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, "ifa
is less thanb
, compare 1 toc
; otherwise, compare 0 toc
."
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.
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.
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 */ |
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 |
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | chained-comparison | Fully checked | |
CC2.EXP13 | Fully implemented | ||
GCC | Option | ||
Helix QAC | C3392, C3401, C4111, C4112, C4113 | ||
LDRA tool suite | 433 S | Fully implemented | |
PC-lint Plus | 503, 731 | Fully supported | |
Polyspace Bug Finder | CERT C: Rec. EXP13-C | Checks for possibly unintended evaluation of expression because of operator precedence rules (rec. fully covered) | |
PVS-Studio | V709 | ||
RuleChecker | chained-comparison | Fully checked |
SEI CERT C++ Coding Standard | VOID EXP17-CPP. Treat relational and equality operators as if they were nonassociative |
[ISO/IEC 9899:2011] | Subclause 6.5.8, "Relational Operators" |