How do I share a global variable between c files?

Question

If I define a global variable in a .c file, how can I use the same variable in another .c file?

file1.c:

#include<stdio.h> int i=10; int main() { printf("%d",i); return 0; }

file2.c:

#include<stdio.h> int main() { //some data regarding i printf("%d",i); return 0; }

How can the second file file2.c use the value of i from the first file file1.c?

possible duplicate of How do I share variables between different .c files? — 7hi4g0
– 7hi4g0, Commented Jun 3, 2015 at 6:16
Possible duplicate of How do I use extern to share variables between source files in C? — user719662
– user719662, Commented Jun 3, 2016 at 5:41

Rocky Pulley · Accepted Answer · 2011-07-22 16:20:04Z

105

file 1:

int x = 50;

file 2:

extern int x; printf("%d", x);

answered Jul 22, 2011 at 16:20

Rocky Pulley

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6 Comments

Rocky Pulley Over a year ago

no, i didn't forget anything. I'm just explaining what you would put in those files.

Rocky Pulley Over a year ago

also, file 1 should NOT be a header file, it should be a .c file. File 2 can be a header file which contains the "extern int x" part though.

Steve Jessop Over a year ago

@fresh_graduate: Putting the line extern int x; in a header file, and including that file from file2.c, has exactly the same effect as putting the line in file2.c. It's up to you how you organise your code, all you need is that after the preprocessor has finished with includes and everything else it does, file 2 has that line in it.

peter_perl Over a year ago

@steve..i can accept your comment as an answer.Knowing about extern,i can find it anywhere in the internet easily but was not able to understand the inclusion of files as how it is done.your comment above cleared everything thanks.Since i cannot accept a comment as an answer +1 for your comment.Nobody has clearly touched the point you mentioned here

Paul G. Over a year ago

Can I do the same with an array?

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mdm · Accepted Answer · 2011-07-22 16:22:05Z

5

Use the extern keyword to declare the variable in the other .c file. E.g.:

extern int counter;

means that the actual storage is located in another file. It can be used for both variables and function prototypes.

answered Jul 22, 2011 at 16:22

mdm

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2 Comments

Platinum Azure Over a year ago

Just for my education, could I see an example of a function prototype declaration using extern? Is it just as simple as putting extern before the prototype? (e.g., extern void doSomething(int, char *);)

Michael Burr Over a year ago

@Platinum Azure: in C a function declaration is extern by default. But it hurts nothing to add the extern explicitly, and it would be done just like in your example.

Murali VP · Accepted Answer · 2011-07-22 16:20:24Z

2

using extern <variable type> <variable name> in a header or another C file.

answered Jul 22, 2011 at 16:20

Murali VP

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2 Comments

peter_perl Over a year ago

but how should i include the files?is there no need for it?

Asha Over a year ago

@fresh_graduate: You need not include the file, linker will figure it out for you.

bharaniprakash tumu · Accepted Answer · 2015-11-26 12:48:40Z

If you want to use global variable i of file1.c in file2.c, then below are the points to remember:

main function shouldn't be there in file2.c
now global variable i can be shared with file2.c by two ways:
a) by declaring with extern keyword in file2.c i.e extern int i;
b) by defining the variable i in a header file and including that header file in file2.c.

For 2. above, if you have two .c files and include the header, how many copies of the variable will there be in your program?

Asha · Accepted Answer · 2011-07-22 16:21:01Z

1

In the second .c file use extern keyword with the same variable name.

answered Jul 22, 2011 at 16:21

Asha

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Comments

Matt Sieker · Accepted Answer · 2011-07-22 22:33:17Z

Do same as you did in file1.c In file2.c:

#include <stdio.h> extern int i; /*This declare that i is an int variable which is defined in some other file*/ int main(void) { /* your code*/

If you use int i; in file2.c under main() then i will be treated as local auto variable not the same as defined in file1.c

Kiran Padwal · Accepted Answer · 2015-11-08 17:13:45Z

1

Use extern keyword in another .c file.

answered Nov 8, 2015 at 17:13

Kiran Padwal

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Comments

Tekshan Madhawa · Accepted Answer · 2023-09-02 04:48:48Z

1

use extern keyword in second while defining variable value of first c file. //in first file double z =50;

//in second file extern double x;

answered Sep 2, 2023 at 4:48

Tekshan Madhawa

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2 Comments

David C. Rankin Over a year ago

Put ```c (3 back-ticks and 'c') above your block of code and ``` below it to format in a fixed font with c syntax highlight. (or indent all by 4-spaces). For code within a sentence, wrap in a single back-tick.

Community Over a year ago

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