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The following attributes internal representations of bit-fields field structures have several properties (such as internal padding) that are implementation-defined. Additionally, bit-field structures have several implementation-defined constraints:

• The alignment of bit-fields in the storage unit . For (for example, the bit-fields may be allocated from the high end or the low end of the storage unit.)
• Whether or not bit-fields can overlap a storage unit boundary.

Consequently, it is impossible to write portable safe code that makes assumptions about regarding the layout of bit-field structuresstructure members.

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Noncompliant Code Example (Bit-Field Alignment)

Bit-fields can be used to allow flags or other integer values with small ranges to be packed together to save storage space.   Bit-fields can improve the storage efficiency of structures. Compilers typically allocate consecutive bit-field structure members into the same int-sized storage, as long as they fit completely into that storage unit. However, the order of allocation within a storage unit is implementation-defined. Some implementations are "right-to-left": the first member occupies the low-order position of the storage unit. Others are "left-to-right": the first member occupies the high-order position of the storage unit. Calculations that depend on the order of bits within a storage unit may produce different results on different implementations.

Consider the following structure made up of four 8-bit bit-field members.:

struct  bf {
  unsigned int m1 : 8;
  unsigned int m2 : 8;
  unsigned int m3 : 8;
  unsigned int m4 : 8;

};	/* 32 bits total */

Right-to-left implementations will allocate struct bf as one storage unit with this format:

m4   m3   m2   m1

Conversely, left-to-right implementations will allocate struct bf as one storage unit with this format:

m1   m2   m3   m4

The following code behaves differently depending on whether the implementation is left-to-right or right-to-left.:

struct  bf {
  unsigned int m1 : 8;
  unsigned int m2 : 8;
  unsigned int m3 : 8;
  unsigned int m4 : 8;
};	 /* 32 bits total */

void function() {
  struct bf data;
  unsigned char *ptr;

  data.m1 = 0;
  data.m2 = 0;
  data.m3 = 0;
  data.m4 = 0;
  char* ptr = (unsigned char *) &data;
  (*ptr)++; /* couldCan increment data.m1 or data.m4 */
}

Compliant Solution (Bit-Field Alignment)

This code compliant solution is explicit about the in which fields it modifies.:

struct  bf {
  unsigned int m1 : 8;
  unsigned int m2 : 8;
  unsigned int m3 : 8;
  unsigned int m4 : 8;
};	 /* 32 bits total */

void function() {
  struct bf data;
  data.m1 = 0;
  data.m2 = 0;
  data.m3 = 0;
  data.m4 = 0;
  data.m1++;
}

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Noncompliant Code Example (Bit-Field Overlap)

In this non-compliant noncompliant code example, assuming eight assuming 8 bits to a byte, if bit-fields of six and four of 6 and 4 bits are declared, is each bit-field contained within a byte, or are the bit-fields split across multiple bytes?

struct  bf {
  unsigned int m1 : 6;
  unsigned int m2 : 4;
};


void function() {
  unsigned char *ptr;
  struct bf data;
  data.m1 = 0;
  data.m2 = 0;
  char* ptr = (unsigned char *) &data;
  ptr++;
  *ptr += 1; /* whatWhat does this increment? */
}

In the above example, if If each bit-field lives within its own byte, then m2 (or m1, depending on alignment) is incremented by 1. If the bit-fields are indeed packed across 8-bit bytes, then m2 might be incremented by 4.

Compliant Solution (Bit-Field Overlap)

This compliant solution is explicit in which fields it modifies:

struct  bf {
  unsigned int m1 : 6;
  unsigned int m2 : 4;
};


void function() {
  struct bf data;
  data.m1 = 0;
  data.m2 = 0;
  data.m2 += 1;
}

Risk Assessment

Making invalid assumptions about the type of a type-cast data, especially bit-field or its layout fields, can result in unexpected data values.

Automated Detection

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2, "Type specifiers"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "BQF Unspecified Behaviour" and "STR Bit Representations"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 3.5
\[[Plum 85|AA. C References#Plum 85]\] Rule 6-5

Related Guidelines

Bibliography

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Image Added Image Added Image AddedINT10-A. Do not assume a positive remainder when using the % operator      04. Integers (INT)       INT12-A. Do not make assumptions about the type of a plain int bit-field when used in an expression