FLP36-C. Beware of precision loss when converting integral types to floating point

Narrower primitive types can be cast to wider types without any effect on the magnitude of numeric values. However, whereas integers represent exact values, floating-point numbers have limited precision. C99 says, in Section 6.3.1.4: "Real floating and integer"

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.

Conversion from integral types to floating-point types without sufficient precision can lead to loss of precision (loss of least significant bits). No runtime exception occurs despite the loss.

Noncompliant Code Example

In this noncompliant example, an int is converted to float.

#include <stdio.h>

int main() {
  int big = 1234567890;
  float approx = big;
  printf("%d\n", (big - (int)approx));
  return 0;
}

When compiled with GCC 4.3.2 on Linux, this program prints the value -46.

Compliant Solution

While not a portable solution, you can improve precision by using double instead of float.

#include <stdio.h>

int main() {
  int big = 1234567890;
  double approx = big;
  printf("%d\n", (big - (int)approx));
  return 0;
}

On the same platform, this program prints 0.

Risk Assessment

Casting numeric types to floating-point types may lose information.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] 6.3.1.4: "Real floating and integer"

INT32-J. Perform conversion from BigInteger to String and back properly      04. Integers (INT)      INT34-J. Perform explicit range checking to ensure integer operations do not overflow