From: shwaresyst <shwaresyst@aol.com>
To: eblake@redhat.com, bruno@clisp.org, cbouchar@redhat.com,
bug-m4@gnu.org, libc-alpha@sourceware.org,
austin-group-l@opengroup.org
Subject: Re: SIGSTKSZ is now a run-time variable
Date: Tue, 9 Mar 2021 21:29:57 +0000 (UTC) [thread overview]
Message-ID: <1569476484.1894162.1615325397167@mail.yahoo.com> (raw)
In-Reply-To: <1569476484.1894162.1615325397167.ref@mail.yahoo.com>
Yes, it's not something an application would expect to need to keep increasing, just that's the part of <limits.h> I'd move it to. The definition could also be the max required by a processor family, with sysconf() reporting a possible lower value for a particular processor stepping. At least that way the application that doesn't use sysconf() won't be getting SIGSEGV faults.
Additionally, I believe the definition can be calculated at compile time as a multiple of ( sizeof(ucontext_t)+sizeof(overhead_struct(s)) ), whatever other overhead applies, so I don't see any real need to use sysconf(). This may mean having to munge a <signal.in> by configure, based on config.guess, but that's not the standard's headache.
The CS, SC, and PC constants are not in the XSH 2.2.2 table deliberately, from Issue 6 TC1, as adding any also requires a bump in POSIX_VERSION or POSIX2_VERSION, and often XSI_VERSION. This is so each usage of a constant doesn't need individual #ifdefs to test option group availability. The previous text was allowing if an implementation wasn't supporting an option group they could skip including the related constants in <unistd.h>. A simple check of VERSION at the top of a source C file suffices now to indicate those constants shall be available.
On Tuesday, March 9, 2021 Eric Blake <eblake@redhat.com> wrote:
On 3/9/21 10:14 AM, shwaresyst wrote:
>
> To me that looks like a conformance violation and should be reverted. There is no _SC_SIGSTKSZ defined in <unistd.h> by the standard, to begin with, so that use of sysconf() is a non-portable extension on its own.
Portable apps can't use _SC_SIGSTKSZ, but the standard generally permits
implementations to define further constants. Then again, re-reading XSH
2.2.2:
" Implementations may add symbols to the headers shown in the following
table, provided the identifiers for those symbols either:
Begin with the corresponding reserved prefixes in the table, or
..."
but the table lacks a row for <unistd.h> with _CS_* and _SC_* constants.
Looks like you found an independent defect.
>
> I could see the definition of SIGSTKSZ being changed to the static minimum a particular processor requires, or is initially allocated as a 'safe' amount, rather than static "default size", and moving SIGSTKSZ to <limits.h>. This would contrast to MINSIGSTKSZ as the lowest value for a platform for all supported processors. Then an application could use sysconf() to query for the maximum size the configuration supports if it wants to use more than that, as a runtime increasable limit.
As I understand it, the concern in glibc is less about runtime
increasability, so much as ABI compatibility with applications compiled
against older headers at a time when the kernel had less state
information to store during a context switch.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
next parent reply other threads:[~2021-03-09 21:30 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1569476484.1894162.1615325397167.ref@mail.yahoo.com>
2021-03-09 21:29 ` shwaresyst [this message]
2021-03-09 23:58 ` H.J. Lu
[not found] <832918739.1734727.1615306471320.ref@mail.yahoo.com>
2021-03-09 16:14 ` shwaresyst
2021-03-09 19:33 ` Paul Eggert
2021-03-09 19:34 ` Eric Blake
2021-03-09 20:12 ` Eric Blake
[not found] <1841269.IEpri3ZHvQ@omega>
2021-03-09 15:23 ` Eric Blake
2021-03-09 15:26 ` Andreas Schwab
2021-03-09 15:48 ` Eric Blake
2021-03-09 19:58 ` Bruno Haible
2021-03-09 21:05 ` H.J. Lu
[not found] ` <20210309211735.GW19922@dragon.sl.home>
2021-03-16 19:46 ` Carol Bouchard
2021-03-26 12:46 ` Carol Bouchard
2021-03-09 15:46 ` Zack Weinberg
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