# V2591. MISRA. Bit fields should only be declared with explicitly signed or unsigned integer type

This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).

This rule is only relevant to C. According to various C language standard versions, bit fields must be declared only with corresponding types. For C90: 'signed int' or 'unsigned int'. For C99: 'signed int', 'unsigned int', '_Bool' or another integral type allowed by the implementation must explicitly specify the modifier 'signed' or 'unsigned'.

It is also allowed to use an alias ('typedef') for a valid type.

A bit field of 'int' type can be 'signed' or 'unsigned' depending on the compiler. If you use 'unsigned int' to represent a bit field, all the bits allocated for the field will be significant. Such a bit field of 'n' bits has a range of values '[0, 2 ^ n - 1]'.

If you use 'signed int' to represent the bit field, 1 bit will be allocated for the sign. To write a significant part of a bit field value a developer will use 1 bit less than allocated. Such a bit field of 'n' bits has a range of values '[-2 ^ (n - 1), 2 ^ (n - 1) - 1]'.

So, depending on the compiler, bit fields of 'int' type may have different ranges of values. To avoid potential errors, explicitly specify the modifier - 'signed' or 'unsigned'.

An example of a bit field misuse:

    struct S
    {
      int b : 3; // <=
    };

    void foo()
    {
      S s;
      s.b = 5;
      if (s.b != 5)
      {
        Boom();
      }
    }

In this example, the compiler may choose an unsigned type for 'b' bit field representation. This way all 3 bits in which 5 is written will be significant. In fact, a modulo 8 residue is written. The code will work as expected — 5 will be written to the bit field.

If the compiler chooses a signed type to represent the 'b' bit field, the latter splits into 1 bit sign and 2 bits of a significant part. When writing 5 in 'b', only 2 low bits will be written in significant part. 1 will be written in the bit field instead of 5. The check will pass, the 'Boom' function will be called.

To fix it, explicitly specify the modifier - 'signed'/'unsigned':

    struct S
    {
      unsigned int b : 3;
    };

    void foo()
    {
      S s;
      s.b = 5;
      if (s.b != 5)
      {
        Boom();
      }
    }

You can explicitly specify which types are allowed to declare bit fields by your compiler that supports C99 standard. To do this, add the following comment to the source code or the pvsconfig file.

    //V_2591_ALLOWED_C99_TYPES:short,long long

After the colon, write allowed types in a comma-separated list. Don't specify signed/unsigned:

- 'char'
- 'short'
- 'long'
- 'long long'

This diagnostic is classified as:

MISRA-C-2012-6.1 MISRA-C-2023-6.1