Question

From C99 Standard:

6.2.1/2

For each different entity that an identifier designates, the
identifier is visible (i.e., can be used) only within a region of
program text called its scope. Different entities designated by the
same identifier either have different scopes, or are in different name
spaces.

I have a confusion with entity and identifier (which I know can be the name of an object). What exactly is an entity according to this quote with example?

Answer 1

Entities are variables, functions, types, etc. Identifiers are names of those entities. For example:

#include <stdlib.h>
#include <stdio.h>

static int foo()
{
    static int x = 0; // "x" is an identifier, the variable "x" inside
                      // this function "foo()" is an entity.
    return x++;
}

int main()
{
    int x = 1; // "x" is an identifier as well, and it is the same name "x"
                   // as of that used inside "foo()". However, this entity
                   // (variable) is not the same as in "foo()", it has different
               // scope and is a totally different entity.
    printf("foo()+x++ = %d\n", foo()+x++);
    printf("foo()+x++ = %d\n", foo()+x++);
    printf("foo()+x++ = %d\n", foo()+x++);
    return EXIT_SUCCESS;
}