CertC-FLP30

Do not use floating-point variables as loop counters

Required inputs: IR

Because floating-point numbers represent real numbers, it is often mistakenly assumed that they can represent any simple fraction exactly. Floating-point numbers are subject to representational limitations just as integers are, and binary floating-point numbers cannot represent all real numbers exactly, even if they can be represented in a small number of decimal digits.

In addition, because floating-point numbers can represent large values, it is often mistakenly assumed that they can represent all significant digits of those values. To gain a large dynamic range, floating-point numbers maintain a fixed number of precision bits (also called the significand) and an exponent, which limit the number of significant digits they can represent.

Different implementations have different precision limitations, and to keep code portable, floating-point variables must not be used as the loop induction variable. See Goldberg's work for an introduction to this topic [ Goldberg 1991].

For the purpose of this rule, a loop counter is an induction variable that is used as an operand of a comparison expression that is used as the controlling expression of a do, while, or for loop. An induction variable is a variable that gets increased or decreased by a fixed amount on every iteration of a loop [ Aho 1986]. Furthermore, the change to the variable must occur directly in the loop body (rather than inside a function executed within the loop).

Noncompliant Code Example

In this noncompliant code example, a floating-point variable is used as a loop counter. The decimal number 0.1 is a repeating fraction in binary and cannot be exactly represented as a binary floating-point number. Depending on the implementation, the loop may iterate 9 or 10 times. 

void func(void) {
  for (float x = 0.1f; x <= 1.0f; x += 0.1f) {
    /* Loop may iterate 9 or 10 times */
  }
}

For example, when compiled with GCC or Microsoft Visual Studio 2013 and executed on an x86 processor, the loop is evaluated only nine times.

Compliant Solution

In this compliant solution, the loop counter is an integer from which the floating-point value is derived:

#include <stddef.h>
 
void func(void) {
  for (size_t count = 1; count <= 10; ++count) {
    float x = count / 10.0f;
    /* Loop iterates exactly 10 times */
  }
}
Noncompliant Code Example

In this noncompliant code example, a floating-point loop counter is incremented by an amount that is too small to change its value given its precision:

void func(void) {
  for (float x = 100000001.0f; x <= 100000010.0f; x += 1.0f) {
    /* Loop may not terminate */
  }
}

On many implementations, this produces an infinite loop.

Compliant Solution

In this compliant solution, the loop counter is an integer from which the floating-point value is derived. The variable x is assigned a computed value to reduce compounded rounding errors that are present in the noncompliant code example. 

void func(void) {
  for (size_t count = 1; count <= 10; ++count) {
    float x = 100000000.0f + (count * 1.0f);
    /* Loop iterates exactly 10 times */
  }
}
Risk Assessment

The use of floating-point variables as loop counters can result in   unexpected behavior.

Rule Severity Likelihood Remediation Cost Priority Level
FLP30-C Low Probable Low P6 L2
Related Guidelines
Taxonomy Taxonomy item Relationship
CERT C FLP30-CPP. Do not use floating-point variables as loop counters Prior to 2018-01-12: CERT: Unspecified Relationship
CERT Oracle Secure Coding Standard for Java NUM09-J. Do not use floating-point variables as loop counters Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013 Floating-Point Arithmetic [PLF] Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012 Directive 1.1 (required) Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012 Rule 14.1 (required) Prior to 2018-01-12: CERT: Unspecified Relationship
Bibliography
[ Aho 1986]
[ Goldberg 1991]
[ Lockheed Martin 05] AV Rule 197
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/floating-point-flp/flp30-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

float_loop_counter

Use of floating-point loop counter.

None

False

Options

loop_counter_model

loop_counter_model : LoopCounterModel = 'cert'

The loop counter model on which the analysis is based.
 

Option Types

These types are used by options listed above:

LoopCounterModel

An enumeration.
 

  • misra_c_2004

  • misra_c_2004_continue

  • misra_c_2012

  • misra_c_2012_continue

  • misra_cpp_2008

  • cert

  • generic