CODEWITHSUNDEEP
c
LambdaG
LambdaG

Reputation: 148

How to use "extern struct" to share variables in c programming and compile with gcc?

I want some shared variables should be accessed among source files, main.c and second.c and my header file is all.h defined the shared data type,

#ifndef ALL_H
#define ALL_H
struct foo {
    double v;
    int i;
};

struct bar {
    double x;
    double y;
};
#endif

main.c is given below

/* TEST*/
#include "all.h"
#include "second.h"

int main(int argc, char* argv[])
{
    struct foo fo; // should be accessed in second.c
    fo.v= 1.1;
    fo.i = 12;

    struct bar ba; // should be accessed in second.c
    ba.x= 2.1;
    ba.y= 2.2;

    sec(); // function defined in second.c

    return 0;
}

second.h is given below

#include <stdio.h>
#include "all.h"

int sec();

second.c is given below

#include "second.h"

extern struct foo fo;
extern struct bar ba;

int sec()
{
    printf("OK is %f\n", fo.v+ba.x);

    return 0;
}

I thought i have all the declaration and include the headers. But when i compile 

    gcc -o main main.c second.c 

or 

    gcc -c second.c
    gcc -c main.c
    gcc -o main main.o second.o

It will give some error like 

second.o: In function `sec':
second.c:(.text+0x8): undefined reference to `fo'
second.c:(.text+0xe): undefined reference to `ba'
collect2: ld returned 1 exit status

I think somewhere of the use of extern was wrong or i use the gcc incorrectly?

Upvotes: 5

Views: 68415

Answers (3)

AlphaGoku
AlphaGoku

Reputation: 1020

main.h:

typedef struct
{
    double v;
    int i;
} Foo;

extern.h:

extern Foo fo; // "fo is a global variable stored somewhere else"

main.c:

#include "main.h"
#include "extern.h"
//use fo.v here

second.c:

#include "second.h"
#include "main.h"
#include "extern.h"
Foo fo; // "store the data of fo here"
//use fo.v here

Just include #include "main.h" and #include "extern.h" in all .c files you want to use fo. Notice that Foo fo is only in second.c, nowhere else.

Upvotes: 3

cdhowie
cdhowie

Reputation: 169191

The error message indicates that the linker is unable to find fo and ba. With the extern declaration you have told the compiler that the variables will exist in some other translation unit, but they don't.

You need to move the struct foo fo; and struct bar ba; outside of the main() function. Right now, they are function local variables. They need to be global variables for this to work.

Upvotes: 3

Jay
Jay

Reputation: 24905

The problem is with the scope. Your variables (fo & ba) have local scope as they are declared within main.So, their visibility is restricted to within main function. Please make them global variables and it should work.

Upvotes: 7

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