The logical AND and logical OR operators (&&
and ||
, respectively) exhibit "short-circuit" operation. That is, the second operand is not evaluated if the result can be deduced solely by evaluating the first operand.
Programmers should exercise caution if the second operand contains side effects because it may not be apparent whether the side effects actually occur.
In the following code, the value of i
is incremented only when i >= 0
:
enum { max = 15 }; int i = /* Initialize to user-supplied value */; if ( (i >= 0) && ( (i++) <= max) ) { /* Code */ } |
Although the behavior is well defined, it is not immediately obvious whether or not i
gets incremented.
In this noncompliant code example, the second operand of the logical OR operator invokes a function that results in side effects:
char *p = /* Initialize; may or may not be NULL */ if (p || (p = (char *) malloc(BUF_SIZE)) ) { /* Perform some computation based on p */ free(p); p = NULL; } else { /* Handle malloc() error */ return; } |
Because malloc()
is called only if p
is NULL
when entering the if
clause, free()
might be called with a pointer to local data not allocated by malloc()
. (See MEM34-C. Only free memory allocated dynamically.) This behavior is partially due to the uncertainty of whether or not malloc()
is actually called.
In this compliant solution, a second pointer, q
, is used to indicate whether malloc()
is called; if not, q
remains set to NULL
. Passing NULL
to free()
is guaranteed to safely do nothing.
char *p = /* Initialize; may or may not be NULL */ char *q = NULL; if (p == NULL) { q = (char *) malloc(BUF_SIZE); p = q; } if (p == NULL) { /* Handle malloc() error */ return; } /* Perform some computation based on p */ free(q); q = NULL; |
Failing to understand the short-circuit behavior of the logical OR or AND operator may cause unintended program behavior.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
EXP02-C | Low | Unlikely | Medium | P2 | L3 |
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | logop-side-effect | Fully checked | |
Axivion Bauhaus Suite | CertC-EXP02 | Fully implemented | |
Compass/ROSE | Could detect possible violations of this recommendation by reporting expressions with side effects, including function calls, that appear on the right-hand side of an | ||
Helix QAC | C3415 | ||
Klocwork | MISRA.LOGIC.SIDEEFF | ||
LDRA tool suite | 35 D | Fully implemented | |
Parasoft C/C++test | CERT_C-EXP02-a | The right-hand operand of a logical && or || operator shall not contain side effects | |
PC-lint Plus | 9007 | Fully supported | |
RuleChecker | logop-side-effect | Fully checked | |
SonarQube C/C++ Plugin | SideEffectInRightHandSideOfLogical |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
SEI CERT C++ Coding Standard | VOID EXP02-CPP. Be aware of the short-circuit behavior of the logical AND and OR operators |
MITRE CWE | CWE-768, Incorrect short circuit evaluation |