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CERT C: Rec. INT02-C

Understand integer conversion rules

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Description

Rule Definition

Understand integer conversion rules.1

Polyspace Implementation

The rule checker checks for Sign change integer conversion overflow.

Extend Checker

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.

Examples

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Issue

Sign change integer conversion overflow occurs when converting an unsigned integer to a signed integer. If the variable does not have enough bytes to represent both the original constant and the sign bit, the conversion overflows.

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

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.

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 - Convert from unsigned char to char
char sign_change(void) {
    unsigned char count = 255;

    return (char)count; //Noncompliant
}

In the return statement, the unsigned character variable count is converted to a signed character. However, char has 8 bits, 1 for the sign of the constant and 7 to represent the number. The conversion operation overflows because 255 uses 8 bits.

Correction — Change conversion types

One possible correction is using a larger integer type. By using an int, there are enough bits to represent the sign and the number value.

int sign_change(void) {
    unsigned char count = 255;

    return (int)count;
}

Check Information

Group: Rec. 04. 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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