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. According to the C Standard [ISO/IEC 9899:2011], C integer promotions also require that "if an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int."

This issue is similar to the signedness of plain char, discussed in INT07-C. 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 behavior is the same 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 previously when the specified width is at least as narrow as CHAR_BIT*sizeof(int), but wider bit-fields are not portable.

Noncompliant Code Example

This noncompliant 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.

Automated Detection

Tool	Version	Checker	Description
Astrée		bitfield-type	Fully checked
Axivion Bauhaus Suite		CertC-INT12
CodeSonar		LANG.TYPE.BFSIGN	Bit-field signedness not explicit
Compass/ROSE
ECLAIR		CC2.INT12	Fully implemented
Helix QAC		C0634, C0635
Klocwork		MISRA.BITFIELD.TYPE
LDRA tool suite		73 S	Fully implemented
Parasoft C/C++test		CERT_C-INT12-a	Bit fields shall only be defined to be of type unsigned int or signed int
PC-lint Plus		846	Fully supported
Polyspace Bug Finder		CERT C: Rec. INT12-C	Checks for bit-field declared without appropriate type (rec. fully covered)
RuleChecker		bitfield-type	Fully checked
SonarQube C/C++ Plugin		S814

Related Vulnerabilities

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

Related Guidelines

Bibliography