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The relational and equality operators are left-associative in C. Consequently, C, unlike many other languages, allows chaining of relational and equality operators. Subclause 6.5.8, footnote 107, of the C Standard [ISO/IEC 9899:2011], says:

The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c; in other words, "if a is less than b, compare 1 to c; otherwise, compare 0 to c."

These operators are left-associative, which means the leftmost comparison is performed first, and the result is compared with the rightmost comparison. This syntax allows a programmer to write an expression (particularly an expression used as a condition) that can be easily misinterpreted.

Noncompliant Code Example

Although this noncompliant code example compiles correctly, it is unlikely that it means what the author of the code intended:

int a = 2;
int b = 2;
int c = 2;
/* ... */
if (a < b < c) /* Misleading; likely bug */
/* ... */
if (a == b == c) /* Misleading; likely bug */

The expression a < b < c evaluates to true rather than, as its author probably intended, to false, and the expression a == b == c evaluates to false rather than, as its author probably intended, to true.

Compliant Solution

Treat relational and equality operators as if it were invalid to chain them:

if ( (a < b) && (b < c) ) /* Clearer and probably what was intended */
/* ... */
if ( (a == b) && (a == c) ) /* Ditto */

Risk Assessment

Incorrect use of relational and equality operators can lead to incorrect control flow.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP13-C

Low

Unlikely

Medium

P2

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04
chained-comparisonFully checked

ECLAIR

1.2

CC2.EXP13

Fully implemented

GCC
4.3.5


Option -Wparentheses warns if a comparison like x<=y<=z appears; this warning is also enabled by -Wall

LDRA tool suite
9.7.1
433 SFully implemented
Polyspace Bug Finder

R2019a

CERT C: Rec. EXP13-CChecks for possibly unintended evaluation of expression because of operator precedence rules (rec. fully covered)


PRQA QA-C
9.5

3392
3401
4111
4112
4113

Fully implemented
PVS-Studio

6.23

V709
RuleChecker
19.04
chained-comparisonFully checked

Related Guidelines

Bibliography

[ISO/IEC 9899:2011]Subclause 6.5.8, "Relational Operators"



2 Comments

  1. A couple of questions:

    1. What does "left-associatvie" mean in C? What's the definition in C spec?
    2. Is is true that in other languages, relational (or equality) operators are associative?

    1. Added some text to address your first question..

      I think most operators are left-associative in most languages by default; that's the easiest way to parse source code. (Some languages choose features to be right-associative, but that's a conscious decision).

      But Java doesn't allow relation chaining (eg a<b<c). It's not forbidden by 'associativity', but rather by Java being more strongly typed than C. It's still parsed as ((a<b)<c), but (a<b) is a boolean type, and the relation operators cannot be applied to boolean types, so a<b<c generates a type mismatch error. I'd guess most langauges that disallow chaining use a similar strategy. C allows this because (a<b) returns an int type (which has the value 0 or 1).