glibc/manual
Joseph Myers 64924422a9 C2x strtol binary constant handling
C2x adds binary integer constants starting with 0b or 0B, and supports
those constants in strtol-family functions when the base passed is 0
or 2.  Implement that strtol support for glibc.

As discussed at
<https://sourceware.org/pipermail/libc-alpha/2020-December/120414.html>,
this is incompatible with previous C standard versions, in that such
an input string starting with 0b or 0B was previously required to be
parsed as 0 (with the rest of the string unprocessed).  Thus, as
proposed there, this patch adds 20 new __isoc23_* functions with
appropriate header redirection support.  This patch does *not* do
anything about scanf %i (which will need 12 new functions per long
double variant, so 12, 24 or 36 depending on the glibc configuration),
instead leaving that for a future patch.  The function names would
remain as __isoc23_* even if C2x ends up published in 2024 rather than
2023.

Making this change leads to the question of what should happen to
internal uses of these functions in glibc and its tests.  The header
redirection (which applies for _GNU_SOURCE or any other feature test
macros enabling C2x features) has the effect of redirecting internal
uses but without those uses then ending up at a hidden alias (see the
comment in include/stdio.h about interaction with libc_hidden_proto).
It seems desirable for the default for internal uses to be the same
versions used by normal code using _GNU_SOURCE, so rather than doing
anything to disable that redirection, similar macro definitions to
those in include/stdio.h are added to the include/ headers for the new
functions.

Given that the default for uses in glibc is for the redirections to
apply, the next question is whether the C2x semantics are correct for
all those uses.  Uses with the base fixed to 10, 16 or any other value
other than 0 or 2 can be ignored.  I think this leaves the following
internal uses to consider (an important consideration for review of
this patch will be both whether this list is complete and whether my
conclusions on all entries in it are correct):

benchtests/bench-malloc-simple.c
benchtests/bench-string.h
elf/sotruss-lib.c
math/libm-test-support.c
nptl/perf.c
nscd/nscd_conf.c
nss/nss_files/files-parse.c
posix/tst-fnmatch.c
posix/wordexp.c
resolv/inet_addr.c
rt/tst-mqueue7.c
soft-fp/testit.c
stdlib/fmtmsg.c
support/support_test_main.c
support/test-container.c
sysdeps/pthread/tst-mutex10.c

I think all of these places are OK with the new semantics, except for
resolv/inet_addr.c, where the POSIX semantics of inet_addr do not
allow for binary constants; thus, I changed that file (to use
__strtoul_internal, whose semantics are unchanged) and added a test
for this case.  In the case of posix/wordexp.c I think accepting
binary constants is OK since POSIX explicitly allows additional forms
of shell arithmetic expressions, and in stdlib/fmtmsg.c SEV_LEVEL is
not in POSIX so again I think accepting binary constants is OK.

Functions such as __strtol_internal, which are only exported for
compatibility with old binaries from when those were used in inline
functions in headers, have unchanged semantics; the __*_l_internal
versions (purely internal to libc and not exported) have a new
argument to specify whether to accept binary constants.

As well as for the standard functions, the header redirection also
applies to the *_l versions (GNU extensions), and to legacy functions
such as strtoq, to avoid confusing inconsistency (the *q functions
redirect to __isoc23_*ll rather than needing their own __isoc23_*
entry points).  For the functions that are only declared with
_GNU_SOURCE, this means the old versions are no longer available for
normal user programs at all.  An internal __GLIBC_USE_C2X_STRTOL macro
is used to control the redirections in the headers, and cases in glibc
that wish to avoid the redirections - the function implementations
themselves and the tests of the old versions of the GNU functions -
then undefine and redefine that macro to allow the old versions to be
accessed.  (There would of course be greater complexity should we wish
to make any of the old versions into compat symbols / avoid them being
defined at all for new glibc ABIs.)

strtol_l.c has some similarity to strtol.c in gnulib, but has already
diverged some way (and isn't listed at all at
https://sourceware.org/glibc/wiki/SharedSourceFiles unlike strtoll.c
and strtoul.c); I haven't made any attempts at gnulib compatibility in
the changes to that file.

I note incidentally that inttypes.h and wchar.h are missing the
__nonnull present on declarations of this family of functions in
stdlib.h; I didn't make any changes in that regard for the new
declarations added.
2023-02-16 23:02:40 +00:00
..
examples Update copyright dates with scripts/update-copyrights 2023-01-06 21:14:39 +00:00
argp.texi manual: Complete @standards in argp.texi. 2017-06-16 01:19:30 -07:00
arith.texi C2x strtol binary constant handling 2023-02-16 23:02:40 +00:00
charset.texi wcrtomb: Make behavior POSIX compliant 2022-05-13 19:15:46 +05:30
check-safety.sh Update copyright dates with scripts/update-copyrights 2023-01-06 21:14:39 +00:00
conf.texi sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305] 2021-02-01 11:00:52 -08:00
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creature.texi Add _FORTIFY_SOURCE implementation documentation [BZ #28998] 2023-01-10 10:22:38 -05:00
crypt.texi Add GRND_INSECURE from Linux 5.6 to sys/random.h 2020-04-09 21:21:16 +00:00
ctype.texi manual: Drop obsolete @refill 2022-01-12 14:28:44 +05:30
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dir
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filesys.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
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io.texi
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libcbook.texi
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message.texi manual: Use @code{errno} instead of @var{errno} [BZ #24063] 2019-01-07 11:42:04 +01:00
nss.texi nss: Use "files dns" as the default for the hosts database (bug 28700) 2021-12-17 12:01:25 +01:00
nsswitch.texi Remove --enable-obsolete-nsl configure flag 2020-07-08 17:25:57 +02:00
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platform.texi x86: Install <bits/platform/x86.h> [BZ #27958] 2021-07-23 05:12:51 -07:00
probes.texi elf: Add _dl_find_object function 2021-12-28 22:52:56 +01:00
process.texi nptl: Handle spurious EINTR when thread cancellation is disabled (BZ#29029) 2022-04-14 12:48:31 -03:00
README.pretty-printers Use gen-as-const.py to process .pysym files. 2018-12-10 22:56:59 +00:00
README.tunables tunables: Simplify TUNABLE_SET interface 2021-02-10 19:08:33 +05:30
resource.texi Move vtimes to a compatibility symbol 2020-10-19 16:44:20 -03:00
search.texi manual: Correct description of ENTRY [BZ #17183] 2021-02-04 15:22:12 +01:00
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signal.texi manual: SA_ONSTACK is ignored without alternate stack 2022-02-28 11:50:41 +01:00
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xtract-typefun.awk

			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 for AT_SECURE
			binaries.  Valid values are:

			SXID_ERASE: (default) Do not read and do not pass on to
			child processes.
			SXID_IGNORE: Do not read, but retain for non-AT_SECURE
			child processes.
			NONE: Read all the time.

2. Use TUNABLE_GET/TUNABLE_SET/TUNABLE_SET_WITH_BOUNDS 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, val)

where 'check' is the tunable name 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, cpu, hwcap_mask, uint64_t, NULL)

  TUNABLE_SET_FULL (glibc, cpu, hwcap_mask, val)

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

The minimum and maximum values can updated together with the tunable value
using:

  TUNABLE_SET_WITH_BOUNDS (check, val, min, max)

where 'check' is the tunable name, 'val' is a value of same type, 'min' and
'max' are the minimum and maximum values of the tunable.

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

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

  TUNABLE_SET_WITH_BOUNDS_FULL (glibc, cpu, hwcap_mask, val, min, max)

where 'glibc' is the top namespace, 'cpu' 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, TUNABLE_SET_FULL,
TUNABLE_SET_WITH_BOUNDS and TUNABLE_SET_WITH_BOUNDS_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