Question

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();

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

void f(int a) exit();

Answer 1

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.