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From: Jeremy Brubaker <jbrubake.362@orionarts.invalid>
Newsgroups: comp.lang.c
Subject: Re: sizeof struct with flexible array: when did it change?
Date: Wed, 9 Oct 2024 12:55:42 -0000 (UTC)
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On 2024-10-07, Kaz Kylheku wrote:
> On 2024-10-07, Nick Bowler <nbowler@draconx.ca> wrote:
>> On Mon, 7 Oct 2024 02:32:13 -0000 (UTC), Kaz Kylheku wrote:
> What GCC seems to be doing is simply nothing special. When determining
> the most strictly aligned member of the struct, it takes the flexible
> array into account (the alignment of its element type). It otherwise
> ignores it (or perhaps treats it as a size zero subobject).  The
> structure is padded after that for the sake of the most strictly
> aligned member.
>
>>> Don't get burned: don't rely on the size of a flexible array struct.
>>> Use the offsetof that flexible member.
>
> If the size is anything other than what the program expects, whether
> it is larger or smaller, that breaks the program.
>
> For instance, if the wrong value is used when displacing a pointer to
> the flexible member to recover a pointer to the struct.
>
> This issue showed up in exactly one program of mine in which I
> experimented with using the flexible array member.
>
> It was reported by a user who ran into a crash.
>

As the user who had the pleasure of running into said crash, here is a
brief demo of the sizes and addresses reported by my system (gcc 13.3.1)
using both methods of determining the start of the struct:


#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>

typedef struct dstr {
  int a;
  size_t b;
  int c;
  char str[];
} dstr;

typedef struct ref {
  int a;
  size_t b;
  int c;
} ref;

#define old_dstr_of(str) ((dstr *) ((str) - sizeof (dstr)))
#define new_dstr_of(s) ((dstr *) ((s) - offsetof (struct dstr, str)))

int main (int argc, char ** argv)
{
    dstr *ds = malloc (sizeof (dstr));

    printf ("sizeof(int)    %zu\n", sizeof (int));
    printf ("sizeof(char)   %zu\n", sizeof (char));
    printf ("sizeof(size_t) %zu\n", sizeof (size_t));
    printf ("sizeof(dstr)   %zu\n", sizeof (dstr));
    printf ("sizeof(ref)    %zu\n", sizeof (ref));
    puts ("");

    puts ("Addresses:");
    printf ("ds          %p\n", ds);
    printf ("ds->str     %p\n", ds->str);
    printf ("old dstr_of %p\n", old_dstr_of(ds->str));
    printf ("new dstr_of %p\n", new_dstr_of(ds->str));

}

And the output on my machine:

sizeof(int)    4
sizeof(char)   1
sizeof(size_t) 8
sizeof(dstr)   24
sizeof(ref)    24

Addresses:
ds          0x9d62a0
ds->str     0x9d62b4
old dstr_of 0x9d629c
new dstr_of 0x9d62a0


-- 
() www.asciiribbon.org  | Jeremy Brubaker
/\  - against html mail | јЬruЬаkе@оrіоnаrtѕ.іо / neonrex on IRC

Success is something I will dress for when I get there, and not until.