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 \[4, paragraph 2 [ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]:9899:2024], says, Wiki Markup
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 When a
double
is demoted tofloat
or along double
is demoted todouble
orfloat
...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 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
This non-compliant code example illustrates possible undefined behavior associated with demoting floating point represented numbers.
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:
Code Block | ||||
---|---|---|---|---|
| ||||
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.
Code Block | ||||
---|---|---|---|---|
| ||||
#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:
Code Block | ||||
---|---|---|---|---|
| ||||
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;
} | ||||
Code Block | ||||
| ||||
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
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.:
Code Block | ||||
---|---|---|---|---|
| ||||
#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)ldbig_d; } } |
Risk
...
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.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
FLP34-C |
1 (low)
1 (unlikely)
Low | Unlikely | Low | P3 | L3 |
Automated Detection
Tool |
---|
...
Version | Checker | Description | |||||||
---|---|---|---|---|---|---|---|---|---|
Astrée |
| Supported Astrée reports all potential overflows resulting from floating-point conversions. | |||||||
Compass/ROSE | Can detect some violations of this rule. However, it does not flag implicit casts, only explicit ones | ||||||||
CodeSonar |
| LANG.TYPE.IAT | Inappropriate Assignment Type | ||||||
| MISRA_CAST (needs verification) | Can detect instances where implicit float conversion is involved: implicitly converting a complex expression with integer type to floating type, implicitly converting a double expression to narrower float type (may lose precision), implicitly converting a complex expression from | |||||||
Helix QAC |
| C4450, C4451, C4452, C4453, C4454, C4462, C4465 C++3011 | |||||||
Klocwork |
| MISRA.CAST.FLOAT.WIDER | |||||||
LDRA tool suite |
| 435 S, 93 S | Partially implemented | ||||||
Parasoft C/C++test |
| CERT_C-FLP34-a CERT_C-FLP34-b | Avoid implicit conversions from wider to narrower floating type | ||||||
PC-lint Plus |
| 735, 736, | Partially supported | ||||||
Polyspace Bug Finder |
| Checks for float conversion overflow (rule partially covered) | |||||||
PVS-Studio |
| V615, V2003, V2004 | |||||||
TrustInSoft Analyzer |
| float_to_int | Exhaustively verified (see one compliant and one non-compliant example). |
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)
Taxonomy | Taxonomy item | Relationship |
---|---|---|
CERT Oracle Secure Coding Standard for Java | NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data | Prior to 2018-01-12: CERT: Unspecified Relationship |
ISO/IEC TR 24772:2013 | Numeric Conversion Errors [FLC] | Prior to 2018-01-12: CERT: Unspecified Relationship |
CWE 2.11 | CWE-681, Incorrect Conversion between Numeric Types | 2017-06-29: CERT: Rule subset of CWE |
CWE 2.11 | CWE-197 | 2017-06-14: CERT: Rule subset of CWE |
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
[IEEE 754 2006] | |
[ISO/IEC 9899:2024] | Subclause |
...
6.3.1. |
...
4, |
...
"Real Floating and Integer" Subclause 6.3.1.5, "Real Floating Types" |
...
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)