I have a very mixed notion of what happens when I compile many files - mostly when it comes to the visibility of things from one file to an other. From what I read, static limits the scope of a variable or function to the file itself. extern does the opposite. From that I would expect to be able and simply read a global extern from any file. That doesn't work in practice though as seen below.
main.c:
#include <stdio.h>
int main(void){
printf("%d\n", b); // b is extern global
return 0;
}
a.c:
static int a = 40;
b.c:
extern int b = 20;
I can't even compile:
> gcc main.c a.c b.c -o test
b.c:1:12: warning: ‘b’ initialized and declared ‘extern’ [enabled by default]
extern int b = 20;
^
main.c: In function ‘main’:
main.c:4:20: error: ‘b’ undeclared (first use in this function)
printf("%d\n", b); // b is extern global
You are doing all wrong.When we writing extern int b it means an integer type variable called b has been declared.And we can do this declaration as many times as needed. (remember that declaration can be done any number of times).By externing a variable we can use the variables anywhere in the program provided we know the declaration of them and the variable is defined somewhere.
The correct way is
main.c:
#include <stdio.h>
extern int b; //declaration of b
int main(void){
printf("%d\n", b); // using b
return 0;
}
b.c:
int b = 20; //definition here
and compile as gcc main.c b.c -o test
I have omitted a.c as it was doing nothing in this example. To understand more about externs see this site.Its having very good content http://www.geeksforgeeks.org/understanding-extern-keyword-in-c/