other/8947: Please add a Warning about "-malign-double" in docs

[Norbert Wolff]

Subject:        stat-calls fail with certain gcc-switches
>Submitter-Id:  net
>Confidential:  no
>Synopsis:      Problems with stat-calls and "-malign-double"
>Severity:      critical
>Priority:      high
>Category:      libc
>Class:         sw-bug
>Release:       libc-2.3.1
>Environment:
	System: linux-2.4.19
	Arch:   i686
>Description:   please see below

>   Please add a note in your docs that is not possible to compile
>    the GNU-libc (i tried it with 2.3.1) with the options
>    "-malign-double" and/or "-m128bit-long-double".
>
>    The increased aligment of long doubles caused by this switches
>    crashes the calculations in glibc-2.3.1/io/fts.c,
>    so every call of one of the stat-functions will fail...


 On 21 Dec 2002 17:40:22 -0000
 bangerth@dealii.org wrote:

>     Can you give any indications as to what exactly goes wrong?
>     It is not very helpful just to document that some option
>     might not work in some circumstances. I think it would be
>     much better if we actually _fixed_ the problem that was
>     causing the problem.

 Hi Wolfgang,

 In the glibc-sources the alignment of long doubles is used
 to calculate the fts_statp component of the FTSENT

   ---------------------------- file: io/fts.c

   /* Largest alignment size needed, minus one.
     Usually long double is the worst case.  */
   #ifndef ALIGNBYTES
   #define ALIGNBYTES	(__alignof__ (long double) - 1)
   #endif
   /* Align P to that size.  */
   #ifndef ALIGN
   #define	ALIGN(p)	(((unsigned long int) (p) + ALIGNBYTES) & ~ALIGNBYTES)
   #endif

   ...

   static FTSENT *
   internal_function
   fts_alloc(sp, name, namelen)
 	FTS *sp;
 	const char *name;
	register int namelen;
   {
 	register FTSENT *p;
	size_t len;

	len = sizeof(FTSENT) + namelen;
	if (!ISSET(FTS_NOSTAT))
		len += sizeof(struct stat) + ALIGNBYTES;
	if ((p = malloc(len)) == NULL)
		return (NULL);

	memmove(p->fts_name, name, namelen);
	p->fts_name[namelen] = '\0';

	if (!ISSET(FTS_NOSTAT))
		p->fts_statp = (struct stat *)ALIGN(p->fts_name + namelen + 2);

    ----------------------------

 I dont understand what long doubles have to do with stat-calls,
 but when you increase the alignment of them to 16 instead of 4, which is
 one consequence of the gcc-switches 
   "-malign-double" and/or "-m128bit-long-double"
 every stat-call will fail with errno set to EOVERFLOW.

 I agree with you that it would be better to _fix_ the problem, than
 adding a note in the docs, but that is a job for the glibc-people,
 as gcc creates correct code.
 
 I have send a copy of this mail to the glibc-buglist and hope for the
 best...

 Thanks,
   Norbert