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Cast the return value of a function that returns a floating point type to ensure predictable program execution.

Section 6.8.6.4, paragraph 2, of the C Standard [ISO/IEC 9899:2011] states:

If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by assignment to an object having the return type of the function.

This paragraph is annotated (footnote 160) as follows:

The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return. The representation of floating-point values may have wider range or precision than implied by the type; a cast may be used to remove this extra range and precision.

Conversion as if by assignment to the type of the function is required if the return expression has a different type than the function, but not if the return expression has a wider value only because of wide evaluation. This allows seemingly inconsistent and confusing behavior. Consider the following code as an example:

float f(float x) {
  return x * 0.1f;
}

float g(float x) {
  return x * 0.1;
}

Function f is allowed to return a value wider than float, but function g (which uses the wider constant) is not.

Although the current text does not require narrowing return expressions of the same type as the function, it does not clearly state what is allowed. Is it allowed to narrow the result? Is it allowed to narrow the result sometimes but not always? Is it allowed to partially narrow the result (for example, if the ABI returns floats in double format, but a float function has a float return expression evaluated to wider than double)? An aggressive implementation could argue €œyes€ to all these questions, though the resulting behavior would complicate debugging and error analysis.

Footnote 160 says a cast may be used to remove extra range and precision from the return expression. This means that a predictable program must have casts on all floating-point function calls (except where the function directly feeds an operator-like assignment that implies the conversion). With type-generic math (tgmath), the programmer has to reason through the tgmath resolution rules to determine which casts to apply. These are significant obstacles to writing predictable code.

NOTE: WG14 voted to include the following text in the C Standard [ISO/IEC 9899:2011]. It impacts only implementations that implement the optional Annex F, "IEC 60559 Floating-Point Arithmetic."

Require return expressions to be converted as if by assignment to the type of the function, but only in Annex F. This is a compromise that addresses the problems for Annex F implementations while not impacting non-Annex F implementations that exercise the license for wide returns.

Insert the following new subclause after F.5 (and increment subsequent subclause numbers):

F.6 The return statement

If the return expression is evaluated in a floating-point format different from the return type, then the expression is converted to the return type of the function and the resulting value is returned to the caller.

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

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

  percentage = calcPercentage(value);
}

Compliant Solution

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 Section 5.2.4.2.2, para. 8, of the C Standard [ISO/IEC 9899:2011].

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

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

  percentage = calcPercentage(value);
}

Compliant Code Example (Alternate)

Unfortunately, not all compilers honor casts. In this case, the range and precision must be forced by assignment to a variable of the correct type. This compliant solution forces the assignment by type-qualifying result as volatile and assigning the result of the floating point operation to result before returning it.

float calcPercentage(float value) {
  volatile float result;

  result = value * 0.1f;

  return result;
}

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

  percentage = calcPercentage(value);
}

Compliant Solution (Outside the Function 1)

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). However, access to the called function may not always be available. This compliant solution shows one way to force the correct range and precision in a situation in which the source of the called function cannot be modified. This is accomplished by casting the return value of the calcPercentage() function to float.

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

  percentage = (float)calcPercentage(value);
}

Compliant Solution (Outside the Function 2)

This compliant solution shows another way to force the correct range and precision. In this case, a temporary variable is used as the forcing mechanism.

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

  percentage = temp = calcPercentage(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