FLP34-C. Ensure that demoted floating point values are within range

From 6.3.1.5 of the C99 standard [[ISO/IEC 9899-1999]]:

When a double is demoted to float [or] a long double is demoted to double or float...if the value being converted is outside the range of values that can be represented, the behavior is undefined.

This rule does not apply to machines that support signed infinity, such as ones that support IEEE-754, as all numbers are representable.

For some non-IEEE-754 machines, conversions of too small numbers (between zero and FLT_MIN) might produce underflow exceptions that might be considered undefined.

Non-Compliant Code Example

This non-compliant code example illustrates possible undefined behavior associated with demoting floating point represented numbers.

long double ld;
double d1;
double d2;
float f1;
float f2;

/* initializations */

f1 = (float)d1;
f2 = (float)ld;
d2 = (double)ld;

As a result of these conversions, it is possible that d1 is outside the range of values that can be represented by a float or that ld is outside the range of values that can be represented as either a float or a double.

Compliant Solution

This compliant solution properly checks to see whether the values to be stored can be represented in the new type.

#include <float.h>

long double ld;
double d1;
double d2;
float f1;
float f2;

/* initializations */

if (d1 > FLT_MAX || d1 < -FLT_MAX) {
  	/* Handle error condition */
} else {
 	f1 = (float)d1;
}
if (ld > FLT_MAX || ld < -FLT_MAX) {
	/* Handle error condition */
} else {
	f2 = (float)ld;
}
if (ld > DBL_MAX || ld < -DBL_MAX) {
	/* Handle error condition */
} else {
	d2 = (double)ld;
}

Risk Analysis

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

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this rule.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 6.3.1.5, "Real floating types"

FLP33-C. Convert integers to floating point for floating point operations      05. Floating Point (FLP)       06. Arrays (ARR)