Suppose we implement a function library in C....
 

A sample function library that is implemented using global variables and functions in C: 

 // C program that implements a function library

 int   var1;
 float var2;

 int f1( int x, float y )
 {
    .... // body is not important in this discussion
 }

 int f2( int A[], int i, int j )
 {
    .... // body is not important in this discussion
 }

Question:   how can other C functions use these (global) variables and functions ???

What we must do to use the library functions

We must declare the global variables and functions in the function library before we use them: 

 // Declaration needed to 
 // use global variables and functions in library 
 extern  int   var1;              
 extern  float var2;		  

 int f1( int x, float y );        
 int f2( int A[], int i, int j ); 

 // Functions that use the subsystem
 int main( ... )
 {
    int a, b;

    var1 = 1234;
    ...
    f1( a, b );
    ...
 }

Unlike Java, we must provide the declaration information by ourselves in C !!!

How to provide declarations using "header" files in C
 

C uses header files as the mechanism to provide the declarations information:

  • Header file = a file that is included into a C program file using the #include command

  • The #include command usually appears at the start (= head) of a C program file

  • Hence, the include file is called a "header" file

How to make a header file for a C program file
 

Every C program file has a header file that contains the declarations of the global variables and functions defined in the C program file:

progFile.c progFile.h 
 int   var1;
 float var2;

 int f1( int x, float y )
 {
    .... // body 
 }

 int f2( int A[], int i, int j )
 {
    .... // body
 }  
 // Declaration:
 extern  int   var1;
 extern  float var2;

 int f1( int x, float y );
 int f2( int A[], int i, int j );

  You (the programmer) has to create the header file yourself.... (yes, it's tedious...)

How to use a header file
 

We can now declare the global variables and the functions using an #include command: 

 // Declaration:
 #include "progFile.h" 

 // Functions that use the subsystem
 int main( ... )
 {
    int a, b;

    var1 = 1234;
    ...
    f1( a, b );
    ...
 }

Note: the "...." quotes tells the C compiler to search the current directory for the included file

Example: implementation of 1 stack in C

The C program that implements one stack of int: 

#include <stdio.h>

int stackTop = -1;
int stackData[100];

void push(int x)
{
   // check if stack is not full. 
   if ( stackTop < 100 )
   {
      stackData[++stackTop] = x;
   }
}

int pop( )
{
   // check if stack is not empty
   if ( stackTop > -1 )
   {    
      return( stackData[stackTop--] );
   }

   return (-999999); // Pop empty stack error
}

Example: implementation of 1 stack in C

The header file stack.h is: 


extern int stackTop;
extern int stackData[100];

void push(int x);
int pop( );

Example: implementation of 1 stack in C

A C program that want to use this stack implemention only need to #include "stack.h to declare the (global) variables and functions in the stack.c program: 

 #include <stdio.h>

 #include "stack.h"  

 int main(int argc, char *argv[])
 {
    push(1); push(4); push(7); push(9);

    printf("%d\n", pop());
    printf("%d\n", pop());
    printf("%d\n", pop());
    printf("%d\n", pop());
    printf("%d\n", pop());
 }

Postscript
 

  • Later we will learn how to define new data types in C

  • The new data type definitions are also included in the header file