun1, x=%d \n",x); } fun2() {     x=20;     printf("In fun2, x=%d \n",x); } int main() {     printf("In main, x=%d \n",x);     fun1();     fun2();     return; }

  The output of the above program is
  In main, x=10
  In fun1, x=30
  In fun2, x=20
• If the initial value is not assigned , the variable will automatically be assigned a value of zero.
  Example

  #include<stdio.h> int x; //Not initialized fun1() {    x=30;    printf("In fun1, x=%d \n",x); } fun2() {     x=20;     printf("In fun2, x=%d \n",x); } int main() {     printf("In main, x=%d \n",x);     fun1();     fun2();     return; }

  
  The output of the above program is
  In main, x=0
  In fun1, x=30
  In fun2, x=20