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Rich Felker 8cca79a72c exit: add back lock to make concurrent calls to exit safe 
per the C and POSIX standards, calling exit "more than once",
including via return from main, produces undefined behavior. this
language predates threads, and at the time it was written, could only
have applied to recursive calls to exit via atexit handlers. C++
likewise makes calls to exit from global dtors undefined. nonetheless,
by the present specification as written, concurrent calls to exit by
multiple threads also have undefined behavior.

originally, our implementation of exit did have locking to handle
concurrent calls safely, but that was changed in commit
2e55da9118 based on it being undefined.
from a standpoint of both hardening and quality of implementation,
that change seems to have been a mistake.

this change adds back locking, but with awareness of the lock owner so
that recursive calls to exit can be trapped rather than deadlocking.
this also opens up the possibility of allowing recursive calls to
succeed, if future consensus ends up being in favor of that.

prior to this change, exit already behaved partly as if protected by a
lock as long as atexit was linked, but multiple threads calling exit
could concurrently "pop off" atexit handlers and execute them in
parallel with one another rather than serialized in the reverse order
of registration. this was a likely unnoticed but potentially very
dangerous manifestation of the undefined behavior. if on the other
hand atexit was not linked, multiple threads calling exit concurrently
could each run their own instance of global dtors, if any, likely
producing double-free situations.

now, if multiple threads call exit concurrently, all but the first
will permanently block (in SYS_pause) until the process terminates,
and all atexit handlers, global dtors, and stdio flushing/position
consistency will be handled in the thread that arrived first. this is
really the only reasonable way to define concurrent calls to exit. it
is not recommended usage, but may become so in the future if there is
consensus/standardization, as there is a push from the rust language
community (and potentially other languages interoperating with the C
runtime) to make concurrent calls to the language's exit interfaces
safe even when multiple languages are involved in a program, and this
is only possible by having the locking in the underlying C exit.
2024-07-23 20:36:58 -04:00
arch stdint.h: derive limits from __LONG_MAX, use common fast16 types 2024-05-07 09:17:51 -04:00
compat/time32 remove LFS64 symbol aliases; replace with dynamic linker remapping 2022-10-19 14:01:31 -04:00
crt aarch64 crti.o: fix alignment of _init/_fini 2024-05-12 12:17:13 -04:00
dist add another example option to dist/config.mak 2012-04-24 16:49:11 -04:00
include syslog: revert LOG_FAC/LOG_FACMASK changes 2024-07-05 13:22:25 -04:00
ldso dynlink: avoid copying to temp buffer in get_lfs64 2024-06-22 16:54:11 -04:00
src exit: add back lock to make concurrent calls to exit safe 2024-07-23 20:36:58 -04:00
tools install.sh: avoid creating symlinks with restricted permissions 2024-02-03 19:57:30 -05:00
.gitignore remove obsolete gitignore rules 2016-07-06 00:21:25 -04:00
.mailmap update contributor name 2019-12-07 12:21:35 -05:00
configure configure: enable riscv32 port 2024-02-29 16:59:06 -05:00
COPYRIGHT add optimized aarch64 memcpy and memset 2020-06-26 17:49:51 -04:00
dynamic.list fix regression in access to optopt object 2018-11-19 13:20:41 -05:00
INSTALL update INSTALL file archs list with riscv32, loongarch64 additions 2024-02-29 19:23:03 -05:00
Makefile make mallocng the default malloc implementation 2020-06-30 15:38:27 -04:00
README update version reference in the README file 2014-06-25 14:16:53 -04:00
VERSION release 1.2.5 2024-02-29 21:07:33 -05:00
WHATSNEW release 1.2.5 2024-02-29 21:07:33 -05:00

README 

    musl libc

musl, pronounced like the word "mussel", is an MIT-licensed
implementation of the standard C library targetting the Linux syscall
API, suitable for use in a wide range of deployment environments. musl
offers efficient static and dynamic linking support, lightweight code
and low runtime overhead, strong fail-safe guarantees under correct
usage, and correctness in the sense of standards conformance and
safety. musl is built on the principle that these goals are best
achieved through simple code that is easy to understand and maintain.

The 1.1 release series for musl features coverage for all interfaces
defined in ISO C99 and POSIX 2008 base, along with a number of
non-standardized interfaces for compatibility with Linux, BSD, and
glibc functionality.

For basic installation instructions, see the included INSTALL file.
Information on full musl-targeted compiler toolchains, system
bootstrapping, and Linux distributions built on musl can be found on
the project website:

    http://www.musl-libc.org/