From: Daniel Kiper <daniel.kiper@oracle.com> To: grub-devel@gnu.org, xen-devel@lists.xenproject.org Cc: jgross@suse.com, eric.snowberg@oracle.com, arvidjaar@gmail.com, andrew.cooper3@citrix.com, seth.goldberg@oracle.com, phcoder@gmail.com Subject: [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section Date: Thu, 9 Jun 2016 22:30:42 +0200 [thread overview] Message-ID: <1465504244-17175-9-git-send-email-daniel.kiper@oracle.com> (raw) In-Reply-To: <1465504244-17175-1-git-send-email-daniel.kiper@oracle.com> Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com> --- doc/multiboot.texi | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/doc/multiboot.texi b/doc/multiboot.texi index c81b2ea..bf02a1b 100644 --- a/doc/multiboot.texi +++ b/doc/multiboot.texi @@ -1384,6 +1384,7 @@ document, but are included for prospective operating system and boot loader writers. @menu +* C structure alignment and padding consideration:: * Notes on PC:: * BIOS device mapping techniques:: * Example OS code:: @@ -1391,6 +1392,22 @@ loader writers. @end menu +@node C structure alignment and padding consideration +@section C structure alignment and padding consideration + +Many C compilers try to optimize memory accesses aligning structure +members properly. Usually they reach the goal by adding some padding. +This is very useful thing in general. However, if you try to mix assembler +with C or use C to implement structure low level access this behavior +may lead, at least, to quite surprising results. Hence, compiler should +be instructed to not optimize such accesses. Usually it is done by special +attribute added to structure definition, e.g. GCC compatible sources use +@samp{__attribute__ ((__packed__))} for this purpose. However, this is not +required if it is known that its members are properly aligned and compiler +does not do any optimization. Very good example of this is shown below in +@file{multiboot2.h} file. + + @node Notes on PC @section Notes on PC -- 1.7.10.4 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
WARNING: multiple messages have this Message-ID (diff)
From: Daniel Kiper <daniel.kiper@oracle.com> To: grub-devel@gnu.org, xen-devel@lists.xenproject.org Cc: andrew.cooper3@citrix.com, arvidjaar@gmail.com, eric.snowberg@oracle.com, jgross@suse.com, konrad.wilk@oracle.com, phcoder@gmail.com, seth.goldberg@oracle.com Subject: [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section Date: Thu, 9 Jun 2016 22:30:42 +0200 [thread overview] Message-ID: <1465504244-17175-9-git-send-email-daniel.kiper@oracle.com> (raw) In-Reply-To: <1465504244-17175-1-git-send-email-daniel.kiper@oracle.com> Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com> --- doc/multiboot.texi | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/doc/multiboot.texi b/doc/multiboot.texi index c81b2ea..bf02a1b 100644 --- a/doc/multiboot.texi +++ b/doc/multiboot.texi @@ -1384,6 +1384,7 @@ document, but are included for prospective operating system and boot loader writers. @menu +* C structure alignment and padding consideration:: * Notes on PC:: * BIOS device mapping techniques:: * Example OS code:: @@ -1391,6 +1392,22 @@ loader writers. @end menu +@node C structure alignment and padding consideration +@section C structure alignment and padding consideration + +Many C compilers try to optimize memory accesses aligning structure +members properly. Usually they reach the goal by adding some padding. +This is very useful thing in general. However, if you try to mix assembler +with C or use C to implement structure low level access this behavior +may lead, at least, to quite surprising results. Hence, compiler should +be instructed to not optimize such accesses. Usually it is done by special +attribute added to structure definition, e.g. GCC compatible sources use +@samp{__attribute__ ((__packed__))} for this purpose. However, this is not +required if it is known that its members are properly aligned and compiler +does not do any optimization. Very good example of this is shown below in +@file{multiboot2.h} file. + + @node Notes on PC @section Notes on PC -- 1.7.10.4
next prev parent reply other threads:[~2016-06-09 20:32 UTC|newest] Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-06-09 20:30 [MULTIBOOT2 DOC PATCH 00/10] multiboot2: Update documentation Daniel Kiper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 01/10] multiboot2: Remove redundant if Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 21:02 ` Andrew Cooper 2016-06-09 21:02 ` Andrew Cooper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 02/10] multiboot2: Clarify meaning of information request header tag Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 21:14 ` Andrew Cooper 2016-06-09 21:14 ` [Xen-devel] " Andrew Cooper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 03/10] multiboot2: Fix description of EFI boot services tag Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 04/10] multiboot2: Add description of support for EFI boot services Daniel Kiper 2016-06-09 21:31 ` Andrew Cooper 2016-06-09 21:31 ` Andrew Cooper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 05/10] multiboot2: Add description of EFI image handle tags Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 06/10] multiboot2: Add description of support for relocatable images Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 21:36 ` Andrew Cooper 2016-06-09 21:36 ` [Xen-devel] " Andrew Cooper 2016-06-10 17:36 ` Daniel Kiper 2016-06-10 17:36 ` Daniel Kiper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 07/10] multiboot2: Say that memory maps may not be available on EFI platforms Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 21:37 ` Andrew Cooper 2016-06-09 21:37 ` Andrew Cooper 2016-06-09 20:30 ` Daniel Kiper [this message] 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section Daniel Kiper 2016-06-09 22:07 ` Andrew Cooper 2016-06-10 17:58 ` Daniel Kiper 2016-06-10 17:58 ` Daniel Kiper 2016-06-09 22:07 ` Andrew Cooper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 09/10] multiboot2: Add me to authors Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper 2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 10/10] multiboot2: Bump version to 2.0 Daniel Kiper 2016-06-09 20:30 ` Daniel Kiper
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