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Avoid performing bitwise and arithmetic operations on the same data. In particular, it is frequently the case that bitwise operations are frequently performed on arithmetic values as a form of premature optimization. Bitwise operators include the unary operator {~}, and the binary operators <<, >>, &, {^}, and |. Although such operations are valid and will compile, they can reduce code readability. Declaring a variable as containing a numeric value or a bitmap makes the programmer's intentions clearer and the code more maintainable.

Bitmapped types may be defined to further separate bit collections from numeric types. This Doing so may make it easier to verify that bitwise operations are performed only performed on variables that represent bitmaps.

typedef uint32_t bitmap32_t;
bitmap32_t bm32x = 0x000007f3;

x = (x << 2) | 3; /* shiftsShifts in two 1-bits from the right */

The typedef name uintN_t designates an unsigned integer type with width N. Consequently, uint32_t denotes an unsigned integer type with a width of exactly 32 bits. Bitmaps are normally should be declared as unsigned. See INT13-C.

Non-Compliant Code Example (left shift)

Use bitwise operators only on unsigned operands.

Left- and right-shift operators are often employed to multiply or divide a number by a power of 2. However, using shift operators to represent multiplication or division is an optimization that renders the code less portable and less readable. Furthermore, most compilers routinely optimize multiplications and divisions by constant powers of 2 with bit-shift operations, and they are more familiar with the implementation details of the current platform.

Noncompliant Code Example (Left Shift)

In this noncompliant In this non-compliant code example, both bit manipulation and arithmetic manipulation are performed on the integer type x. The result is a (prematurely) optimized statement that assigns 5x + 1 to x for implementations where integers are represented as two's complement values.

int compute(int x) {
  int y = x =<< 502;
  x += (x << 2) + 1;
y + 1;
  return x;
}
// ...
 
int x = compute(50);

Although this is a legal valid manipulation, the result of the shift depends on the underlying representation of the integer type and is consequently implementation-defined. Additionally, the readability of the code is reduced.

Compliant Solution (

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Left Shift)

In this compliant solution, the assignment statement is modified to reflect the arithmetic nature of x, resulting in a clearer indication of the programmer's intentions.:

int compute(
int x) = 50;
x ={
  return 5 * x + 1;
}
// ...
 
int x = compute(50);

A reviewer may now recognize that the operation should also be checked for integer overflowwrapping. This might not have been apparent in the original, non-compliant noncompliant code example.

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Noncompliant Code Example (

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Right Shift)

In this non-compliant noncompliant code example, the programmer prematurely optimizes code by replacing a division with a right shift.:

intint compute(int x) {
  x >>= -502;
  return x >>= 2;
}
// ...
 
int x = compute(-50);

Although this code is likely to perform the division correctly, it is not guaranteed to. If x has a signed type and a negative value, the operation is implementation-defined and could can be implemented as either an arithmetic shift or a logical shift. In the event of a logical shift, if the integer is represented in either one's complement or two's complement form, the most significant bit (which controls the sign in a different way for both representations) will be set to zero. This will cause 0, causing a once negative number to become a possibly very large, positive number. For more details, see INT13-C. Use bitwise operators only on unsigned operands.

For example, if the internal representation of x is 0xFFFF FFCE (two's complement), an arithmetic shift results in 0xFFFF FFF3 (-13 −13 in two's complement), while  whereas a logical shift results in 0x3FFF FFF3 (1,073,741,811 in two's complement).

Compliant Solution (

...

Right Shift)

In this compliant solution, the right shift is replaced by division.:

int compute(int x) {
  return
int x =/ -50;
x /= 44;
}
// ...
 
int x = compute(-50);

The resulting value is now more likely to be consistent with the programmer's expectations.

Exceptions

INT14-C-EX0: Routines may treat integers as bit vectors for I/O purposes. That is, they may treat an integer as a series of bits in order to write it to a file or socket. They may also read a series of bits from a file or socket and create an integer from the bits. Bitwise operations are also permitted when reading or writing the data from a tightly packed data structure of bytes.

int value = /* Interesting value */
unsigned char bytes[sizeof(int)];
for (int i = 0; i < sizeof(int); i++) {
  bytes[i] = value >> (i*8) & 0xFF;
}
/* bytes[] now has same bit representation as value  */

NUM01-J-EX1: Bitwise operations may be used to construct constant expressions.

int limit = (1 << 17) - 1; // 2^17 - 1 = 131071

Nevertheless, as a matter of style, it is preferable to replace such constant expressions with the equivalent hexadecimal constants.

int limit = 0x1FFFF; // 2^17 - 1 = 131071

Risk Assessment

Performing bit manipulation and arithmetic operations on the same variable obscures the programmers programmer's intentions and reduces readability. This in turn makes  It also makes it more difficult for a security auditor or maintainer to determine which checks must be performed to eliminate security flaws and ensure data integrity.

Automated Detection

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.6.2, "Integer types"

Related Guidelines

Bibliography

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