How does a compiler know that a variable is global or local(C )

Question

I want to know that what happens inside the compiler... like whether it store global variable at different location.

Get yourself a good book on basics of compiler. Might get some resources here - http://stackoverflow.com/questions/1669/learning-to-write-a-compiler — CCoder, Nov 12 '12 at 11:07
Get yorself any book on C or C++. Got the the chapter about *scope*. — Benjamin Bannier, Nov 12 '12 at 11:07
@honk a variable can be a global but not be in the global scope... scope and lifetime are different. (assuming that's what you meant) — Luchian Grigore, Nov 12 '12 at 11:08
@LuchianGrigore: I was pretty sure that random book would mention `static` near that spot, but yes, my comment wasn't too helpful. — Benjamin Bannier, Nov 12 '12 at 11:24

score 3 · Answer 1 · edited Jun 20 '20 at 09:12

The Wikipedia page on symbol tables can provide you a basic understanding.

http://en.wikipedia.org/wiki/Symbol_table

In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and sometimes its location.

[...]

A common implementation technique is to use a hash table implementation. A compiler may use one large symbol table for all symbols or use separated, hierarchical symbol tables for different scopes.

Emphasis mine.

score 0 · Answer 2 · answered Nov 12 '12 at 11:07

0

It knows that the variable is global or local by how you declare it.

//declared at namespace scope - global
extern int x;     

int main()
{
   //declared inside a method - local
   int y;
};

answered Nov 12 '12 at 11:07

Luchian Grigore

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score 0 · Answer 3 · answered Nov 12 '12 at 11:13

Generally there are 4 scopes of any variables. Using extern keyword you are making that variable extern explicitly.(by default global variables are extern)

Using staticlimiting the variable or function scope to it's current file

In accordance of that the memory are allocated in different segments

        global: visible in more than one source file
 -- data segement(also differs whether initialised or uninitialized)


        local : visible with in { } it also called function(){} scope
 -- on stack 


        block : {} inside any function another scope of block valiables with in {}

     -- on stack if with in function 


        file : making static variable is limited to it's file scope or 
    current translation unit. -- again data section

How does a compiler know that a variable is global or local(C )

3 Answers3