Unlike static keyword, extern is used to only declare the variable rather than allocating memory. Just after declaring var, you cannot initialize it, you need to define it as well. In case of function declarations, they are implicitly extern. But, incase of variables, you need to explicitly write extern keyword.

external_vars_defined.c

#include <stdio.h>
#include "../cspec/cspec.c"

// Withour this line, compiler wont compile your code.
// error: `some_var_declared_somewhere` undeclared (first use in this function)
extern int some_var_declared_somewhere;

void test_runner(void);

void main(){
 printf("value of some_var_declared_somewhere is %d", some_var_declared_somewhere);
}

void test_runner(void){

}

// This var has no visibility in main() function
// because, its defined below main() function; so compiler has no idea
//  where and what it is. However, we can tell compiler that the var is
//  defined somewhere in some of the source files; dont worry, for now compile the code.
int some_var_declared_somewhere = 10;