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CWE Rule 196

Unsigned to Signed Conversion Error

Since R2023a

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Description

Rule Description

The software uses an unsigned primitive and performs a cast to a signed primitive, which can produce an unexpected value if the value of the unsigned primitive can not be represented using a signed primitive.

Polyspace Implementation

The rule checker checks for Sign change integer conversion overflow.

Examples

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Issue

This issue 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 (or source code tooltips in Polyspace as You Code) 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 this event history, you can search for previous references of variables relevant to the defect using right-click options in the source code and find related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface or Interpret Bug Finder Results in Polyspace Access Web Interface (Polyspace Access).

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:

Extend Checker

A default Bug Finder analysis might not raise this defect when the input values are unknown and only a subset of inputs cause an issue. To check for defects 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.

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

Category: Others

Version History

Introduced in R2023a

See Also

Topics

External Websites
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