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From: bader@spice.cs.cmu.edu (Miles Bader)
Newsgroups: comp.lang.c
Subject: structure element offsets
Message-ID: <1096@spice.cs.cmu.edu>
Date: Mon, 24-Nov-86 02:53:05 EST
Article-I.D.: spice.1096
Posted: Mon Nov 24 02:53:05 1986
Date-Received: Mon, 24-Nov-86 22:00:33 EST
Organization: Carnegie-Mellon University, CS/RI
Lines: 29
Keywords: structure, offset

Is there any way of finding the offset of a structure element from
the beginning of a structure in a portable AND efficient way?  I have
a structure that looks like this:

struct fentry{
    FPOOL   *pool;
    union{
	FENTRY	*next;
	char    buf[1]; /* extended as needed ... */
    }	    data;
};

And I want to find the base of the structure given address of buf.
My first one went like:

static fentry calc;
#define FALLOC_OFFSET    ((long)calc.data.buf-(long)&calc)

This worked, and with pcc on an sun, compiled into the correct
constant, 4.  However on an ibm rt (under both pcc and another
non-pcc compiler), it did the calculation at run time (despite the
fact that the two quantities being subtracted were the same address
with offsets differing by 4).

I have given up and defined this as the constant 4 (which it is on
any machine around here), but am just curious if there's a better
way.

				    -Miles