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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. Consequently,

Programmers should exercise caution if the second operand should not contain contains side effects because , if it does, it is not apparent if the side effect occurs.

Non-Compliant Code Example

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 */

int i;
int max;

if ( (i >= 0) && ( (i++) <= max) ) {
  /* codeCode */
}

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 calledIt is unclear whether the value of i will be incremented as a result of evaluating the condition.

Compliant Solution

In this compliant solution, the behavior is much clearer, 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.

int i;
int max;

if ( (i >= 0 && (i + 1) <= max) ) {
  i++;
  /* code */
}

Risk Assessment

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 Attempting to modify an object that is the second operand to the logical OR or AND operator may cause that object to take on an unexpected value. This can lead to unintended program behavior.

Automated Detection

Related Vulnerabilities

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

References

Related Guidelines

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.5.13, "Logical AND operator," and Section 6.5.14, "Logical OR operator"