INT12-A. Do not make assumptions about the type of a plain int bit-field when used in an expression

Bit-fields can be used to allow flags or other integer values with small ranges to be packed together to save storage space.

It is implementation-defined whether the specifier int designates the same type as signed int or the same type as unsigned int for bit-fields. C integer promotions also require that "If an int can represent all values of the original type, the value is converted to an int; otherwise, it is converted to an unsigned int."

This is a similar issue to the signedness of plain char, discussed in INT07-A. Use only explicitly signed or unsigned char type for numeric values.  A plain int bit-field that is treated as unsigned will promote to int as long as its field width is less than that of int, because int can hold all values of the original type.  This is the same behavior as that of a plain char treated as unsigned.  However, a plain int bit-field treated as unsigned will promote to unsigned int if its field width is the same as that of int.  This difference makes a plain int bit-field even trickier than a plain char.

Bit-field types other than _Bool, int, signed int, and unsigned int are implementation-defined.  They still obey the integer promotions quoted above when the specified width is at least as narrow as CHAR_BIT*sizeof(int), but wider bit-fields are not portable.

Non-Compliant Code Example

This non-compliant code depends on implementation-defined behavior.  It prints either -1 or 255 depending on whether a plain int bit-field is signed or unsigned.

struct {
  int a: 8;
} bits = {255};

int main(void) {
  printf("bits.a = %d.\n", bits.a);
  return 0;
}

Compliant Solution

This compliant solution uses an unsigned int bit-field and does not depend on implementation-defined behavior.

struct {
  unsigned int a: 8;
} bits = {255};

int main(void) {
  printf("bits.a = %d.\n", bits.a);
  return 0;
}

Risk Assessment

Making invalid assumptions about the type of a bit-field or its layout can result in unexpected program flow.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 6.7.2, "Type specifiers"
[[ISO/IEC PDTR 24772]] "STR Bit Representations"
[[MISRA 04]] Rule 12.7

INT11-A. Do not make assumptions about the layout of bit-field structures      04. Integers (INT)       INT13-A. Do not assume that a right shift operation is implemented as a logical or an arithmetic shift