Header file and extern keyword

Question

I am having a lot of issue using extern variable and header files. I have read through sections of books and searched the web for hours but I haven't been able to figure out. Any help in understanding this problem will be greatly appreciated. The following is the code and the error when I try to compile

 #include <stdio.h> #include <stdlib.h> #include <string.h> #include "sample.h" int main() { int i; int gI = 0; double recv; i = 10; gI = i; recv = AnotherFunc(); return 0; }

And the sample.h is the following

 #ifndef SAMPLE #define SAMPLE extern int gI; extern double AnotherFunc(); #endif

And the other is function is

 #include <math.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include "sample.h" double AnotherFunc() { double someVar; int test; test = gI; someVar = 10.0; return someVar; }

When I compile the following way, I get the following errors which I don't understand why I am getting those errors. sample.h has the variable declaration and it should be visible in the AnotherFunc.

 gcc -Wall -ansi -pedantic -c Main.c AnotherFunc.c gcc Main.o AnotherFunc.o -o test AnotherFunc.o: In function `AnotherFunc': AnotherFunc.c:(.text+0x6): undefined reference to `gI' collect2: ld returned 1 exit status

I only added the int gI = 0; because I wanted to define it. If I modify the code in the following way I get error in main also. Please see below.

 #include <stdio.h> #include <stdlib.h> #include <string.h> #include "sample.h" int main(int argc, char *argv[]) { int i; double recv; i = 10; gI = i; recv = AnotherFunc(); return 0; } gcc -Wall -Wstrict-prototypes -ansi -pedantic -c Main.c AnotherFunc.c gcc Main.o AnotherFunc.o -o test Main.o: In function `main': Main.c:(.text+0x1b): undefined reference to `gI' AnotherFunc.o: In function `AnotherFunc': AnotherFunc.c:(.text+0x6): undefined reference to `gI' collect2: ld returned 1 exit status

You have defined gI as local to main() - you need to declare it outside of main(), at global level. — Andrew
– Andrew, Commented Jul 12, 2016 at 6:05

Manoj Awasthi · Accepted Answer · 2014-02-26 18:12:42Z

3

Move int gI = 0 outside main() so that it is globally available:

int gI = 0; int main() { int i; .... }

answered Feb 26, 2014 at 18:12

Manoj Awasthi

3,5202 gold badges25 silver badges26 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Samuel Edwin Ward · Accepted Answer · 2014-02-26 18:30:36Z

You need to declare your variables before using them, and define them exactly once.

This is a declaration:

extern int gI;

Basically this just says that there is an int named gI that will be defined elsewhere.

This is a definition:

int gI;

This actually creates an int named gI. (Technically it is a declaration as well as a definition.)

At the moment you have an int gI line inside of your main function, but this is just a shadowing definition. It is a local variable whose name happens to be the same as the declared global gI, but it is not the global gI. So you have a problem where you declared a variable (the global gI) and defined it zero times.

If you were to put int gI in your sample.h file, it would then be included in both of your .c files. This would also be a problem because the rule is to define variables exactly once, and you will have defined it twice (once in each file).

The solution is to place the extern declaration in the .h file, and the definition in one of your .c files.

Community · Accepted Answer · 2017-05-23 12:16:25Z

1

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;

check this link. well explained How do I use extern to share variables between source files?

edited May 23, 2017 at 12:16

CommunityBot

11 silver badge

answered Feb 26, 2014 at 18:23

tesseract

9011 gold badge7 silver badges16 bronze badges

2 Comments

user3357091 Over a year ago

I understand and that's why I included the sample.h in AnotherFunc. But I get the error saying undefined reference

tesseract Over a year ago

extern keyword declares a variable, but doesn't define it, you should define the variable in 1 .c file and use the extern in the header.

Albatropy · Accepted Answer · 2014-02-26 18:14:41Z

You defined gI inside the scope of the main() function, which makes it visible only from there. I suspect what you really wanted is a global gI variable (hence the extern int gI declaration).

If you want to have AnotherFunc() see it, move int gI = 0 outside, for instance in the same file than the definition of AnotherFunc().

Wouldn't including sample.h serve the purpose of declaring a variable outside the scope of main. Please look at the following code and error when I compile

Fiddling Bits · Accepted Answer · 2014-02-26 18:49:37Z

gI must be declared above main:

int gI = 0; int main(void) { ... }

By doing this gI has file scope and external linkage.

Perhaps a better place to declare gI would be in sample.c, if there is such a file.

Collectives™ on Stack Overflow

Header file and extern keyword

5 Answers 5

Comments

Comments

2 Comments

1 Comment

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

2 Comments

1 Comment

Comments

Linked

Related