richardm@runx.oz.au (Richard Murnane) (02/17/91)
Hi all,
Does anybody know a decent macro to find the offset of an element
in a structure, e.g. if I have a structure
typedef struct ABC
{
int a;
char b[5];
float c;
} ABC_STRUCT;
I need a macro hich will let me say OFFSET(ABC_STRUCT,c), which will
give me the offset of the float in the above structure.
I have tried something like this:
#define OFFSET(s,e) & (((s *) NULL)->e)
and the above followed by -NULL to force the compiler to evaluate the
macro as the difference of two addresses (and to avoid problems with
different memory models).
Microsoft C 5.1 chokes on the macro, when I try to initialise elements
of a table with such an expression (although if I remove the "&", as I
might to evaluate the address of the array b in the structure, the
compiler accepts it. However, I'm looking for something that will work
for the general case).
I vaguely remember doing something like this for a brain-dead Intermetrics
compiler a few years ago, so it must be possible :-) I just don't remember
how!
Thanks in advance,
- Richard
--
Richard P. Murnane "Let's just be friends." ?
Internet: richardm@runxtsa.runx.oz - Just Say No.
Packet Radio: VK2JRM @ VK2OP.NSW.AUS.OCenag@ifi.uio.no (Erik Naggum) (02/19/91)
In article <1991Feb16.232713.6001@runx.oz.au>, Richard Murnane writes: > Does anybody know a decent macro to find the offset of an element > in a structure I snatched this from /local/lib/gcc-include/stddef.h. #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) This works like a charm on a vast number of compilers I've used. -- [Erik Naggum] <enag@ifi.uio.no> Naggum Software, Oslo, Norway <erik@naggum.uu.no>
gwyn@smoke.brl.mil (Doug Gwyn) (02/20/91)
In article <ENAG.91Feb19131201@holmenkollen.ifi.uio.no> enag@ifi.uio.no (Erik Naggum) writes: >#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) What I would recommend is to key on __STDC__, #including <stddef.h> in a Standard C environment and using a definition like that one in other environments. It should work in most non-standard C environments (but not all of them!).
ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) (02/21/91)
In article <ENAG.91Feb19131201@holmenkollen.ifi.uio.no>, enag@ifi.uio.no (Erik Naggum) writes: > I snatched this from /local/lib/gcc-include/stddef.h. > #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) > This works like a charm on a vast number of compilers I've used. offsetof() being a required part of ANSI C (if you use the right header), it's clearly the right interface. You'll want an #if __STDC__ so that you get the "official" version when it's there, as the cast of zero is not portable. -- Professional programming is paranoid programming
bengsig@dk.oracle.com (Bjorn Engsig) (02/21/91)
In article <1991Feb16.232713.6001@runx.oz.au>, Richard Murnane writes: |Does anybody know a decent macro to find the offset of an element |in a structure And in article <ENAG.91Feb19131201@holmenkollen.ifi.uio.no> enag@ifi.uio.no (Erik Naggum) replied: |#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) |This works like a charm on a vast number of compilers I've used. Actually, ANSI C includes an offsetof macro in <stddef.h>. And please do not enter the usual discussion of wheather the macro definition above is valid ANSI C or not. The rationale of the ANSI C will fill you in with the details of this. -- Bjorn Engsig, ORACLE Corporation, E-mail: bengsig@oracle.com, bengsig@oracle.nl "Stepping in others footsteps, doesn't bring you ahead"
dcurtis@crc.ac.uk (Dr. David Curtis) (02/21/91)
In article <1991Feb16.232713.6001@runx.oz.au> richardm@runx.oz.au (Richard Murnane) writes: >Hi all, >Does anybody know a decent macro to find the offset of an element >in a structure? How about: #define offset(str,el) ((char*) &((str*)NULL)->el - (char*) NULL)