4.8 Block Structure

C is not a block-structured language in the sense of Pascal or similar languages, because functions may not be defined within other functions. On the other hand, variables can be defined in a block-structured fashion within a function. Declarations of variables (including initializations) may follow the left brace that introduces any compound statement, not just the one that begins a function. Variables declared in this way hide any identically named variables in outer blocks, and remain in existence until the matching right brace. For example, in

 

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   if (n > 0) {
       int i;  /* declare a new i */

       for (i = 0; i < n; i++)
           ...
   }

the scope of the variable i is the ``true'' branch of the if; this i is unrelated to any i outside the block. An automatic variable declared and initialized in a block is initialized each time the block is entered.

Automatic variables, including formal parameters, also hide external variables and functions of the same name. Given the declarations

 

   int x;
   int y;

   f(double x)
   {
       double y;
   }

then within the function f, occurrences of x refer to the parameter, which is a double; outside f, they refer to the external int. The same is true of the variable y.

As a matter of style, it's best to avoid variable names that conceal names in an outer scope; the potential for confusion and error is too great.