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If a floating-point value is to be converted 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 representable in the destination type.

The C Standard, From 6.3.1.5 of the C99 standard:

Wiki Markup
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.

Non-Compliant Code Example

This non-compliant code 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;

4, paragraph 2 [ISO/IEC 9899:2024], says,

When a finite value of decimal 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 "invalid" floating-point exception shall be raised and the result of the conversion is unspecified.

Paragraph 2 of the same subclause says,

When a value of integer type is converted to a standard 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+2, 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.

When a value of real floating type is converted to a standard floating type, 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.

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 cannot be represented. */
  i_a = f_a;
}

Compliant Solution (float to int)

This compliant solution tests to ensure that the float value will fit within the int variable before performing the assignment.

#include <float.h>
#include <limits.h>
#include <math.h>
#include <stddef.h>
#include <stdint.h>
 
extern size_t popcount(uintmax_t); /* See INT35-C */
#define PRECISION(umax_value) popcount(umax_value)
 
void func(float f_a) {
  int i_a;
 
  if (isnan(f_a) ||
      PRECISION(INT_MAX) < log2f(fabsf(f_a)) ||
      (f_a != 0.0F && fabsf(f_a) < FLT_MIN)) {
    /* Handle error */
  } else {
    i_a = f_a;
  }
}

Noncompliant Code Example (Narrowing Conversion)

This noncompliant code example attempts to perform conversions that may result in truncating values outside the range of the destination 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 In the assignments above, it is possible that the variable d1 is outside the range of values that can be represented by a float or that the variable ld 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 on implementations that do not support Annex F, "IEC 60559 Floating-Point Arithmetic."

Compliant Solution (Narrowing Conversion)

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

#include <float.h>
#include <float<math.h>
 
void func(double d_a, long double ld;
double d1;
double d2;
float f1;
float f2;

/* initializations */

if(d1 > FLT_MAX || d1 < -FLT_MAX) {
	 big_d) {
  double d_b;
  float f_a;
  float f_b;

  if (d_a != 0.0 &&
      (isnan(d_a) ||
       isgreater(fabs(d_a), FLT_MAX) ||
       isless(fabs(d_a), FLT_MIN))) {
    /* Handle error condition */
  } else {
	f1    f_a = (float)d1d_a;
  }
  if(ld > (big_d != 0.0 &&
      (isnan(big_d) ||
       isgreater(fabs(big_d), FLT_MAX) || ld < -FLT_MAX
       isless(fabs(big_d), FLT_MIN))) {
	    /* Handle error condition */
  } else {
	f2    f_b = (float)ldbig_d;
  }
  if(ld > (big_d != 0.0 &&
      (isnan(big_d) ||
       isgreater(fabs(big_d), DBL_MAX) || ld < -DBL_MAX
       isless(fabs(big_d), DBL_MIN))) {
	    /* Handle error condition */
  } else {
	d2    d_b = (double)ld;
}

References

big_d;
  }  
}

Risk Assessment

Converting a floating-point value to a floating-point value of a smaller range and precision or to an integer type, or converting an integer type to a floating-point type, can result in a value that is not representable in the destination type and is undefined behavior on implementations that do not support Annex F.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-197 and FLP34-C

Independent( FLP34-C, INT31-C) FIO34-C = Subset( INT31-C)

CWE-197 = Union( FLP34-C, INT31-C)

CWE-195 and FLP34-C

Intersection( CWE-195, FLP34-C) = Ø

Both conditions involve type conversion. However, CWE-195 explicitly focuses on conversions between unsigned vs signed types, whereas FLP34-C focuses on floating-point arithmetic.

CWE-681 and FLP34-C

CWE-681 = Union( FLP34-C, INT31-C)

Bibliography

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