---

Making global variables accessible in multi-files C programs through declaration

---

• Making sure that the program variables are accessible to all functions in the program
  • When you write a computer program, you will use variables to store information needed/used by the statements in the program

  • One important task when you write a (correct) computer program is:

    Making sure that a program variable is accessible in the function(s) that need to make use (= access) of the information stored in that variable

    The scoping rules (that we just learned) will determine which function(s) will be able to access a variable

  ---

  ---

  ---

  ---

• You don't have to worry about the accessibility of local variables, parameters and static local variables
  • Because the scope of local variables, parameter variables and static local variables is:

    Local variables, parameter variables and static local variables are only accessible in one function (i.e.: inside the function where the variable is defined)

    Therefore:

    We do not need to worry about making these kinds of variables accessible in other functions (because they cannot be accessed by other functions....)

  ---

  ---

  ---

  ---

• Making static global variables accessible to all functions (in the same program file)
  • Where can a static global varaible be accessed:

    static global variables are accessible by functions in the same program file

    ---

  • Best practice to ensure that static global variables are most accessible:

    If we define the static global variables at the top of the program file, the all the static global variables will be accessible by all functions in the program file

    Because when the static global variables are defined at the top:

    All functions in the program file will follow after the definition of the static global variables Consequently: All functions in the program file can access the static global variables

  ---

  ---

  ---

  ---

• Important fact about global variables
  • Property of global variables:

    Only the global variables are accessible (= used) by functions (= program statements) in different files !!!

    ---

    ---

    ---

  • Therefore:

    We need to organize the global variables carefully so they can accessed by all functions in all program files

  ---

  ---

  ---

  ---

• How to make globals variables accessible to (all) functions in all program files
  • What you need to do to make global variables accessible to (all) functions in all program files:

    When global variables are defined in some program file: All the global variables must be declared (at the top) in all other program files

    ---

  • Example:

    Suppose a C program consists of 3 program files p1.c, p2.c and p3.c and has 2 globals variables x and y: Then we must declare them in other program files as follows:

    There is an easier way to declare global variables which I will dicuss next

  ---

  ---

  ---

  ---

• Easier way to declare global variables
  • Because you can declare a variable as long as the declaration does not conflict with its definition, it is easier to:

    Declare all global variable in all program files, even when the global variable is defined in that program file....

    Example:

    Suppose a C program consists of 3 program files p1.c, p2.c and p3.c and has 2 globals variables x and y: Then we simply declare all global variables inside all program files as follows:

    Even though the global variable x is defined and declared in the program file p1.c, there is no error as long as the declaration and definition do not conflict.

    The same is true for the global variable y in the program file p2.c

  ---

  ---

  ---

  ---

• Using header files to declare global variables defined inside a program file
  • Previously, we saw this common practice in C programming: ( click here)

    Write a header file prog.h for each (library) C source program file prog.c The header file prog.h will contain: Declaration of all functions that are defined in the C program file Some other stuff that will be discussed later (specifically: user-defined types and global variables ) Include the header file prog.h in every C program file that calls one or more functions defined in the C program file prog.c This is done to declare the functions so the C compiler will not use implicit function prototyping

  ---

  ---

  ---

  ---

• Accessible global variables defined in a different file
  • Suppose we have defined the following global variables in file glob2.c:

    glob2.c: int a, b; float c, d; void print() { printf( "print( )>>> a = %d, b = %d\n", a, b); printf( "print( )>>> c = %f, d = %f\n", c, d); }

    ---

  • We like to use these global variables in another program file:

    glob1.c: int main( int argc, char *argv[] ) { a = 2; b = 3; printf( "a = %d, b = %d\n", a, b); c = 2.0; d = 3.0; printf( "c = %f, d = %f\n", c, d); print( ); // Calls print( ) in glob2.c }

    Then:

    We must first declare the global variables in the program file where they are used The variable declaration is used to inform the C compiler of the data type of the global variables (So the C compiler can perform type conversion !!!)

    We will study how to declare global variables next....

  ---

  ---

  ---

  ---

• Declaring global variables
  • Syntax to declare a global variable in C:

    extern DataType varName ;

    ---

    ---

  • Example:

    File: glob1.c	File: glob2.c
    extern int a, b; extern float c, d; int main( int argc, char *argv[] ) { a = 2; b = 3; printf( "a = %d, b = %d\n", a, b); c = 2.0; d = 3.0; printf( "c = %f, d = %f\n", c, d); print( ); // Calls print( ) in glob2.c }	int a, b; float c, d; void print() { printf( "print( )> a = %d, b = %d\n", a, b); printf( "print( )> c = %f, d = %f\n", c, d); }

    Demo:

    cd /home/cs255000/demo/c/multiple-files gcc -c glob1.c gcc -c glob2.c gcc glob1.o glob2.o a.out a = 2, b = 3 c = 2.000000, d = 3.000000 print( )>>> a = 2, b = 3 print( )>>> c = 2.000000, d = 3.000000

  ---

  ---

  ---

  ---

• Summary
  • Global variables in C:

    A global variable can be defined in any C program file Functions in the same file as the global variable can access the variable !!! If you want to access the global variable in a different file: You must declare the global variable in that C program file

    ---

    ---

  • This sounds tedious...

    But we have a C-preprocessor trick to make thing easy !!!!!