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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 C99 to the C Standard [ISO/IEC 9899:19992011], 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 is a similar issue This issue is similar to the signedness of plain char, discussed in recommendation 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 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 previously when the specified width is at least as narrow as CHAR_BIT*sizeof(int), but wider bit-fields are not portable.

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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;
}

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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;
}

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Bibliography

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Image Added Image Added

ISO/IEC TR 24772 "STR Bit Representations"

MISRA Rule 12.7

Bibliography

INT11-C. Take care when converting from pointer to integer or integer to pointer      04. Integers (INT)      Image Modified