Clarify to understand C variables

Question

I am a newbie and trying to understand program in C. In the program, there is one header file typedef.h which declares typedef struct { int i; char b;} rdi;

Then there are 3 folders and each folder has several programs that do a particular task and each folder has one header file. In the header file of each 3 folders it has

#include typfedef.h 

and then declared

extern rdi *rdi_x;

In all 3 folders it did it. But I do not see it any where in the program defined:

  rdi rdi_x;

My question is whether it is not required to define rdi rdi_x; in the program.

As I understand from c text books, somewhere in the program it needs to define rdi rdi_x; or my understanding is not good.

Answer 1

As I understand from c text book, somewhere in the program it needs to define rdi rdi_x; or my understanding is not good.

You need to define rdi_x only if it is used. Just declaring the variable does not require that it be defined.

Take a much simpler case of one .c file:

Program that fails to build:

extern int i;
extern int j;
extern int k;

int main()
{
   k = 10;
}

This program will fail to build since k is used. If we provide the definition of just k, it will build just fine.

Program that builds successfully:

extern int i;
extern int j;
extern int k;

int main()
{
   k = 10;
}

int k;

Here, i, and j are declared but not used. Hence, they need not be defined.

Answer 2

Firstly, the #include in the three source files will be #include "typfedef.h", not #include typfedef.h. The double quotes are required.

Second, extern rdi *rdi_x is a declaration, but not a definition. It can be repeated in any number of source files (or in header files), and will never result in rdi_x being defined.

It is necessary to also create a corresponding definition, for example providing the same declaration without the extern keyword, viz.

 rdi *rdi_x;

Note the * is still present. Your description incorrectly suggests that a definition rdi rdi_x; (leaving out the *) is required.

Bear in mind that, formally, a definition is a type of declaration. The extern rdi *rdi_x is a declaration but not a definition - which is why it can be repeated. The declaration rdi *rdi_x is a corresponding definition - and is subject to the "one definition rule" - in other words, such a definition must be supplied once (and only once) among all compilation units in a project.