[metadata]
id = "INT34-C"
type = "rule"
category = "INT"
number = 34
title = "Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand"
description = """
Bitwise shifts include left-shift operations of the formshift-
expression<<additive-expressionand right-shift operations of the formshift-
expression>>additive-expression. The standard integer promotions are first
performed on the operands, each of which has an integer type. The type of the
result is that of the promoted left operand. If the value of the right operand
is negative or is greater than or equal to the width of the promoted left
operand, the behavior isundefined. (Seeundefined behavior 48.) Do not shift an
expression by a negative number of bits or by a number greater than or equal to
theprecisionof the promoted left operand. The precision of an integer type is
the number of bits it uses to represent values, excluding any sign and padding
bits. For unsigned integer types, the width and the precision are the same;
whereas for signed integer types, the width is one greater than the precision.
This rule uses precision instead of width because, in almost every case, an
attempt to shift by a number of bits greater than or equal to the precision of
the operand indicates a bug (logic error). A logic error is different from
overflow, in which there is simply a representational deficiency. In general,
shifts should be performed only on unsigned operands. (SeeINT13-C. Use bitwise
operators only on unsigned operands.) The result ofE1 << E2isE1left-shiftedE2bit
positions; vacated bits are filled with zeros. The following diagram illustrates
the left-shift operation.
"""
severity = "Low"
likelihood = "Unlikely"
priority = "P2"
level = "L3"
cert_version = "2016 Edition (Wiki)"
last_modified = "Aug 31, 2025"
[rules.cert_c.INT34-C]
enabled = true
[references]
wiki = "https://wiki.sei.cmu.edu/confluence/display/c/INT34-C.+Do+not+shift+an+expression+by+a+negative+number+of+bits+or+by+greater+than+or+equal+to+the+number+of+bits+that+exist+in+the+operand"
cwe = ["CWE-682", "CWE-758"]