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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, Wiki MarkupFrom 6.3.1.5 of the C99 standard \[[4, paragraph 2 [ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]:

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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

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This rule does not apply to machines demotions of floating-point types on implementations that support signed infinity, such as ones that support IEEE - 754, as all numbers values 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

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

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

/* initializations */

f1 = (float)d1big_d;
f2  float f_a = (float)ldd_a;
d2  float f_b = (doublefloat)ldbig_d;
}

As a result of these conversions, it is possible that d1 d_a is outside the range of values that can be represented by a float or that 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 solution properly checks to see whether the values to be stored can be represented 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_MAXbig_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 (big_d != 0.0 &&
      (ld >(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)ldbig_d;
  }  
}

Risk

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Assessment

Converting a floating-point value to Failing to check that a floating-point value fits within a demoted type 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 too large to be represented by the new type, resulting in undefined behaviorthat is not representable in the destination type and is undefined behavior on implementations that do not support Annex F.

Automated Detection

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Related Vulnerabilities

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

References

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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Image Added Image Added Image Added floating types" [IEEE 754-1985] IEEE 754-1985 Standard for Binary Floating-Point ArithmeticFLP33-C. Convert integers to floating point for floating point operations      05. Floating Point (FLP)       06. Arrays (ARR)