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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 !=).

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

Code Block

bgColor	#FFCCCC
lang	c

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

...

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

Code Block

bgColor	#ccccff
lang	c

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

...

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.:

Code Block

bgColor	#ffcccc
lang	c

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

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

Code Block

bgColor	#ccccff
lang	c

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

Anchor

	MSC21-EX1
	MSC21-EX1

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.

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