Question

I declared a variable i in temp2.h
extern i; which contains just one above line
and made another file
temp3.c

 #include<stdio.h>
#include<temp2.h>
int main ()
{
extern i;
i=6;
printf("The i is %d",i);
}

When I compiled above as
cc -I ./ temp3.c I got following errors

 /tmp/ccJcwZyy.o: In function `main':
temp3.c:(.text+0x6): undefined reference to `i'
temp3.c:(.text+0x10): undefined reference to `i'
collect2: ld returned 1 exit status

I had declared extern in temp3.c above as K R page 33 says as I mentioned in above post.
I tried another way for temp3.c with same header file temp2.h

 #include<stdio.h>
#include<temp2.h>
int main ()
{

i=6;
printf("The i is %d",i);
}

and compiled it cc -I ./ temp3.c and got following error

/tmp/ccZZyGsL.o: In function `main':
temp3.c:(.text+0x6): undefined reference to `i'
temp3.c:(.text+0x10): undefined reference to `i'
collect2: ld returned 1 exit status

I also tried

 #include<stdio.h>
#include<temp2.h>
int main ()
{
extern i=6;
printf("The i is %d",i);
} 

compiled this one

 cc -I ./ temp3.c

got same error as in post 1

 temp3.c: In function ‘main’:
temp3.c:5: error: ‘i’ has both ‘extern’ and initializer

So I have tried at least 3 different ways to use extern but non of them worked.

Answer 1

When you declare a variable using extern , you are telling the compiler that the variable was defined elsewhere and the definition will be provided at the time of linking. Inclusion is a different thing altogether.

extern

An external variable must be defined, exactly once, outside of any function; this sets aside storage for it. The variable must also be declared in each function that wants to access it; this states the type of the variable. The declaration may be an explicit extern statement or may be implicit from context.

-The C Programming Language

A variable must be defined once in one of the modules(in one of the Translation Units) of the program. If there is no definition or more than one, an error is produced, possibly in the linking stage (as in example 1 and 2).

Try something like the following

a.c

int i =10; //definition

b.c

extern int i; //declaration
int main()
{
    printf("%d",i);
}

Compile, link and create an executable using

gcc a.c b.c -o executable_name

or

---

gcc -c a.c // creates a.o
gcc -c b.c // creates b.o

Now link the object files and create an executable

gcc a.o b.o -o executable_name