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.

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.

Noncompliant Code Example

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.

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 = /* 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;

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