EXP14-C. Beware of integer promotion when performing bitwise operations on chars or shorts

All 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 example demonstrates how promotion to int yields surprising effects.

uint8_t port = 0x5aU;
uint8_t result_8 = ( ~port ) >> 4;

In this example, port is negated, and shifted 4 bits to the right. If these operations were performed on a 8-bit integer, then result_8 would have the value 0x0aU. However, port will first be promoted to a 32-bit integer (signed or unsigned, depending on implementation), with the following results:

Compliant Solution

In this compliant solution, we truncate the negation back down to 8 bits. Consequently, result_8 receives the expected value of 0x0aU.

uint8_t port = 0x5aU;
uint8_t result_8 = (static_cast<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

C++ Secure Coding Standard: EXP15-CPP. Beware of integer promotion when performing bitwise operations on chars or shorts

Bibliography

[MISRA 2004] Rule 10.5

      03. Expressions (EXP)