Dictate order that operands are evaluated in c

Question

The statement puts("a") + puts("b") is undefined. This is because it is not specified in the C Standard whether these ought to be executed left to right or right to left so you could get

a
b

or

b
a

Is there a clean way to dictate the order of operations in an expression?

The only thing I can think of is to use a compound statement such as

({
    int temp = puts("a");
    temp += puts("b");
    temp;
})

though this is non-portable and a little longer than I was hoping.

How could this best be achieved?

Answer 1

If you declare an int variable before the expression, you can force order portably with the comma operator while computing the sum inside an expression:

int temp;
...

(temp = puts("a"), temp + puts("b"))

As specified in the C Standard:

6.5.17 Comma operator

Syntax

 expression:
     assignment-expression
     expression , assignment-expression

Semantics

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

Note however that the value of the expression will not be very useful given the semantics of puts(), as commented by Jonathan Leffler.