C defines <, >, <=, and >= to be relational operators, and it defines == and != to be equality operators.

If a for or while statement uses a loop counter, than it is safer to use a relational operator (such as <) to terminate the loop than to use an equality operator (such as !=).

Noncompliant Code Example (Equality Operators)

This noncompliant code example appears to have five iterations, but in fact, the loop never terminates:

size_t i;
for (i = 1; i != 10; i += 2) {
  /* ... */
}

Compliant Solution (Relational Operators)

Using the relational operator <= instead of an equality operator guarantees loop termination:

size_t i;
for (i = 1; i <= 10; i += 2 ) {
  /* ... */
}

Noncompliant Code Example (Equality Operators)

It is also important to ensure termination of loops where the start and end values are variables that might not be properly ordered. The following function assumes that begin < end; if this is not the case, the loop will never terminate:

void f(size_t begin, size_t end) {
  size_t i;
  for (i = begin; i != end; ++i) {
    /* ... */
  }
}

Compliant Solution (Relational Operators)

Again, using a relational operator instead of equivalence guarantees loop termination. If begin >= end, the loop never executes its body.

void f(size_t begin, size_t end) {
  size_t i;
  for (i = begin; i < end; ++i) {
    /* ... */
  }
}

Noncompliant Code Example (Boundary Conditions)

Numerical comparison operators do not always ensure loop termination when comparing against the minimum or maximum representable value of a type, such as SIZE_MAX:

void f(size_t begin, size_t step) {
  size_t i;
  for (i = begin; i <= SIZE_MAX; i += step) {
    /* ... */
  }
}

Compliant Solution (Boundary Conditions)

A compliant solution is to compare against the difference between the maximum representable value of a type and the increment:

void f(size_t begin, size_t step) {
  if (0 < step) {
    size_t i;
    for (i = begin; i <= SIZE_MAX - step; i += step) {
      /* ... */
    }
  }
}

Exceptions

MSC21-C-EX1: If the loop counter is incremented by 1 on each iteration, and it is known that the starting value of a loop is less than or equal to the ending value, then an equality operator may be used to terminate the loop. Likewise, if the loop counter is decremented by 1 on each iteration, and it is known that the starting value of the loop is greater than or equal to the ending value, then an equality operator may be used to terminate the loop.

size_t i;
for (i = 1; i != 5; ++i) {
  /* ... */
}

Risk Assessment

Testing for exact values runs the risk of a loop terminating much longer than expected or never terminating at all.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported: Astrée reports potential infinite loops.
CodeSonar		LANG.STRUCT.LOOP.HR LANG.STRUCT.LOOP.UB	High risk loop Potential unbounded loop
Compass/ROSE
LDRA tool suite		510 S	Partially implemented
PC-lint Plus		440, 442, 443, 444, 445, 2650	Partially supported
Polyspace Bug Finder		CERT C: Rec. MSC21-C	Checks for loop bounded with tainted value (rec. partially covered)
PVS-Studio		V621

Related Vulnerabilities

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

Related Guidelines