INT19-C. Correctly compute integer widths

Integer types in C have two metrics associated with them, a size and a width. The size indicates the number of bytes used by a variable, and can be retrieved for any variable or type using the sizeof operator, while the width indicates the number of bits that are actually used to encode the integer's value. The C standard permits an integer to contain internal padding bits; these contribute to the integer's size, but not to its width. Consequently, an integer type may have a size that could be used to incorrectly assume its width is larger than it actually is.  Do not use the sizeof operator to compute the width of an integer type.

Noncompliant Code Example

This noncompliant code example illustrates a function that produces 2 raised to the power of the function argument. To prevent undefined behavior, in compliance with INT34-C. Do not shift a negative number of bits or more bits than exist in the operand, the function ensures that the argument is less than the number of bits used to store an unsigned int.

unsigned int pow2(unsigned int exp) {
  if (exp >= sizeof( unsigned int) * CHAR_BIT) {
    /* handle error */
  }
  return 1 << exp;
}

Compliant Solution

This compliant solution uses a popcount() function, which counts the number of bits set on any unsigned integer. This allows this code to count the number of width bits used by the unsigned int type.

/* Returns the number of set bits */
size_t popcount(uintmax_t num) {
  size_t width = 0;
  while (num != 0) {
    if (num % 2 == 1) {
      width++;
    }
    num >>= 1;
  }
  return width;
}

#define UWIDTH(type, umax_value) popcount(umax_value)

unsigned int pow2(unsigned int exp) {
  if (exp >= UWIDTH(unsigned int, UINT_MAX)) {
    /* handle error */
  }
  return 1 << exp;
}

#define UWIDTH(type, umax_value) _popcnt(umax_value)

Exceptions

INT19-EX1: Platforms in which integer types have no padding may use the sizeof operator to compute integer widths. This includes x86 and x86-64 platforms.

#define UWIDTH(type, umax_value) (sizeof(type) * CHAR_BIT)

Mistaking an integer size for its width can permit invalid width arguments to operations such as bitwise shifts, resulting in undefined behavior.

Bibliography