The relational and equality operators are left-associative, 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 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 false as its author probably intended, and the expression a == b == c evaluates to false, rather than true as its author probably intended.

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 */

Automated Detection

The gcc option -Wparentheses warns if a comparison like `x<=y<=z' appears. This warning is also enabled by -Wall.

Other Languages

This rule appears in the C++ Secure Coding Standard as EXP17-CPP. Treat relational and equality operators as if they were nonassociative.

Risk Assessment

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

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