obstack: first object of each chunk not aligned

[duret_g at epita dot fr]

>Submitter-Id:	net
>Originator:	Alexandre Duret-Lutz
>Organization:
  Alexandre Duret-Lutz
>
>Confidential:	no
>Synopsis:	the first object of each obstack chunk can be unaligned
>Severity:	non-critical
>Priority:	low
>Category:	libc
>Class:		sw-bug
>Release:	libc-2.1.95
>Environment:
	
Host type: i386-pc-linux-gnu
System: Linux phobos 2.4.0-test7 #2 Sat Sep 9 14:25:27 CEST 2000 i586 unknown
Architecture: i586

Addons: linuxthreads

Build CC: gcc
Compiler version: 2.95.2 20000220 (Debian GNU/Linux)
Kernel headers: UTS_RELEASE
Symbol versioning: yes
Build static: yes
Build shared: yes
Build pic-default: no
Build profile: yes
Build omitfp: no
Build bounded: no
Build static-nss: no
Stdio: libio

>Description:
When asking an obstack to return object with a given alignment,
the resulting objects are *not always* correctly aligned.  More
precisely, the first object allocated in a chunk can be
unaligned.

Here is an output of the included test (here the first object of
a chunk is sometimes aligned because by chance malloc
return a pointer which will be aligned once the two 
chunk-header-values are added):

~/tmp % gcc obstest.c
~/tmp % ./a.out
malloc (4072) => 0x8049998
obstack_alloc (1000) => 0x80499a0       
obstack_alloc (1000) => 0x8049da0       
obstack_alloc (1000) => 0x804a1a0       
malloc (4072) => 0x804a988
obstack_alloc (1000) => 0x804a990       (not aligned)
obstack_alloc (1000) => 0x804ad80       
obstack_alloc (1000) => 0x804b180       
obstack_alloc (1000) => 0x804b580       
malloc (4072) => 0x804b978
obstack_alloc (1000) => 0x804b980       
obstack_alloc (1000) => 0x804bd80       
obstack_alloc (1000) => 0x804c180       
malloc (4072) => 0x804c968
obstack_alloc (1000) => 0x804c970       (not aligned)
obstack_alloc (1000) => 0x804cd60       
obstack_alloc (1000) => 0x804d160       
obstack_alloc (1000) => 0x804d560       
malloc (4072) => 0x804d958
obstack_alloc (1000) => 0x804d960       
free (0x804d958)
free (0x804c968)
free (0x804b978)
free (0x804a988)
free (0x8049998)

>How-To-Repeat:
#include <stdlib.h>
#include <stdio.h>
#include <obstack.h>

#define obstack_chunk_alloc verbose_malloc
#define obstack_chunk_free verbose_free
#define ALIGN_BOUNDARY 32
#define ALIGN_MASK (ALIGN_BOUNDARY - 1)
#define OBJECT_SIZE 1000

void *
verbose_malloc (size_t size)
{
  void *buf = malloc (size);
  fprintf (stderr, "malloc (%u) => %p\n", size, buf);
  return buf;
}

void
verbose_free (void *buf)
{
  free (buf);
  fprintf (stderr, "free (%p)\n", buf);
}

int main (void)
{
  struct obstack obs;
  int i;

  obstack_init (&obs);
  obstack_alignment_mask (&obs) = ALIGN_MASK;
  /* finish an empty object to take alignment into account */
  obstack_finish (&obs);

  /* let's allocate some objects and print their addresses */
  for (i = 15; i; --i) {
    void *obj = obstack_alloc (&obs, OBJECT_SIZE);
    fprintf (stderr, "obstack_alloc (%u) => %p \t%s\n", OBJECT_SIZE, obj,
	     ((int)obj & ALIGN_MASK)?"(not aligned)":"");
  }    

  /* clean up */
  obstack_free (&obs, 0);
  return 0;
}
>Fix:
My understanding of the obstack source is that an
alignment test is missing in `_obstack_newchunk'.