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This applies only to implementations that conforms to the optional Annex F, "IEC 60559 Floating-Point Arithmetic."  The macro __STDC_IEC_559__ can be used to determine whether an implementation conforms to Annex F.

Noncompliant Code Example

This noncompliant code example fails to cast the result of the expression in the return statement and thereby guarantee that the range or precision is no wider than expected. The uncertainty in this example is introduced by the constant 0.1f. This constant may be stored with a range or precision that is greater than that of float. Consequently, the result of x * 0.1f may also have a range or precision greater than that of float. As described previously, this range or precision may not be reduced to that of a float, so the caller of calcPercentage() may receive a value that is more precise than expected. This may lead to inconsistent program execution across different platforms.

float calc_percentage(float value) {
  return value * 0.1f;
}

void float_routine(void) {
  float value = 99.0f;
  long double percentage;

  percentage = calc_percentage(value);
}

Compliant Solution (

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within the Function)

This compliant solution casts the value of the expression in the return statement. It forces the return value to have the expected range and precision, as described in subclause 5.2.4.2.2, paragraph 9, of the C Standard [ISO/IEC 9899:2011].

float calc_percentage(float value) {
  return (float)(value * 0.1f);
}

void float_routine(void) {
  float value = 99.0f;
  long double percentage;

  percentage = calc_percentage(value);
}

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Forcing the range and precision inside the calcPercentage() function is a good way to fix the problem once without having to apply fixes in multiple locations (every time calcPercentage() is called).  This compliant solution uses a temporary variable as the forcing mechanism:

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Compliant Solution (Outside the Function)

Source code to the called function may not always be available. This compliant solution casts the return value of the calcPercentage() function to float to force the correct range and precision when the source of the called function cannot be modified.

void float_routine(void) {
  float value = 99.0f;
  long double percentage;

  percentage = (float) calc_percentage(value);
}

Risk Assessment

Failure to follow this guideline can lead to inconsistent results across different platforms.

Related Vulnerabilities

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

Bibliography

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