EXP02-C. Be aware of the short-circuit behavior of the logical AND and OR operators

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.

One 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 i gets incremented or not.

Noncompliant Code Example

In this 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)) ) {
  /* do stuff with p */
  free(p);
  p = NULL;
}
else {
  /* handle malloc() error */
  return;
}

Because malloc() is only called if p is NULL when entering the if clause, free() might be called with a pointer to local data not allocated by malloc(). (See rule MEM34-C. Only free memory allocated dynamically.) This is partially due to the uncertainty of whether malloc() is actually called or not.

Compliant Solution

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;
char *q = NULL;
if (p == NULL) {
  q = (char *) malloc(BUF_SIZE);
  p = q;
}
if (p == NULL) {
  /* handle malloc() error */
  return;
}

/* do stuff with p */
free(q);
q = NULL;

Risk Assessment

Failing to understand the short-circuit behavior of the logical OR or AND operator may cause unintended program behavior.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

CERT C++ Secure Coding Standard: EXP02-CPP. Be aware of the short-circuit behavior of the logical AND and OR operators

MITRE CWE: CWE-768, "Incorrect Short Circuit Evaluation"

ISO/IEC 9899:1999 Section 6.5.13, "Logical AND operator," and Section 6.5.14, "Logical OR operator"

Bibliography

EXP01-C. Do not take the size of a pointer to determine the size of the pointed-to type      03. Expressions (EXP)