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Right operand of shift operation outside allowed bounds

Overflow from shifting operation

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Description

This defect occurs when a shift operation 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 data types depends on your processor. See Target processor type (-target).

Risk

Shift operation overflows can result in undefined behavior.

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).

You can fix the defect by:

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

  • Checking for values that lead to the overflow and performing appropriate error handling.

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.

Examples

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int left_shift(void) {

    int foo = 33;
    return 1 << foo; 
}

In the return statement of this function, bit-wise shift operation is performed shifting 1 foo bits to the left. However, an int has only 32 bits, so the range of the shift must be between 0 and 31. Therefore, this shift operation causes an overflow.

Correction — Different storage type

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

long long left_shift(void) {

    int foo = 33;
    return 1LL << foo; 
}

Result Information

Group: Numerical
Language: C | C++
Default: Off
Command-Line Syntax: SHIFT_OVFL
Impact: Low

Version History

Introduced in R2013b

See Also

Topics