How to declare global variables and functions defined in a C program file with ease

Programming practice of C programmers:

For each C program file named myProgFile.c:
The header file myProgFile.h contains the declarations of:

#Include myProgFile.h in:

myProgFile.c (to verify the declarations in myProgFile.h) and
Any C program file that uses a global variable or a function defined in myProgFile.c

Example using header file to declare global item in a C program file

Suppose we have a global variable someGlobalVar and 3 functions defined in program file func3.c:

`func3.c`	`func3.h`
*double someGlobalVar = 4; int f( float x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*

Example using header file to declare global item in a C program file

We first create a header file containing the declarations of all the global variables and all the functions:

func3.c func3.h

`func3.c`	`func3.h`
*double someGlobalVar = 4; int f( float x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*	extern double someGlobalVar; int f( float x ); float g( int x ); float h( float x );

double someGlobalVar = 4; int f( float x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }

extern double someGlobalVar; int f( float x ); float g( int x ); float h( float x );

Example using header file to declare global item in a C program file

It is a good practice to avoid a recursive include chain by using the ifndef "trick" (see )

func3.c func3.h

`func3.c`	`func3.h`
*double someGlobalVar = 4; int f( float x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*	#ifndef func3_h #define func3_h extern double someGlobalVar; int f( float x ); float g( int x ); float h( float x ); #endif

double someGlobalVar = 4; int f( float x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }

#ifndef func3_h #define func3_h extern double someGlobalVar; int f( float x ); float g( int x ); float h( float x ); #endif

Example using header file to declare global item in a C program file

Always #include the header file in the source C program file:

func3.c func3.h

`func3.c`	`func3.h`
*#include "func3.h" double someGlobalVar = 4; int f( float x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*	#ifndef func3_h #define func3_h extern double someGlobalVar; int f( float x ); float g( int x ); float h( float x ); #endif

#include "func3.h" double someGlobalVar = 4; int f( float x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }

#ifndef func3_h #define func3_h extern double someGlobalVar; int f( float x ); float g( int x ); float h( float x ); #endif

Reason: this practice allows the C compiler to verify that the declarations and the definitions are consistent

Example using header file to declare global item in a C program file

Example declation verification: suppose we change the function definition of f(float x) to f(double x):

func3.c func3.h

`func3.c`	`func3.h`
*#include "func3.h" double someGlobalVar = 4; int f( double x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*	#ifndef func3_h #define func3_h extern double someGlobalVar; int f( float x ); // Wrong !! float g( int x ); float h( float x ); #endif

#include "func3.h" double someGlobalVar = 4; int f( double x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }

#ifndef func3_h #define func3_h extern double someGlobalVar; int f( float x ); // Wrong !! float g( int x ); float h( float x ); #endif

The C compiler will detect the error when you re-compile func3.c

DEMO: demo/C/set4/func3.c

Example using header file to declare global item in a C program file

Suppose a C program file main3.c uses some global variable or function defined in func3.c:

func3.c main3.c

`func3.c`	`main3.c`
*#include "func3.h" double someGlobalVar = 4; int f( double x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*	#include <stdio.h> int main( int argc, char *argv[]) { short a = 2; int b; printf("someGlobalVar = %f\n", someGlobalVar); b = f( a ); printf("f(%d) = %d\n", a, b); b = g( a ); printf("g(%d) = %d\n", a, b); }

#include "func3.h" double someGlobalVar = 4; int f( double x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }

#include <stdio.h> int main( int argc, char *argv[]) { short a = 2; int b; printf("someGlobalVar = %f\n", someGlobalVar); b = f( a ); printf("f(%d) = %d\n", a, b); b = g( a ); printf("g(%d) = %d\n", a, b); }

Example using header file to declare global item in a C program file

We can easily declare all global variables and functions defined in func3.c using an #include directive:

func3.c main3.c

`func3.c`	`main3.c`
*#include "func3.h" double someGlobalVar = 4; int f( double x ) { return(xx); } float g( int x ) { return(xxx); } float h( float x ) { return(xxxx); }*	#include <stdio.h> #include "func3.h" int main( int argc, char *argv[]) { short a = 2; int b; printf("someGlobalVar = %f\n", someGlobalVar); b = f( a ); printf("f(%d) = %d\n", a, b); b = g( a ); printf("g(%d) = %d\n", a, b); }

#include "func3.h" double someGlobalVar = 4; int f( double x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }

#include <stdio.h> #include "func3.h" int main( int argc, char *argv[]) { short a = 2; int b; printf("someGlobalVar = %f\n", someGlobalVar); b = f( a ); printf("f(%d) = %d\n", a, b); b = g( a ); printf("g(%d) = %d\n", a, b); }

DEMO: demo/C/set4/main3.c + func3.c