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From: jwf@munsell.UUCP (Jim Franklin)
Newsgroups: comp.lang.c
Subject: sizeof(bit-field structures)
Message-ID: <1190@munsell.UUCP>
Date: Mon, 17-Aug-87 12:12:07 EDT
Article-I.D.: munsell.1190
Posted: Mon Aug 17 12:12:07 1987
Date-Received: Wed, 19-Aug-87 04:20:10 EDT
Organization: Eikonix Corp., Bedford, MA
Lines: 31

Is the size of a bit-field structure required to be at least sizeof(int)?
K&R is vague about this -- all I can find is

        "Field members are packed into machine integers; they do not
        straddle words." (p. 196)

It doesn't say anything about the size of the resulting structure.
As a specific example, consider

#include <sys/types.h>
union xtbl_entry {
	u_short         x_entry;
	struct {
		u_int   x_spare : 5;
		u_int   x_val : 11;
	} x_bits;
};
main ()
{
        printf("sizeof(union xtbl_entry) = %d\n", sizeof(union xtbl_entry));
}

This prints out 2 on my SUN-3 (Release 3.3).  In this case, this is exactly
what I wanted - the union of a short int and a bit-field structure.  But is
this portable?  I suspect not, since another compiler might allocate a
32-bit int for struct x_bits.  Does ANSI-C address this?
-----
{harvard!adelie,{decvax,allegra,talcott}!encore}!munsell!jwf

Jim Franklin, Eikonix Corp., 23 Crosby Drive, Bedford, MA 01730
Phone: (617) 663-2115 x415