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.
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 | 24.04 | chained-comparison | Fully checked |
1.2 | CC2.EXP13 | Fully implemented | |
GCC | 4.3.5 | Option | |
Helix QAC | 2024.2 | C3392, C3401, C4111, C4112, C4113 | |
LDRA tool suite | 9.7.1 | 433 S | Fully implemented |
PC-lint Plus | 1.4 | 503, 731 | Fully supported |
Polyspace Bug Finder | R2024a | CERT C: Rec. EXP13-C | Checks for possibly unintended evaluation of expression because of operator precedence rules (rec. fully covered) |
PVS-Studio | 7.33 | V709 | |
RuleChecker | 24.04 | chained-comparison | Fully checked |
Related Guidelines
SEI CERT C++ Coding Standard | VOID EXP17-CPP. Treat relational and equality operators as if they were nonassociative |
Bibliography
[ISO/IEC 9899:2011] | Subclause 6.5.8, "Relational Operators" |
2 Comments
Masaki Kubo
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?
David Svoboda
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).