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

If a for or while or for statement uses a loop counter, and increments or decrements it by more than one, it should use an inequality operator 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 may appear appears to have 5 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

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(Relational Operators)

Again, using a relational operator instead of equivalence An inequality comparison 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= 1; i <= 10SIZE_MAX; i += 2 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

ROSE can detect violations of this rule.

Related Vulnerabilities

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

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Related Guidelines

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References

\[[MISRA 04|AA. C References#MISRA 04]\]

MSC06-C. Be aware of compiler optimization when dealing with sensitive data      49. Miscellaneous (MSC)      APP00-C. Functions should validate their parameters