Float overflow

Overflow from operation between floating points

Description

This defect occurs when an operation on floating point variables can result in values that cannot be represented by the result data type. The data type of a variable determines the number of bytes allocated for the variable storage and constrains the range of allowed values.

The exact storage allocation for different floating point types depends on your processor. See Target processor type (-target).

Risk

Overflows can result in unpredictable values from computations. The result can be infinity or the maximum finite value depending on the rounding mode used in the implementation. If you use the result of an overflowing computation in subsequent computations and do not account for the overflow, you can see unexpected results.

Fix

The fix depends on the root cause of the defect. Often the result details show a sequence of events that led to the defect. Use this event list to determine how the variables in the overflowing computation acquire their current values. You can implement the fix on any event in the sequence. If the result details do not show the event history, you can trace back using right-click options in the source code and see previous related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface.

See examples of fixes below.

If you do not want to fix the issue, for instance, when you handle infinities in your code, add comments to your result or code to avoid another review. See Address Polyspace Results Through Bug Fixes or Justifications.

Examples

Multiplication of Floats

#include <float.h>

float square(void) {

   float val = FLT_MAX;
   return val * val;  
}

In the return statement, the variable val is multiplied by itself. The square of the maximum float value cannot be represented by a float (the return type for this function) because the value of val is the maximum float value.

Correction — Different Storage Type

One possible correction is to store the result of the operation in a larger data type. In this example, by returning a double instead of a float, the overflow defect is fixed.

#include <float.h>

double square(void) {
    float val = FLT_MAX;

    return (double)val * (double)val;  
}

Result Information

Group: Numerical

Language: C | C++

Default: Off

Command-Line Syntax: FLOAT_OVFL

Impact: Low

CWE ID: 189, 682, 873