glibc/manual
Joseph Myers 644d38570a Remove add-ons mechanism.
glibc has an add-ons mechanism to allow additional software to be
integrated into the glibc build.  Such add-ons may be within the glibc
source tree, or outside it at a path passed to the --enable-add-ons
configure option.

localedata and crypt were once add-ons, distributed in separate
release tarballs, but long since stopped using that mechanism.
Linuxthreads was always an add-on.  Ports spent some time as an add-on
with separate release tarballs, then was first moved into the glibc
source tree, then had its sysdeps files moved into the main sysdeps
hierarchy so the add-ons mechanism was no longer used.  NPTL spent
some time as an add-on in the main glibc tree before stopping using
the add-on mechanism.  libidn used to have separate release tarballs
but no longer does so, but still uses the add-ons mechanism within the
glibc source tree.  Various other software has supported building with
the add-ons mechanism at times in the past, but I don't think any is
still widely used.

Add-ons involve significant, little-used complexity in the glibc build
system, and make it hard to understand what the space of possible
glibc configurations is.  This patch removes the add-ons mechanism.
libidn is now built via the Subdirs mechanism to cause any
configuration using sysdeps/unix/inet to build libidn; HAVE_LIBIDN
(which effectively means shared libraries are available) is now
defined via sysdeps/unix/inet/configure.  Various references to
add-ons around the source tree are removed (in the case of maint.texi,
the example list of sysdeps directories is still very out of date).

Externally maintained ports should now put their files in the normal
sysdeps directory structure rather than being arranged as add-ons;
they probably need to change e.g. elf.h anyway, rather than actually
being able to work just as a drop-in subtree.  Hurd libpthread should
be arranged similarly to NPTL, so some files might go in a
hurd-pthreads (or similar) top-level directory in glibc, while sysdeps
files should go in the normal sysdeps directory structure (possibly in
hurd or hurd-pthreads subdirectories, just as there are nptl
subdirectories in the sysdeps tree).

Tested for x86_64, and with build-many-glibcs.py.

	* configure.ac (--enable-add-ons): Remove option.
	(machine): Do not mention add-ons in comment.
	(LIBC_PRECONFIGURE): Likewise.
	(add_ons): Remove variable and sanity checks and logic to locate
	add-ons.
	(add_ons_automatic): Remove variable.
	(configured_add_ons): Likewise.
	(add_ons_sfx): Likewise.
	(add_ons_pfx): Likewise.
	(add_on_subdirs): Likewise.
	(sysnames_add_ons): Likewise.  Remove loop over add-ons and
	consideration of add-ons in Implies handling.
	(sysdeps_add_ons): Likewise.
	* configure: Regenerated.
	* libidn/configure.ac: Remove.
	* libidn/configure: Likewise.
	* sysdeps/unix/inet/configure.ac: New file.
	* sysdeps/unix/inet/configure: New generated file.
	* sysdeps/unix/inet/Subdirs: Add libidn.
	* Makeconfig (sysdeps-srcdirs): Remove variable.
	(+sysdep_dirs): Do not include $(sysdeps-srcdirs).
	($(common-objpfx)config.status): Do not depend on add-on files.
	($(common-objpfx)shlib-versions.v.i): Do not mention add-ons in
	comment.
	(all-subdirs): Do not include $(add-on-subdirs).
	* Makefile (dist-prepare): Do not use $(sysdeps-add-ons).
	* config.make.in (add-ons): Remove variable.
	(add-on-subdirs): Likewise.
	(sysdeps-add-ons): Likewise.
	* manual/Makefile (add-chapters): Remove.
	($(objpfx)texis): Do not depend on $(add-chapters).
	(nonexamples): Do not handle $(add-chapters).
	(examples): Do not handle $(add-ons).
	(chapters.% top-menu.%): Do not pass '$(add-chapters)' to
	libc-texinfo.sh.
	* manual/install.texi (Installation): Do not mention add-ons.
	(--enable-add-ons): Do not document configure option.
	* INSTALL: Regenerated.
	* manual/libc-texinfo.sh: Do not handle $2 add-ons argument.
	* manual/maint.texi (Hierarchy Conventions): Do not mention
	add-ons.
	* scripts/build-many-glibcs.py (Glibc.build_glibc): Do not use
	--enable-add-ons.
	* scripts/gen-sorted.awk: Do not handle Subdirs files from
	add-ons.
	* scripts/test-installation.pl: Do not handle glibc-compat add-on.
	* sysdeps/nptl/Makeconfig: Do not mention add-ons in comment.
2017-10-05 15:58:13 +00:00
..
examples Update copyright dates with scripts/update-copyrights. 2017-01-01 00:14:16 +00:00
argp.texi manual: Complete @standards in argp.texi. 2017-06-16 01:19:30 -07:00
arith.texi Obsolete matherr, _LIB_VERSION, libieee.a. 2017-08-21 17:45:10 +00:00
charset.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
check-safety.sh Update copyright dates with scripts/update-copyrights. 2017-01-01 00:14:16 +00:00
conf.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
contrib.texi Remove Banner mechanism. 2017-09-22 17:43:42 +00:00
creature.texi manual: Complete @standards in creature.texi. 2017-07-27 03:21:56 -07:00
crypt.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
ctype.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
debug.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
dir .. 2005-11-21 15:45:19 +00:00
errno.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
fdl-1.3.texi BZ#13738: Switch manual to FDL 1.3. 2012-02-24 12:58:10 -08:00
filesys.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
freemanuals.texi Update to canonical freemanuals.texi file. 2013-09-24 14:06:56 -07:00
getopt.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
header.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
install-plain.texi BZ #15941: Fix INSTALL file regeneration failure with makeinfo 5.x 2013-12-05 09:58:20 +05:30
install.texi Remove add-ons mechanism. 2017-10-05 15:58:13 +00:00
intro.texi manual: fix typo in the introduction 2016-05-19 23:22:59 -04:00
io.texi Clean up glibc manual references to "GNU system" (bug 6911). 2012-03-08 01:27:38 +00:00
ipc.texi manual/ipc.texi: Fix AC-safety notes. 2014-04-08 17:12:15 -04:00
job.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
lang.texi manual: Rewrite the section on widths of integer types. 2017-08-10 20:28:28 -07:00
lgpl-2.1.texi Use canonical FSF .texi files for LGPL and FDL texts. 2011-06-06 16:16:55 -07:00
libc-texinfo.sh Remove add-ons mechanism. 2017-10-05 15:58:13 +00:00
libc.texinfo Update copyright dates not handled by scripts/update-copyrights. 2017-01-01 00:26:24 +00:00
libcbook.texi
libdl.texi * manual/libdl.texi: New. 2014-01-31 23:23:59 -02:00
libm-err-tab.pl Prepare the manual to display math errors for float128 functions 2017-06-23 10:31:09 -03:00
llio.texi posix: Add p{read,write}v2 RWF_NOWAIT flag (BZ#21738) 2017-07-11 11:06:44 -03:00
locale.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
macros.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
maint.texi Remove add-ons mechanism. 2017-10-05 15:58:13 +00:00
Makefile Remove add-ons mechanism. 2017-10-05 15:58:13 +00:00
math.texi Obsolete pow10 functions. 2017-09-01 21:13:18 +00:00
memory.texi malloc: Remove check_action variable [BZ #21754] 2017-08-30 20:08:34 +02:00
message.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
nss.texi Remove compat from DEFAULT_CONFIG lookup strings 2017-09-12 10:21:48 -07:00
nsswitch.texi
pattern.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
pipe.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
platform.texi PowerPC: Extend Program Priority Register support 2015-08-19 17:43:26 -03:00
probes.texi malloc: Remove check_action variable [BZ #21754] 2017-08-30 20:08:34 +02:00
process.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
README.pretty-printers Remove obsolete notes at top level of source tree. 2017-09-01 08:04:22 -04:00
README.tunables Remove obsolete notes at top level of source tree. 2017-09-01 08:04:22 -04:00
resource.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
search.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
setjmp.texi manual: Document getcontext uc_stack value on Linux [BZ #759] 2017-08-08 16:16:43 -03:00
signal.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
socket.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
startup.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
stdio-fp.c
stdio.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
string.texi manual: Complete @standards in string.texi. 2017-06-16 01:23:17 -07:00
summary.pl manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
sysinfo.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
syslog.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
terminal.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
texinfo.tex Update miscellaneous files from upstream sources. 2016-12-21 16:05:55 +00:00
texis.awk Correct close statement. 2001-05-18 13:01:32 +00:00
threads.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
time.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
tsort.awk Update copyright dates with scripts/update-copyrights. 2017-01-01 00:14:16 +00:00
tunables.texi Add thunderx2t99 and thunderx2t99p1 CPU names to tunables list 2017-09-08 11:02:09 -07:00
users.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
xtract-typefun.awk Make shebang interpreter directives consistent 2016-01-07 04:03:21 -05:00

			TUNABLE FRAMEWORK
			=================

Tunables is a feature in the GNU C Library that allows application authors and
distribution maintainers to alter the runtime library behaviour to match their
workload.

The tunable framework allows modules within glibc to register variables that
may be tweaked through an environment variable.  It aims to enforce a strict
namespace rule to bring consistency to naming of these tunable environment
variables across the project.  This document is a guide for glibc developers to
add tunables to the framework.

ADDING A NEW TUNABLE
--------------------

The TOP_NAMESPACE macro is defined by default as 'glibc'.  If distributions
intend to add their own tunables, they should do so in a different top
namespace by overriding the TOP_NAMESPACE macro for that tunable.  Downstream
implementations are discouraged from using the 'glibc' top namespace for
tunables they don't already have consensus to push upstream.

There are three steps to adding a tunable:

1. Add a tunable to the list and fully specify its properties:

For each tunable you want to add, make an entry in elf/dl-tunables.list.  The
format of the file is as follows:

TOP_NAMESPACE {
  NAMESPACE1 {
    TUNABLE1 {
      # tunable attributes, one per line
    }
    # A tunable with default attributes, i.e. string variable.
    TUNABLE2
    TUNABLE3 {
      # its attributes
    }
  }
  NAMESPACE2 {
    ...
  }
}

The list of allowed attributes are:

- type:			Data type.  Defaults to STRING.  Allowed types are:
			INT_32, UINT_64, SIZE_T and STRING.  Numeric types may
			be in octal or hexadecimal format too.

- minval:		Optional minimum acceptable value.  For a string type
			this is the minimum length of the value.

- maxval:		Optional maximum acceptable value.  For a string type
			this is the maximum length of the value.

- default:		Specify an optional default value for the tunable.

- env_alias:		An alias environment variable

- security_level:	Specify security level of the tunable.  Valid values:

			SXID_ERASE: (default) Don't read for AT_SECURE binaries and
				    removed so that child processes can't read it.
			SXID_IGNORE: Don't read for AT_SECURE binaries, but retained for
				     non-AT_SECURE subprocesses.
			NONE: Read all the time.

2. Use TUNABLE_GET/TUNABLE_SET to get and set tunables.

3. OPTIONAL: If tunables in a namespace are being used multiple times within a
   specific module, set the TUNABLE_NAMESPACE macro to reduce the amount of
   typing.

GETTING AND SETTING TUNABLES
----------------------------

When the TUNABLE_NAMESPACE macro is defined, one may get tunables in that
module using the TUNABLE_GET macro as follows:

  val = TUNABLE_GET (check, int32_t, TUNABLE_CALLBACK (check_callback))

where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
'check_callback' is the function to call if the tunable got initialized to a
non-default value.  The macro returns the value as type 'int32_t'.

The callback function should be defined as follows:

  void
  TUNABLE_CALLBACK (check_callback) (int32_t *valp)
  {
  ...
  }

where it can expect the tunable value to be passed in VALP.

Tunables in the module can be updated using:

  TUNABLE_SET (check, int32_t, val)

where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
'val' is a value of same type.

To get and set tunables in a different namespace from that module, use the full
form of the macros as follows:

  val = TUNABLE_GET_FULL (glibc, tune, hwcap_mask, uint64_t, NULL)

  TUNABLE_SET_FULL (glibc, tune, hwcap_mask, uint64_t, val)

where 'glibc' is the top namespace, 'tune' is the tunable namespace and the
remaining arguments are the same as the short form macros.

When TUNABLE_NAMESPACE is not defined in a module, TUNABLE_GET is equivalent to
TUNABLE_GET_FULL, so you will need to provide full namespace information for
both macros.  Likewise for TUNABLE_SET and TUNABLE_SET_FULL.

** IMPORTANT NOTE **

The tunable list is set as read-only after the dynamic linker relocates itself,
so setting tunable values must be limited only to tunables within the dynamic
linker, that too before relocation.

FUTURE WORK
-----------

The framework currently only allows a one-time initialization of variables
through environment variables and in some cases, modification of variables via
an API call.  A future goals for this project include:

- Setting system-wide and user-wide defaults for tunables through some
  mechanism like a configuration file.

- Allow tweaking of some tunables at runtime