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The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. According to C99, if E1 has an unsigned type, the value of the result is E1 * 2E2, reduced modulo one more than the maximum value representable in the result type. Although C99 specifies modulo behavior for unsigned integers, unsigned integer overflow frequently results in unexpected values and resultant security vulnerabilities (see INT32-C. Ensure that operations on signed integers do not result in overflow). Consequently, unsigned overflow is generally noncompliant, and E1 * 2E2 must be representable in the result type. Modulo behavior is allowed under exception INT36INT34-EX1.
This noncompliant code example can result in undefined behavior because there is no check to ensure that the right operand is less than or equal to the width of the promoted left operand.
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