declare extern variable within a C function?

Question

I define a variable in a C file: int x, and I know I should use extern int x to declare it in other files if I want to use it in other files.

My question is: where should I declare it in other files?

1. Outside of all functions,

   // in file a.c:
   int x;
   
   // in file b.c:
   extern int x;
   void foo() { printf("%d\n", x); }
   

2. within the function(s) that will use it?

   // in file b.c:
   void foo() {
      extern int x;
      printf("%d\n", x);
   }
   

My doubts are:

• Which one is correct?, or
• Which is preferred if both are correct?

Answer 1

1. Both are correct.

2. Which one is preferred depend on the scope of variable's use.

   • If you only use it in one function, then declare it in the function.

     void foo() 
     {
          extern int x;   <--only used in this function.
          printf("%d",x);   
     }
     

   • If it is used by more than one function in file, declare it as a global value.

     extern int x;   <-- used in more than one function in this file
     void foo()
     {
         printf("in func1 :%d",x);   
     }    
     void foo1() 
     {
         printf("in func2 :%d",x);   
     }  
     

Answer 2

Suppose if you declare within function:

// in file b.c:
void foo() {
    extern int x;
    printf("%d\n", x);
}
void foo_2() {
    printf("%d\n", x);  <-- "can't use x here"
}

then x is visible locally inside function foo() only and if I have any other function say foo_2(), I can't access x inside foo_2().

Whereas if you declares x outside before all function then it would be visible/accessible globally in complete file (all functions).

  // in file b.c:
  extern int x;
  void foo() { printf("%d\n", x); }
  void foo_2() { printf("%d\n", x); }  <--"visible here too"

So if you need x in only single function then you can just declare inside that function but if x uses in several function then declare x outside all function (your first suggestion).