🚀 go-pugleaf

In article <nzRPxQ9ytZZA084yn@csn.net>, thads@csn.net (Thad Smith) writes:
> Can the size of a bit-field, measured in bits, exceed the size of an
> int?  The description of bit-fields describes it as an integral
> type with a (programmer-)specified number of bits, not specifically
> constrained in size.  ANSI Classic section 3.2.1.1 says, however, that
> bit-fields are promoted to either int or unsigned int.  This would
> seem to prohibit promoting a large bit-field to a long.  If so, why
> was this constraint imposed?

No.  ANSI Classic 3.5.2.1:

	The expression that specifies the width of a bit-field shall be
	an integral constant expression that nas nonnegative value that
	shall not exceed the number of bits in an ordinary object of
	compatible type.

The constraint is historical, probably because extracting a bit-field
that is longer than a word is sufficiently complicated as to be contrary
to the ``spirit of C''.

> Secondly, is the storage unit within which bit-fields are allocated
> constrained to be a single size or can it vary, depending on the size
> of the bit fields?  I see nothing in the standard prohibiting
> different size storage units for bit-fields.  Also bit-fields are
> explicitly allowed to span multiple storage units, at least if the
> bit-field is not the first in a structure (the use of the word "may"
> makes the requirement for the first bit-field ambiguous with regard to
> storage in a single unit).

It can vary (my emphasis):

	An implementation may allocate *any* addressable storage unit
	large enough to hold a bit-field.

Thus, an implementation is free to allocate a char for a 1 bit field and
an int for a 14 bit field.

> If different size storage units are allowed or any bit-field is
> allowed to span multiple units, the size of a struct containing
> bit-fields might be 1 byte, while bit-fields could be allocated up to
> the size (in bits) of ints, possibly four bytes in a particular
> implementation, by choosing larger storage units for larger
> bit-fields.

Correct.

> Is anyone aware of an implementation using bit-field storage units
> smaller than int?

Lots of them.
----
Larry Jones, SDRC, 2000 Eastman Dr., Milford, OH  45150-2789  513-576-2070
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