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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 Section section 6.3.1.5 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.

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This noncompliant code example leads to undefined behavior if the integral part of f1 cannot be represented as an integer.:

float f1;
int i1;

/* initialize fl */

i1 = f1; /* Undefined if the integral part of f1 > INT_MAX */

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

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

/* initializations */

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

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This compliant solution checks 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;
}

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Related Vulnerabilities

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

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