Is the “true” result of >, <, !, &&, || or == defined?

Question

When I for instance write 7>1 in C (say C99 if this is not an always-been feature), can I expect the result will be exactly 1 or just some non-zero value? D

Answer 1

All C operators that yield logically true/false values always yield a result of type int with the value 0 for false, 1 for true.

That's not the case for all C expressions that yield logically true/false values. For example, the is*() character classification functions declared in <ctype.h> (isdigit(), isupper(), etc.) return 0 if the condition is false, but may return any non-zero value if the condition is true.

As long as you use the result directly as a condition:

if (isdigit(c)) ...
if (!isdigit(c)) ...
if (isdigit(c) || islower(c)) ...

and don't attempt to compare it to something:

if (isdigit(c) == 1) ...    /* WRONG */
if (isdigit(c) == true) ... /* ALSO WRONG */

this shouldn't cause any problems.

(You can safely compare the result to 0 or false, but there's no good reason to do so; that's what the ! operator is for.)

Answer 2

In C99 §6.5.8 Relational Operators, item 6 (<,>,<= and >=):

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) The result has type int.

As for equality operators, it's a bit further in §6.5.9 (== and !=):

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. The result has type int. For any pair of operands, exactly one of the relations is true.

The logical AND and logical OR are yet a bit further in §6.5.13 (&&)

The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

... and §6.5.14 (||)

The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

And the semantics of the unary arithmetic operator ! are over at §6.5.3.3/4:

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. The result has type int. The expression !E is equivalent to (0==E).

Result type is int across the board, with 0 and 1 as possible values. (Unless I missed some.)

Answer 3

C follows Postel's Law for its boolean operators: be conservative in what you do, be liberal in what you accept from others. It will treat any non-zero value as true in boolean expressions, but it will always produce either a 0 or a 1 itself. 2 != 3 is always 1.

Answer 4

From the ISO C99 standard, section 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. The result has type int.

From section 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. The result has type int. For any pair of operands, exactly one of the relations is true.

Same thing happens with the logical conjunction (&&) and disjunction (||) operators.

PS: Incidentally, this is why the bitwise operators (& and |) can usually be used as non-short-circuiting versions of the logical operators.