FLP30-C. Do not use floating-point variables as loop counters

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.

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 incremented to maintain the same value it held at each iteration of the loop in the noncompliant code example.

void func(void) {
  float x = 100000001.0f;
  for (size_t count = 1; count <= 10; ++count, x += 1.0f) {
    /* Loop iterates exactly 10 times */
  }
}

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography