19.4 Blocks

A block is a construct that contains multiple statements of any kind. It begins with ‘{’ and ends with ‘}’, and has a series of statements and declarations in between. Another name for blocks is compound statements.

Is a block a statement? Yes and no. It doesn’t look like a normal statement—it does not end with a semicolon. But you can use it like a statement; anywhere that a statement is required or allowed, you can write a block and consider that block a statement.

So far it seems that a block is a kind of statement with an unusual syntax. But that is not entirely true: a function body is also a block, and that block is definitely not a statement. The text after a function header is not treated as a statement; only a function body is allowed there, and nothing else would be meaningful there.

In a formal grammar we would have to choose—either a block is a kind of statement or it is not. But this manual is meant for humans, not for parser generators. The clearest answer for humans is, “a block is a statement, in some ways.”

A block that isn’t a function body is called an internal block or a nested block. You can put a nested block directly inside another block, but more often the nested block is inside some complex statement, such as a for statement or an if statement.

There are two uses for nested blocks in C:

• To specify the scope for local declarations. For instance, a local variable’s scope is the rest of the innermost containing block.
• To write a series of statements where, syntactically, one statement is called for. For instance, the execute-if-true of an if statement is one statement. To put multiple statements there, they have to be wrapped in a block, like this:

  if (x < 0)
    {
      printf ("x was negative\n");
      x = -x;
    }
  

This example (repeated from above) shows a nested block which serves both purposes: it includes two statements (plus a declaration) in the body of a while statement, and it provides the scope for the declaration of q.

void
free_intlist (struct intlistlink *p)
{
  while (p)
    {
      struct intlistlink *q = p;
      p = p->next;
      free (q);
    }
}