Where does the C standard define the else if statement?

Question

I was just curious about the else if statement in C so I looked at the C99 standard and found nothing. Then I looked at the grammar but again no else if

selection_statement
    : IF '(' expression ')' statement
    | IF '(' expression ')' statement ELSE statement
    | SWITCH '(' expression ')' statement
    ;

How is the else if declared in C. By reading the standard, how can I notice it is part of it?

Answer 1

The else if statement is officially defined as byproduct of the if statement in §6.8.4/1, C18 by declaring the syntax:

Syntax

1 selection-statement:

if ( expression ) statement

if ( expression ) statement else statement

Source: C18, §6.8.4/1

The last "statement" in the latter form describes that another if statement can be followed after an else.

---

Beside that, you can find an, of course non-normative, example of its use in the C standard in normative appendix G in the code example at G.5.1/8:

if (isnan(x) && isnan(y)) { ... }
else if ((isinf(a) ||isinf(b)) && isfinite(c) && isfinite(d)) { ... }
else if ((logbw == INFINITY) && isfinite(a) && isfinite(b)) { ... } 

This is the only place where an else if statement appears as it is in the C18 standard.

So regarding:

By reading the standard, how can I notice it is part of it?

There at least although examples are non-normative it is part of it.

Answer 2

C 2018 6.8.4 says a selection-statement may be “if ( expression ) statement else statement”. C 2018 6.8 says that latter statement may be “selection-statement”, and so it may be an if or if … else statement, which results in a statement containing else if.