Are all parameters in ternary operator mandatory or can you do "(exp1 ? : value)"?

Question

I would like to know if in the ternary operator in language C all parameters are mandatory? e.g.:

(exp1 ? : value2);

or you need to write:

(expr1 ? value1: value2);

I asked that because if you write: (exp1 ? : value2); What return if the expr1 is TRUE?

Answer 1

It not a standard, but GCC extension (may be some other compilers do the same):

5.7 Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

 x ? : y 

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

 x ? x : y

Edit:

As @MadPhysicist pointed, that shortened form would evaluate the x once, while the traditional form would re-evaluate the x second time when x is non-zero

Answer 2

The statement

(exp1 ? : value2);   

is equivalent to

(exp1 ? exp1 : value2);

It is a GCC extension. The only difference in both of them is that exp1 will be evaluated only once in the former statement.