Syntax: Single statement in function declaration

In the C programming language, it is possible to omit a code block in the case of a single statement, for example:

if(1) exit();开发者_开发技巧

Now, does this only apply to conditionals ? Why is this not valid in the case of functions:

void f(int a) exit();

It's a feature of C syntax. In BNF, a function definition is something like

FUNC_DEF ::= TYPE IDENTIFIER "(" PARAM_LIST ")" BLOCK

while a statement is

STATEMENT ::= (EXPRESSION | DECLARATION | CONTROL | ) ";" | BLOCK
BLOCK ::= "{" STATEMENT* "}"

(simplifying to allow intermixed declarations and statements, which C++ allows but C doesn't), and an if statement is

CONDITIONAL ::= "if" "(" EXPRESSION ")" STATEMENT

omitting the else part for now.

The reason for this is that otherwise, you could write the function

void no_op() {}

as

void no_op();

but the latter syntax is already in use to denote a declaration.

• The syntax of a conditional statement is this:

  if(expression) statement
  

• A compound statement is a statement.

• A compound statement is defined as

  { zero or more statements }
  

• The syntax of a function definition is this

   function_declaration compound_statement
  

• So, by definition a function body must be a compound statement and have {}

• QED :)

There is a very old dialect of C, the K&R C. In this dialect the function declaration may look like this:

fun_a(a,b)
char a;
float b;
{
        fun_b(b,a);
}

I think it would be too hard to parse it without { and }.