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Scope

Where a program's fingers can and can't reach.

Imagine that a function is a building with a person (Fred) standing in the doorway. This person can see certain things: other people and other buildings, out in the open. But Fred cannot see certain other things, such as the people inside the other buildings. Just so, some variables in a C program, like the people standing outside, are visible to nearly every other part of the program (these are called global variables), while other variables, like the people indoors, are hidden behind the "brick walls" of curly brackets (these are called local variables).

Where a variable is visible to other C code is called the scope of that variable. There are two main kinds of scope, global and local, which stem from the two kinds of places in which you can declare a variable:

  1. Global scope is outside all of the functions, that is, in the space between function definitions -- after the #include lines, for example. Variables declared in global scope are called global variables. Global variables can be used in any function, as well as in any block within that function. 
    #include <stdio.h>

int global_integer;
float global_floating_point;

int main ()
{
  exit (0);
}
  2. You can also declare variables immediately following the opening bracket ({) of any block of code. This area is called local scope, and variables declared here are called local variables. A local variable is visible within its own block and the ones that block contains, but invisible outside its own block. 
    #include <stdio.h>

int main()
{
  int foo;
  float bar, bas, quux;

  exit (0);
}