Page History

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 not non-associative as they often are in other languages. A comparison such as{{x<=y<=z}}, for example, is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation. This allows a programmer to write an expression (particularly an expression used as a condition) that can be easily misinterpreted.

Noncompliant Code Example

While 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,Misleading; likely bug */
//* ... */
if (a == b == c) /* misleading,Misleading; likely bug */

The expression a < b < c evaluates to true , rather than false , as its author probably intended, to false, and the expression a == b == c evaluates to false , rather than true , 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,Clearer and probably what was intended */
//* ... */
if ( (a == b) && (a == c) ) /* dittoDitto */

Risk Assessment

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

Automated Detection

...

Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	chained-comparison	Fully checked
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.EXP13	Fully implemented
GCC	Include Page GCC_V GCC_V		Option -Wparentheses warns if a comparison like

...

x<=y<=z

...

appears

...

; this warning is also enabled by -Wall
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3392, C3401, C4111, C4112, C4113
LDRA tool suite	Include Page LDRA_V LDRA_V	433 S	Fully implemented
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	503, 731	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec.

...

EXP13-C	Checks for possibly unintended evaluation of expression because of operator precedence rules (rec. fully covered)
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V709
RuleChecker	Include Page RuleChecker_V RuleChecker_V	chained-comparison	Fully checked

Related Guidelines

Other Languages

...

...

Bibliography

...

Image Added Image Added Image Added

Risk Assessment

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

Image Removed      03. Expressions (EXP)      EXP30-C. Do not depend on order of evaluation between sequence points