Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!hp4nl!esatst!arne
From: arne@yc.estec.nl (Arne Lundberg)
Newsgroups: comp.std.c
Subject: Void pointers and pointer arithmetic
Message-ID: <1055@esatst.yc.estec.nl>
Date: 2 Jan 90 12:59:20 GMT
Reply-To: arne@yc.estec.nl (Arne Lundberg)
Organization: ESTEC/YCV, Noordwijk, The Netherlands
Lines: 39


I am trying to write some code that tries to access members in a structure
by knowing the start address and the offset to a particular member element.
The following program shows a small example (the real program does not 
hardcode the offsets etc.) Is this program legal in ANSI C, will it
produce the desired result? 

The (Non-ANSI) compilers I have tested either gives the value for `a'
three times or complains about ``unknown size for pointer to void''.
It works perfectly well if I change the type of p to be ``char *''.

---------------------------------------
struct x {
    int a, b, c;
} x = {
    1, 2, 3
};

main()
{
    void *p = &x;
    printf("a %d\n", *(int *)(p + 0));
    printf("b %d\n", *(int *)(p + 4));
    printf("c %d\n", *(int *)(p + 8));
}
---------------------------------------


BTW,
is it possible to write an ANSI compatible ``offsetof'' macro
for traditional C compilers? I am now using the XtOffset macro
from the X11/Toolkit but would like to have something that doesn't
require an additional name for the ``pointer to the structure'' type.

Arne Lundberg

European Space Technology Centre, Noordwijk, the Netherlands
arne@yc.estec.nl or ALUNDBER@ESTEC.BITNET
Phone: +31 1719 84865, Fax: +31 1719 12142, Telex: 39098