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. C99, Section Subclause 6.5.8 "Relational operators", paragraph 6 has a footnote (92), which saysfootnote 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

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

Related Guidelines

...

...

Bibliography

...

...

...

Bibliography

...

Image Removed      03. Expressions (EXP)      EXP14-C. Beware of integer promotion when performing bitwise operations on chars or shortsImage Added Image Added Image Added