FLP34-C. Ensure that floating-point conversions are within range of the new type

If a floating-point value is to be demoted to a floating-point value of a smaller range and precision or to an integer type, or if an integer type is to be converted to a floating-point type, the value must be represented in the new type.

Subclause 6.3.1.4 paragraphs 1 and 2 of the C Standard [ISO/IEC 9899:2011] says,

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.

When a value of integer type is converted to a real floating type, if the value being converted can be represented exactly in the new type, it is unchanged. If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined.

And subclause 6.3.1.5 paragraph 1 says,

When a value of real floating type is converted to a real floating type, if the value being converted can be represented exactly in the new type, it is unchanged. If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined.

See undefined behaviors 17 and 18.

Consequently, in implementations that do not allow for the representation of all numbers, conversions of values between zero and FLT_MIN may result in undefined behavior.

This rule does not apply to demotions of floating-point types on implementations that support signed infinity, such as IEEE 754, as all values are within range.

Noncompliant Code Example (float to int)

This noncompliant code example leads to undefined behavior if the integral part of f_a cannot be represented as an integer:

void func(float f_a) {
  int i_a;
 
  /* Undefined if the integral part of f_a >= INT_MAX */
  i_a = f_a;
}

Compliant Solution (float to int)

This compliant solution assumes that the range of values of type float is greater than that of an int, as is the case in most implementations. Unfortunately, there is no safe way to inquire about this assumption in the code short of already knowing the implementation. Converting INT_MAX to float is a problem on many implementations, resulting in a power of 2, rather than 1 less. 

#include <limits.h>
 
void func(float f_a) {
  int i_a;

  if (f_a >= (float)INT_MAX || f_a < (float)INT_MIN || (f_a >= 0F && f_a < FLT_MIN)) {
    /* Handle error */
  } else {
    i_a = f_a;
  }
}

Noncompliant Code Example (Demotions)

This noncompliant code example contains conversions that may be outside the range of the demoted types:

void func(double d_a, long double big_d) {
  double d_b = (float)big_d;
  float f_a = (float)d_a;
  float f_b = (float)big_d;
}

As a result of these conversions, it is possible that d_a is outside the range of values that can be represented by a float or that big_d is outside the range of values that can be represented as either a float or a double. If this is the case, the result is undefined.

Compliant Solution (Demotions)

This compliant solution checks whether the values to be stored can be represented in the new type:

#include <float.h>
 
void func(double d_a, long double big_d) {
  double d_b;
  float f_a;
  float f_b;

  if (d_a > FLT_MAX || d_a < -FLT_MAX) {
    /* Handle error condition */
  } else {
    f_a = (float)d_a;
  }
  if (big_d > FLT_MAX || big_d < -FLT_MAX) {
    /* Handle error condition */
  } else {
    f_b = (float)big_d;
  }
  if (big_d > DBL_MAX || big_d < -DBL_MAX) {
    /* Handle error condition */
  } else {
    d_b = (double)big_d;
  }  
}

Risk Assessment

Failing to check that a floating-point value fits within a demoted type can result in a value too large to be represented by the new type, resulting in undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography