glibc/manual
Paul Eggert bdc674d97b Improve documentation for malloc etc. (BZ#27719)
Cover key corner cases (e.g., whether errno is set) that are well
settled in glibc, fix some examples to avoid integer overflow, and
update some other dated examples (code needed for K&R C, e.g.).
* manual/charset.texi (Non-reentrant String Conversion):
* manual/filesys.texi (Symbolic Links):
* manual/memory.texi (Allocating Cleared Space):
* manual/socket.texi (Host Names):
* manual/string.texi (Concatenating Strings):
* manual/users.texi (Setting Groups):
Use reallocarray instead of realloc, to avoid integer overflow issues.
* manual/filesys.texi (Scanning Directory Content):
* manual/memory.texi (The GNU Allocator, Hooks for Malloc):
* manual/tunables.texi:
Use code font for 'malloc' instead of roman font.
(Symbolic Links): Don't assume readlink return value fits in 'int'.
* manual/memory.texi (Memory Allocation and C, Basic Allocation)
(Malloc Examples, Alloca Example):
* manual/stdio.texi (Formatted Output Functions):
* manual/string.texi (Concatenating Strings, Collation Functions):
Omit pointer casts that are needed only in ancient K&R C.
* manual/memory.texi (Basic Allocation):
Say that malloc sets errno on failure.
Say "convert" rather than "cast", since casts are no longer needed.
* manual/memory.texi (Basic Allocation):
* manual/string.texi (Concatenating Strings):
In examples, use C99 declarations after statements for brevity.
* manual/memory.texi (Malloc Examples): Add portability notes for
malloc (0), errno setting, and PTRDIFF_MAX.
(Changing Block Size): Say that realloc (p, 0) acts like
(p ? (free (p), NULL) : malloc (0)).
Add xreallocarray example, since other examples can use it.
Add portability notes for realloc (0, 0), realloc (p, 0),
PTRDIFF_MAX, and improve notes for reallocating to the same size.
(Allocating Cleared Space): Reword now-confusing discussion
about replacement, and xref "Replacing malloc".
* manual/stdio.texi (Formatted Output Functions):
Don't assume message size fits in 'int'.
* manual/string.texi (Concatenating Strings):
Fix undefined behavior involving arithmetic on a freed pointer.
2021-04-13 12:17:56 -07:00
..
examples Update copyright dates with scripts/update-copyrights 2021-01-02 12:17:34 -08:00
argp.texi manual: Complete @standards in argp.texi. 2017-06-16 01:19:30 -07:00
arith.texi Use C2x return value from getpayload of non-NaN (bug 26073). 2020-07-06 16:18:02 +00:00
charset.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
check-safety.sh Update copyright dates with scripts/update-copyrights 2021-01-02 12:17:34 -08:00
conf.texi sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305] 2021-02-01 11:00:52 -08:00
contrib.texi Add more contributors to the manual 2020-02-01 17:16:54 +05:30
creature.texi sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305] 2021-02-01 11:00:52 -08:00
crypt.texi Add GRND_INSECURE from Linux 5.6 to sys/random.h 2020-04-09 21:21:16 +00:00
ctype.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
debug.texi Add manual documentation for threads.h 2018-07-24 14:07:31 -03:00
dir
errno.texi manual: Put the istrerrorname_np and strerrordesc_np return type in braces 2020-08-07 17:14:49 -03:00
fdl-1.3.texi Sync FDL from https://www.gnu.org/licenses/fdl-1.3.texi 2021-01-02 12:46:25 -08:00
filesys.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
freemanuals.texi Prefer https to http for gnu.org and fsf.org URLs 2019-09-07 02:43:31 -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 tst: Extend cross-test-ssh.sh to specify if target date can be altered 2021-03-08 22:37:16 +01:00
intro.texi manual: Revise crypt.texi. 2018-06-29 16:53:37 +02:00
io.texi
ipc.texi manual/ipc.texi: Fix AC-safety notes. 2014-04-08 17:12:15 -04:00
job.texi manual/jobs.texi: remove unused var from example code 2020-06-03 18:09:45 -04:00
lang.texi manual: Rewrite the section on widths of integer types. 2017-08-10 20:28:28 -07:00
lgpl-2.1.texi
libc-texinfo.sh Remove add-ons mechanism. 2017-10-05 15:58:13 +00:00
libc.texinfo Update copyright dates with scripts/update-copyrights 2021-01-02 12:17:34 -08:00
libcbook.texi
libdl.texi * manual/libdl.texi: New. 2014-01-31 23:23:59 -02:00
llio.texi manual: Clarify File Access Modes section and add O_PATH 2020-12-03 10:59:50 +01:00
locale.texi Use STRFMON_LDBL_IS_DBL instead of __ldbl_is_dbl. 2018-11-16 09:21:14 -02:00
macros.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
maint.texi Fix the manual for old texinfo 2019-01-04 11:45:13 +00:00
Makefile Update copyright dates with scripts/update-copyrights 2021-01-02 12:17:34 -08:00
math.texi manual: Use Unicode instead HTML entities for characters (bug 19737) 2020-07-16 10:17:31 +02:00
memory.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
message.texi manual: Use @code{errno} instead of @var{errno} [BZ #24063] 2019-01-07 11:42:04 +01:00
nss.texi Remove --enable-obsolete-rpc configure flag 2020-07-13 19:36:35 +02:00
nsswitch.texi Remove --enable-obsolete-nsl configure flag 2020-07-08 17:25:57 +02:00
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 x86: Add PTWRITE feature detection [BZ #27346] 2021-02-07 08:01:14 -08:00
probes.texi math: Remove mpa files [BZ #15267] 2021-03-11 14:26:36 +00:00
process.texi manual: Fix typos in the fexecve description 2020-04-29 10:15:51 +02:00
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
setjmp.texi manual: Use @code{errno} instead of @var{errno} [BZ #24063] 2019-01-07 11:42:04 +01:00
signal.texi manual: Fix sigdescr_np and sigabbrev_np return type (BZ #26343) 2020-08-08 16:51:26 -03:00
socket.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
startup.texi Argument Syntax: Use "option", @option, and @command. 2020-10-30 13:08:38 -04:00
stdio-fp.c
stdio.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
string.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
summary.pl Update copyright dates with scripts/update-copyrights 2021-01-02 12:17:34 -08:00
sysinfo.texi manual: Correct argument order in mount examples [BZ #27207] 2021-01-22 14:22:41 -05:00
syslog.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
terminal.texi Remove obsolete, never-implemented XSI STREAMS declarations 2019-03-14 15:44:15 +01:00
texinfo.tex Update miscellaneous files from upstream sources. 2019-01-01 00:52:59 +00:00
texis.awk
threads.texi nptl: Return EINVAL for invalid clock for pthread_clockjoin_np 2020-11-25 10:46:25 -03:00
time.texi manual: Fix some @code/@var formatting glitches chapter Date And Time 2020-08-05 09:22:21 +02:00
tsort.awk Update copyright dates with scripts/update-copyrights 2021-01-02 12:17:34 -08:00
tunables.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -07:00
users.texi Improve documentation for malloc etc. (BZ#27719) 2021-04-13 12:17:56 -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 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