Using char in different files in C

Question

I have 3 .c files main.c, fun1.c, fun2.c

char buff[50];//in fun1.c

char *arg;  //in fun2.c

arg = strstr(buff, "001"); //in fun2.c

I want to print buff in fun2.c but it gives an error buff undeclared, even though I declared it in fun1.h as extern char buff[]; There are functions in fun1.c and fun2.c each

I include fun1.h file in fun1.c. Do i have to include it in fun2.c? — saurabh, Feb 17 '17 at 06:11
@saurabh It would be easier if we could roughly see how you are `#include`ing in each of your `.c` files. — RoadRunner, Feb 17 '17 at 06:24
@RoadRunner I declared buff in fun1.h as extern char buff[]; — saurabh, Feb 17 '17 at 06:30
Yes, you shall include `fun1.h` in `fun2.c` (and in all other files using symbols from fun1.c). That is the meaning of `.h` files. — Marian, Feb 17 '17 at 06:42
@RoadRunner ok now i have included fun1.h in fun2.c still the error is displaying [undefined reference to 'buff' ] — saurabh, Feb 17 '17 at 06:50
Any symbol (function, variable) you want to use in a module (file) has to be announced to the compiler. This can be done by including a header with the declaration, or by declaring it explicitly. Note: the compiler knows nothing about any symbol in another module, unless you "tell" about it. — Matthias, Feb 17 '17 at 07:28
See also [this answer](http://stackoverflow.com/a/42135581/841108) to a related question — Basile Starynkevitch, Feb 17 '17 at 08:12
what command are you using for compiling the stuff? How do I get the feeling that you're compiling fun2.c and *linking it* into a single-module program? — Antti Haapala -- Слава Україні, Feb 17 '17 at 09:19

RoadRunner · Accepted Answer · 2017-02-17T08:50:02.867

It is hard to say what is wrong with your particular program, but here is an example which links 2 .c files with one .h file.

1. A header file functions.h:

#include <stdio.h>

extern void func();

Where I use extern to provide definitions for another file.

2. Now, a functions.c file which uses this header file:

#include "functions.h"

void func() {
    printf("hello");
}

This needs to #include the header file, and use the function void() to print a message.

3. Finally, a main.c file which links it all together:

#include <stdio.h>
#include <stdlib.h>
#include "functions.h"

int main(void) {
    func();
    return 0;
}

Which also needs function.h as it uses func(). You then can compile the code as:

gcc -Wall -Wextra -g main.c functions.c -o main

You could also look into makefiles, which would reduce this long compilation line to simply make.

The compilation command should really be `gcc -Wall -g main.c functions.c -o main` — Basile Starynkevitch, Feb 17 '17 at 07:46

Using char in different files in C

1 Answers1