Why is Left Shifting a Negative Number Undefined Behavior in C but Well-Defined in C ?

Undefined Behavior in Left Shift Operation with Negative Left Operand in C

In C, bitwise left shift operation (<<) invokes undefined behavior when the left side operand has a negative value. This is specified in ISO C99 (6.5.7/4), which indicates that the result of left shifting a negative value (signed type) is undefined.</p><p><strong>Rationale for Undefined Behavior in C</strong></p><p>The Standard does not provide an explicit reason for this undefined behavior. However, it can be inferred that this is due to the difficulty in defining a consistent and predictable result for left shifting negative values, particularly considering the implementation-dependent behavior of negative values in two's complement representation.</p><p><strong>Well-Defined Behavior in C </strong></p><p>In contrast to C, the left shift operation in C (ISO C -03 5.8/2) does not invoke undefined behavior for signed types if the result can be represented in the result type. This change was made by the ISO C committee to ensure consistency in the behavior of left shift operation regardless of the operand's sign.</p><p><strong>Implementation-Defined Behavior in Right Shift Operation with Negative Left Operand</strong></p><p>Unlike left shift operation, the right shift operation (<code >>>) invokes implementation-defined behavior when the left operand is negative. According to the Standard, this is because the truncation of bits from the left is implementation-dependent for negative values in two's complement representation.

Conclusion

The reason for undefined behavior in left shift operation with negative operands in C and well-defined behavior in C is primarily due to the complexity of defining a consistent result for negative values. The implementation-defined behavior of right shift operation on negative operands reflects the fact that the truncation of bits from the left in two's complement representation is implementation-specific.

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