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CERT C++: INT30-C

Ensure that unsigned integer operations do not wrap

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Description

Rule Definition

Ensure that unsigned integer operations do not wrap.1

Polyspace Implementation

The rule checker checks for these issues:

  • Unsigned integer overflow.

  • Unsigned integer constant overflow.

A default Bug Finder analysis might not raise a violation of this rule when the input values are unknown and only a subset of inputs can cause an issue. To check for violations caused by specific system input values, run a stricter Bug Finder analysis. See Extend Bug Finder Checkers to Find Defects from Specific System Input Values.

Extend Checker

When the input values are unknown and only a subset of inputs cause an issue, Bug Finder might not detect an Unsigned integer overflow or Unsigned integer constant overflow. To check for violations caused by specific system input values, run a stricter Bug Finder analysis. See Extend Bug Finder Checkers to Find Defects from Specific System Input Values.

Examples

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Issue

Unsigned integer overflow occurs when an operation on unsigned integer 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

The C11 standard states that unsigned integer overflows result in wrap-around behavior. However, a wrap around behavior might not always be desirable. For instance, if the result of a computation is used as an array size and the computation overflows, the array size is much smaller than expected.

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. 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.

You can fix the defect by:

  • Using a bigger data type for the result of the operation so that all values can be accommodated.

  • Checking for values that lead to the overflow and performing appropriate error handling. In the error handling code, you can override the default wrap-around behavior for overflows and implement saturation behavior, for instance.

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

Example - Add One to Maximum Unsigned Integer
#include <limits.h>

unsigned int plusplus(void) {

    unsigned uvar = UINT_MAX;
    uvar++; //Noncompliant
    return uvar;
}

In the third statement of this function, the variable uvar is increased by 1. However, the value of uvar is the maximum unsigned integer value, so 1 plus the maximum integer value cannot be represented by an unsigned int. The C programming language standard does not view unsigned overflow as an error because the program automatically reduces the result by modulo the maximum value plus 1. In this example, uvar is reduced by modulo UINT_MAX. The result is uvar = 1.

Correction — Different Storage Type

One possible correction is to store the operation result in a larger data type. In this example, by returning an unsigned long long instead of an unsigned int, the overflow error is fixed.

#include <limits.h>

unsigned long long plusplus(void) {

    unsigned long long ullvar = UINT_MAX;
    ullvar++;
    return ullvar;
}
Issue

Unsigned integer constant overflow occurs when you assign a compile-time constant to an unsigned integer variable whose data type cannot accommodate the value. An n-bit unsigned integer holds values in the range [0, 2n-1].

For instance, c is an 8-bit unsigned char variable that cannot hold the value 256.

unsigned char c = 256;

To determine the sizes of fundamental types, Bug Finder uses your specification for Target processor type (-target).

Risk

The C standard states that overflowing unsigned integers must be wrapped around (see, for instance, the C11 standard, section 6.2.5). However, the wrap-around behavior can be unintended and cause unexpected results.

Fix

Check if the constant value is what you intended. If the value is correct, use a wider data type for the variable.

Example - Overflowing Constant from Macro Expansion
#define MAX_UNSIGNED_CHAR 255 //Noncompliant
#define MAX_UNSIGNED_SHORT 65535 //Noncompliant

void main() {
    unsigned char c1 = MAX_UNSIGNED_CHAR + 1;
    unsigned short c2 = MAX_UNSIGNED_SHORT + 1;
}

In this example, the defect appears on the macros because at least one use of the macro causes an overflow.

Correction — Use Wider Data Type

One possible correction is to use a wider data type for the variables that overflow.

#define MAX_UNSIGNED_CHAR 255
#define MAX_UNSIGNED_SHORT 65535

void main() {
    unsigned short c1 = MAX_UNSIGNED_CHAR + 1;
    unsigned int c2 = MAX_UNSIGNED_SHORT + 1;
}

Check Information

Group: 03. Integers (INT)

Version History

Introduced in R2019a

See Also

Topics

External Websites

1 This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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