From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Dilger <mark.dilger@enterprisedb.com>
cc: Robert Haas <robertmhaas@gmail.com>, Peter Geoghegan <pg@bowt.ie>,
        "Andrey M. Borodin" <x4mmm@yandex-team.ru>,
        Stephen Frost <sfrost@snowman.net>,
        Michael Paquier <michael@paquier.xyz>, Amul Sul <sulamul@gmail.com>,
        Dilip Kumar <dilipbalaut@gmail.com>,
        Andres Freund <andres@anarazel.de>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: new heapcheck contrib module
In-reply-to: <95051.1603406715@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Thu, 22 Oct 2020 18:45:15 -0400"
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Content-ID: <113946.1603417275.1@sss.pgh.pa.us>
Date: Thu, 22 Oct 2020 21:41:15 -0400
Message-ID: <113947.1603417275@sss.pgh.pa.us>
Precedence: bulk

I wrote:
> So now I think this is a REDIRECT on either architecture, but the
> offset and length fields have different values, causing the redirect
> pointer to point to different places.  Maybe it happens to point
> at a DEAD tuple in the big-endian case.

Just to make sure, I tried this test program:

#include <stdio.h>
#include <string.h>

typedef struct ItemIdData
{
    unsigned    lp_off:15,      /* offset to tuple (from start of page) */
                lp_flags:2,     /* state of line pointer, see below */
                lp_len:15;      /* byte length of tuple */
} ItemIdData;

int main()
{
    ItemIdData lp;

    memset(&lp, 0x77, sizeof(lp));
    printf("off = %x, flags = %x, len = %x\n",
           lp.lp_off, lp.lp_flags, lp.lp_len);
    return 0;
}

I get

off = 7777, flags = 2, len = 3bbb

on a little-endian machine, and

off = 3bbb, flags = 2, len = 7777

on big-endian.  It'd be less symmetric if the bytes weren't
all the same ...

			regards, tom lane