- The global variables in a
multi-files
C program are
organized as
follows:
- Each global variable
must be defined inside
exactly one of the files
- Each global variable must be declared inside every C program files
- Each global variable
must be defined inside
exactly one of the files
- A simple way to achieve this is:
- Define
each global variable inside
one of the C program files
- Declare each global variable inside every C program files (except in the one in which they have been defined)
- Define
each global variable inside
one of the C program files
- Schematically:
- In order to understand how to organize the definitions and declarations of the global variables in a C program with ease, we must first learn about a new feature of the C pre-processor.
- Conditional clause of
the C pre-processor:
- The C pre-processor has a conditional clause that tells the C processor whether it should skip the processing of a portion of the C program file
- The C pre-processor has the following
3 conditional clauses:
- #if
- #ifdef
- #ifndef
- #ifndef:
- The #ifndef
conditional is similar to
the #ifdef, except
the meaning is
reversed:
#ifndef MACRO-NAME text segment #endifthe text segment is kept (= included) when the macro name MACR-NAME is not defined
The text segment is omitted otherwise (when the macro name MACR-NAME is defined)
- The #ifndef
conditional is similar to
the #ifdef, except
the meaning is
reversed:
- Recal, we
want make it
easy to
achieve this simple way to
organize
definitions/declarations
of global variables:
- Define
each global variable inside
one of the C program files
- Declare each global variable inside every C program files (except in the one in which they have been defined)
- Define
each global variable inside
one of the C program files
- Schematically:
- Programming trick:
- Create a header file containing
all the global variables
as follows:
#ifndef EXTERN #define EXTERN extern #endif EXTERN type1 globalvar1; EXTERN type2 globalvar2; ....
- Include the
header file vars.h in
every C program file
that make use of the
global variables
- In one of the
C program files, add this definition
at the start of the file:
#define EXTERN(This defines EXTERN as the empty string).
Example:
- Create a header file containing
all the global variables
as follows:
- How the trick works:
- In the one
program file that
contains the
definition:
#define EXTERN
the content of #include "vars.h" will be processed as follows:
EXTERN double x; ⇒ double x; EXTERN int y; ⇒ int y;
Because:
- The macro EXTERN was
defined (as the empty string)
- Hence, the #ifndef EXTERN will be skipped and the C pre-processor will replace EXTERN with the empty string in the rest of the file
Result:
- The global variables are defined inside this one program file.
- The macro EXTERN was
defined (as the empty string)
- In all other
program files
the content of
#include "vars.h" will be
processed as follows:
EXTERN double x; ⇒ extern double x; EXTERN int y; ⇒ extern int y;
Because:
- The macro EXTERN is
not defined
- Hence, the #ifndef EXTERN
will be keep the
then portion and the
header file vars.h becomes:
#define EXTERN extern EXTERN double x; EXTERN int y;and the C pre-processor will replace EXTERN with extern !!!!
Result:
- The global variables are declared inside all other program files.
- The macro EXTERN is
not defined
- In the one
program file that
contains the
definition:
- Example Program:
(Demo above code)
- The main.c Prog file: click here
- The help.c Prog file: click here
- The vars.h header file: click here
How to run the program:
- Right click on links and
save in a scratch directory
- To compile:
gcc -c main.c gcc -c help.c gcc main.o help.o
- To run: ./a.out