Use of extern variable in c

Question

I have made two files in c i.e. file1.c file2.c. In file1.c I wrote

#include< stdio.h >                           
 int s=10;                                                                        
 void main()         
 {    
   printf("This is file 1");    
 }

In file2.c

include < stdio.h >                                                             
 extern int s;                                                                
 void main()    {                                                                         
    printf("%d",s);                                                                      
}

When I compiled file2.c in ubuntu terminal I got undefined referenced to s error. How can I resolve this error?

Answer 1

In file1.c

#include <stdio.h>

void myfunction( void );

int s=10;
void myfunction()
{
    printf("This is file 1");
}

In file2.c

#include <stdio.h>

void myfunction( void );
extern int s;

int main( void )
{
    myfunction();
    printf("%d",s);
}

then compile (the example uses gcc

gcc -g -Wall -Wextra -pedantic -Wconversion -std=gnu11 -c file1.c -o file1.o
gcc -g -Wall -Wextra -pedantic -Wconversion -std=gnu11 -c file2.c -o file2.o

then link using:

gcc -g file1.o file2.o -o myexec

then run it as

./myexec

Of course, if your using Visual Studio, the command line statements will be slightly different