NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c/510

Internet Message Format | 1996-08-06 | 1.6 KB

Path: teal.csn.net!not-for-mail
From: [e-mail] (Thad Smith)
Newsgroups: comp.std.c
Subject: Bit-field sizes
Date: 5 Mar 1996 23:49:35 -0700
Organization: T3 Systems
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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?

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).

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.

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

Thad