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Integer types smaller than int are promoted when an operation is performed on them. If all values of the original type can be represented as an int, the value of the smaller type is converted to an int; otherwise, it is converted to an unsigned int (see INT02-C. Understand integer conversion rules). If the conversion is to a wider type, the original value is zero-extended for unsigned values or sign-extended for signed types. Consequently, bitwise operations on integer types smaller than int may have unexpected resultsAll char and short integer types are promoted to int or unsigned int before they are used in expressions. Consequently, they receive high-order bits. These bits are either zero-extended for unsigned chars and shorts, or are sign-extended for signed chars and shorts. Arithmetic operations performed on ints yield the same values as on chars and shorts (at least in the low-order bits). However, bitwise operations may yield surprising effects.

Noncompliant Code Example

This noncompliant code example demonstrates how promotion to int yields surprising effectsperforming bitwise operations on integer types smaller than int may have unexpected results.

uint8_t port = 0x5aU0x5a;
uint8_t result_8 = ( ~port ) >> 4;

In this example, a bitwise complement of port is negated, first computed and then shifted 4 bits to the right. If both of these operations were are performed on a an 8-bit unsigned integer, then result_8 would will have the value 0x0aU 0x0a. However, port will is first be promoted to a 32-bit integer (signed or unsigned, depending on implementation), signed int, with the following results (on a typical architecture where type int is 32 bits wide):

Compliant Solution

In this compliant solution, we truncate the negation back down the bitwise complement of port is converted back to 8 bits. Consequently, result_8 receives is assigned the expected value of 0x0aU.

uint8_t port = 0x5aU0x5a;
uint8_t result_8 = (static_cast<uint8_t>uint8_t) (~port)) >> 4;

Risk Assessment

Bitwise operations on shorts and chars can produce incorrect data.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

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Bibliography

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