mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-24 11:41:07 +00:00
777d75fbc0
This patch starts preparation for C2X support in glibc headers by adding a feature test macro _ISOC2X_SOURCE and corresponding __GLIBC_USE (ISOC2X). (I chose to use the newer __GLIBC_USE style for this rather than the older __USE_* macros tested with #ifdef.) As with other such macros, C2X features are also enabled by compiling for a standard newer than C17, or by using _GNU_SOURCE. This patch does not itself enable anything new in the headers for C2X; that is to be done in followup patches. (For example, most of the TS 18661-1 functions should be declared for C2X without any __STDC_WANT_IEC_60559_BFP_EXT__ being needed, but the ones that 18661-1 adds to Annex F because of their close relation to IEEE 754 formats do still need the WANT macro in C2X.) Once C2X becomes an actual standard we'll presumably move to using the actual year in the feature test macro and __GLIBC_USE, with some period when both macro spellings are accepted, as was done with _ISOC9X_SOURCE. Tested for x86_64. * include/features.h (_ISOC2X_SOURCE): New feature test macro. Undefine and define to 1 if [_GNU_SOURCE]. (__GLIBC_USE_ISOC2X): New macro. Undefine and redefine depending on [_ISOC2X_SOURCE] and [__STDC_VERSION__ > 201710L]. (__USE_ISOC11): Also define to 1 if [_ISOC2X_SOURCE]. (__USE_ISOC99): Likewise. (__USE_ISOC95): Likewise. * manual/creature.texi (_ISOC2X_SOURCE): Document. |
||
---|---|---|
.. | ||
examples | ||
argp.texi | ||
arith.texi | ||
charset.texi | ||
check-safety.sh | ||
conf.texi | ||
contrib.texi | ||
creature.texi | ||
crypt.texi | ||
ctype.texi | ||
debug.texi | ||
dir | ||
errno.texi | ||
fdl-1.3.texi | ||
filesys.texi | ||
freemanuals.texi | ||
getopt.texi | ||
header.texi | ||
install-plain.texi | ||
install.texi | ||
intro.texi | ||
io.texi | ||
ipc.texi | ||
job.texi | ||
lang.texi | ||
lgpl-2.1.texi | ||
libc-texinfo.sh | ||
libc.texinfo | ||
libcbook.texi | ||
libdl.texi | ||
llio.texi | ||
locale.texi | ||
macros.texi | ||
maint.texi | ||
Makefile | ||
math.texi | ||
memory.texi | ||
message.texi | ||
nss.texi | ||
nsswitch.texi | ||
pattern.texi | ||
pipe.texi | ||
platform.texi | ||
probes.texi | ||
process.texi | ||
README.pretty-printers | ||
README.tunables | ||
resource.texi | ||
search.texi | ||
setjmp.texi | ||
signal.texi | ||
socket.texi | ||
startup.texi | ||
stdio-fp.c | ||
stdio.texi | ||
string.texi | ||
summary.pl | ||
sysinfo.texi | ||
syslog.texi | ||
terminal.texi | ||
texinfo.tex | ||
texis.awk | ||
threads.texi | ||
time.texi | ||
tsort.awk | ||
tunables.texi | ||
users.texi | ||
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. 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, cpu, hwcap_mask, uint64_t, NULL) TUNABLE_SET_FULL (glibc, cpu, hwcap_mask, uint64_t, val) 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 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