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Deprecated

This guideline has been deprecated by

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 results.

Noncompliant Code Example

This noncompliant code example demonstrates how performing bitwise operations on integer types smaller than int may have unexpected results.

uint8_t port = 0x5a;
uint8_t result_8 = ( ~port ) >> 4;

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

Expression

Type

Value

Notes

port

uint8_t

0x5a


~port

int

0xffffffa5


~port >> 4

int

0x0ffffffa

Whether or not value is negative is implementation-defined.

result_8

uint8_t

0xfa


Compliant Solution

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

uint8_t port = 0x5a;
uint8_t result_8 = (uint8_t) (~port) >> 4;

Risk Assessment

Bitwise operations on shorts and chars can produce incorrect data.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

EXP14-C

low

likely

high

P3

L3

Automated Detection

Tool

Version

Checker

Description

CodeSonar
4.5p1
LANG.CAST.RIPRisky integer promotion
Compass/ROSE




ECLAIR

1.2

CC2.EXP14

Fully implemented

Parasoft C/C++test
10.4

CERT_C-EXP14-a

Avoid mixing arithmetic of different precisions in the same expression

Related Vulnerabilities

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

Related Guidelines



  

7 Comments

  1. Since this is a C guideline, the compliant example should be changed to:

    uint8_t result_8 = (uint8_t) (~port) >> 4;
    
    1. Thanks, I fixed this problem and also tried to make this a little more precise.

      I'm giving some thought as to if this should be a separate guideline, or if it should be eliminated and the example integrated with INT02-C. Understand integer conversion rules.

      1. This has now been cleaned up (a bunch). If this is going to remain an "informational" recommendation then I would combine it with INT02-C.

        If we keep it as a separate guideline it should be strengthened to "Do not perform bitwise operations on integer types smaller than int" and doing so should be diagnosed.

        Of course, this is not right either because the compliant solution still violates this. At the end of the day, I think this guideline is just going to say "understand integer promotions/conversion rules" and consequently should be combined.

        1. Since this is a subset of INT02-C. Understand integer conversion rules merging the two makes sense to me.

        2. I agree with combining them. I actually think the example given here was more of a gotcha due to the use of '~' in particular, and not really related to bitwise operations in general.

          1. I've merged this guideline with INT02-C. Understand integer conversion rules and marked it as deprecated.

  2. gcc 4.8.4 warns about 1u << 63ul (assuming 64-bits long and 32-bits int) and computes 0. Rightfully so? No promotion from 1u to 1ul.

    ISO/IEC 9899:201x, 6.3.1.8 (Usual arithmetic conversions): "Many operators that expect operands of arithmetic type cause conversions"; 6.5.7 (Bitwise shift operators): "The integer promotions are performed on each of the operands...". But I am not able conclude from that.