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

Question

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?

Answer 1

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.