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. Subclause 6.3.1.4 of the C Standard [ISO/IEC 9899:2011] states:

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 4.8.1 on Linux, this program prints the value -46.

Compliant Solution

This solution replaces the float with a double. Furthermore, it uses a static assertion to guarantee that the double type can represent any int without loss of precision. (See DCL03-C. Use a static assertion to test the value of a constant expression.)

#include <assert.h>
#include <stdio.h>
#include <float.h>
 
/* 8 = bits / char */
static_assert(sizeof(int) * 8 <= DBL_MANT_DIG);

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 can lose information.

Related Vulnerabilities

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

Related Guidelines

Bibliography