Most variables in C have a size that is an integral number of bytes. Bit-fields are a part of a structure that don't necessarily occupy a integral number of bytes; they can any number of bits. Multiple bit-fields can be packed into a single storage unit. They are a part of standard C, but there are many aspects that are implementation defined. They are one of the least portable parts of C.
|identifier||The name for this field in the structure|
|size||The number of bits to use for this field|
The only portable types for bit-fields are
_Bool. The plain
int type can be used, but the standard says (§6.7.2¶5) … for bit-fields,
it is implementation-defined whether the specifier
int designates the same type as
signed int or the same type as
Other integer types may be allowed by a specific implementation, but using them is not portable.