Question

As anything non-zero means true, but the >, <, == etc. operators returning 1 for true, I’m curious if there are any notable C compilers where these operators can result in a value greater than 1.

In other words, is there any compiler where int i = (a==b); would result in undefined behavior if I intended to use i not as a boolean value, but as an integer, and was assuming it would be either 0 or 1 ?

Answer 1

No, if there are, they’re not C compilers 🙂 The standard mandates that relational and equality operators return 1 for true and 0 for false.

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The rule for interpretation of integral values as boolean values by C states that 0 is false and any other value is true. See C11 sections dealing with if/while/do/for, which all contain language like "while the expression compares unequal to zero". Specifically:

6.8.4.1/2: In both forms [of the if statement, one with and one without an else clause], the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0.

6.8.5/4: An iteration statement [while, do and for] causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to 0.

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However, it’s quite explicit what result you will get for the comparison-type expressions, you either get 0 or 1. The relevant bits of the C11 standard for these are all under 6.5 Expressions:

6.5.8/6: Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.

6.5.9/3: The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence. Each of the operators yields 1 if the specified relation is true and 0 if it is false.

6.5.13/3: The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0.

6.5.14/3: The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0.

6.5.3.3/5: The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0.

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And this behaviour goes way back to C99 and C89 (ANSI days). The C99 sections dealing with relational and equality operators also states that the return values is either 0 or 1.

And, while the C89 draft doesn’t explicitly dictate the return values for equality operators, it does say:

The == (equal to) and the != (not equal to) operators are analogous to the relational operators except for their lower precedence.

And the relational operators section does state:

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.

Reference: http://flash-gordon.me.uk/ansi.c.txt since I don’t have any copies of the C89 standards floating around. I do have the second edition of K&R (the ANSI one from 1988) which basically says the same thing, in sections A7.9 and A7.10 of Appendix A, the Reference Manual. If you want a definitive answer from the first edition, that’ll have to come from someone with a wife less prone to throwing out old junk.

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Addendum:

According to Michael Burr, who is either not married or has a more accommodating wife than I in terms of keeping old books 🙂

K&R 1st Edition (1978) also says the same in 7.6 and 7.7: "The [relational operators] all yield 0 if the specified relation is false and 1 if it is true." … "The [equality operators] are exactly analogous to the relational operators except for their lower precedence."