A commonly used C preprocessor trick to handle definition and declaration of global variables

What do we want to achieve ???

Suppose we have a (one) global variable x
We want to achieve the following:

Schematically:

Important note:

It does not matter in which file the global variable is defined
All that matters is:

Recall: #ifndef of the C pre-processor

#ifndef:

The #ifndef conditional clause:
the text segment is kept (= included) when the macro name MACR-NAME is not defined
The text segment is omitted otherwise (i.e.: when the macro name MACR-NAME is defined)

A programming trick to manage definitions/declarations of global variables used in multiple C program files

Recall: header file for function declarations

We should create a header file prog.h containing the function declarations of all functions in the C program file prog.c

Example:

File: glob2.c File: glob2.h

File: `glob2.c`	File: `glob2.h`
int a, b; float c, d; void print() { printf( ">> a = %d, b = %d\n", a, b); printf( ">> c = %f, d = %f\n", c, d); }	// Prevent recursive include..... #ifndef h_glob_h #define h_glob_h void print(); // Function declaration #endif

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

// Prevent recursive include..... #ifndef h_glob_h #define h_glob_h void print(); // Function declaration #endif

Programming trick to maintain global variables used in a C program file glob2.c:

Move all the global variables into the corresponding header file using the following construct:

#ifndef EXTERN #define EXTERN extern #endif EXTERN type1 globalvar1; EXTERN type2 globalvar2; ....

Example:

File: glob2.c File: glob2.h

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

// Prevent recursive include..... #ifndef h_glob_h #define h_glob_h #ifndef EXTERN #define EXTERN extern #endif EXTERN int a, b; EXTERN float c, d; void print(); // Function declaration #endif

Include the header file glob2.h in every C program file
(You should have done this already to declare the functions !!! I just state this for emphasis)

In one of the C program files (typically, the program file containing the main( ) function) , add this definition at the start of the file:

#define EXTERN // This defines EXTERN as the empty string)

(This defines EXTERN as the empty string).

Example:

File: glob1.c File: glob2.c

#define EXTERN #include "glob2.h" 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( ); // Call print( ); }

#include "h-glob2.h" void print() { printf( "a = %d, b = %d\n", a, b); printf( "c = %f, d = %f\n", c, d); }

Summary:

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.

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:
and the C pre-processor will replace EXTERN with extern !!!!

Result:

The global variables are declared inside all other program files.

Demo:

cd /home/cs255000/demo/c/multiple-files gcc -E h-glob1.c # 10 #include "h-glob2.h" got processed to: int a, b; float c, d; void print( ); gcc -E h-glob2.c # 10 #include "h-glob2.h" got processed to: extern int a, b; extern float c, d; void print( );