Fwd: gcc structures

[JimJoyce <jim at jimjoyce dot co dot uk>]

Thanks, again.  JJ.


---------- Forwarded message ----------
From: Jim Joyce <jim@jimjoyce.co.uk>
Date: 8 September 2013 14:21
Subject: Re: gcc structures
To: "Jonathan Wakely-4 [via gcc]" <ml-node+s1065356n966604h15@n5.nabble.com>


Thanks, Jonathan, for your speedy reply.

However, I'm surprised, That 'C' can pad structures as it sees fit.
I thought the point and value of user-defined structures was to suit user's
needs.
not the whim of the compiler..

I think you're telling me that   '__attribute__((__packed__))'   is what I
need to do  to force the compiler to do things my way?

I learned my coding in the old days, using IBM 360 Assembler.
We programmers told the system how we wanted things.
Happy Days !

Thanks, JJ


On 8 September 2013 13:30, Jonathan Wakely-4 [via gcc] <
ml-node+s1065356n966604h15@n5.nabble.com> wrote:

> On 8 September 2013 11:10, JimJoycewrote:
> > Is this a bug?
>
> No.
>
> > I'm coding in plain 'C'.
> > I am trying to read and write GIS shapefiles.
> > They start with a header of 9 ints and 8 doubles.
> >
> > So I used a structure eg:
> > struct hdr { int g1[9]; double g2[8] }; struc;
> >  fread ( struc, 100, 1, fp1);
> > It was screwing up my doubles.
> > --
> > When, eventually, I experimented:
> > sizeof(g1); sizeof(g2), sizeof (struc).
> > I got 36, 64, 104.
> > Note 104, not 100.
> >
> > Where is gcc placing the redundant 4 bytes?
>
> Between the two arrays. The array of doubles is 8-byte aligned,
> presumably because that's what your architecture requires.
>
> > Is 'struct' insisting on a doubleword boundary?
>
> For the double array, yes.  The C standard doesn't say struct members
> must be adjacent, padding is allowed.
>
> > NB I got round the problem by using 2 fread()s
> > fread(struc.g1,36,1,fp1);
> > fread(struc.g2,64,1,fp1);
>
> That's the right thing to do.
>
> You could also try this:
>
> struct __attribute__((__packed__)) hdr { int g1[9]; double g2[8]; };
>
> Although the compiler adds the padding for good reasons so it's best
> not to force it to use a different layout unless you really need to.
>
>
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-- 


JimJoyce
T: 0(044) 1280 813 899
E: jim@jimjoyce.co.uk



My question  'Are Structures doublewoord aligned?' was not about the second
half of the structure, but the beginning: an array of 9 ints.
It appears, having decided to place them on a doubleword boundary, it then
had to pad after the 9 ints to get back to an 8-byte boundary.
Was it pure mischance that the structure happened to start on a doubleword
that the extra int was needed. Had it started 4 bytes later, there would be
no padding? Or do structures always start on a doubleword?

Thanks, again,
JJ
-- 


JimJoyce
T: 0(044) 1280 813 899
E: jim@jimjoyce.co.uk




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