From: Andrew Haley <aph@redhat.com>
To: gcc@gcc.gnu.org
Subject: g++: __attribute__ ((aligned)) weirdness
Date: Tue, 18 Mar 2003 16:16:00 -0000 [thread overview]
Message-ID: <15991.15030.380667.367246@cuddles.cambridge.redhat.com> (raw)
The doc says
The `aligned' attribute can only increase the alignment; but you
can decrease it by specifying `packed' as well. See below.
but this doesn't seem to be true. For example, on x86 g++ you have a non-POD
class and a subclass:
-----------------------------------------------------------------------------
class Boof
{
int arse ()
{
return 25;
}
void *prickle;
};
class foo : Boof
{
public: long long i;
};
-----------------------------------------------------------------------------
then try:
-----------------------------------------------------------------------------
foo f;
fprintf(stderr, "offset: %d\n", (char *)&f.i - (char *)&f);
-----------------------------------------------------------------------------
offset: 4
Fair enough, this long long is probably 4-aligned.
Now do this:
-----------------------------------------------------------------------------
class bar : Boof
{
public: long long __attribute__((aligned(4))) i;
};
bar b;
fprintf(stderr, "offset: %d\n", (char *)&b.i - (char *)&b);
-----------------------------------------------------------------------------
offset: 8
So you have a field at offset 4. You ask for it to be 4-aligned, and
it jumps to offset 8.
What is going on?
Andrew.
next reply other threads:[~2003-03-18 15:26 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-03-18 16:16 Andrew Haley [this message]
2003-03-18 20:24 ` Piotr Wyderski
2003-03-18 20:28 ` Andrew Haley
2003-03-18 22:34 ` Andreas Tobler
2003-04-08 8:01 ` PPC support in GCC ? A.R. Ashok Kumar
2003-04-08 14:45 ` Falk Hueffner
2003-04-08 14:51 ` A.R. Ashok Kumar
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