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musl - an implementation of the standard library for Linux-based systems
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Rich Felker 167390f055 lift child restrictions after multi-threaded fork 
as the outcome of Austin Group tracker issue #62, future editions of
POSIX have dropped the requirement that fork be AS-safe. this allows
but does not require implementations to synchronize fork with internal
locks and give forked children of multithreaded parents a partly or
fully unrestricted execution environment where they can continue to
use the standard library (per POSIX, they can only portably use
AS-safe functions).

up until recently, taking this allowance did not seem desirable.
however, commit 8ed2bd8bfc exposed the
extent to which applications and libraries are depending on the
ability to use malloc and other non-AS-safe interfaces in MT-forked
children, by converting latent very-low-probability catastrophic state
corruption into predictable deadlock. dealing with the fallout has
been a huge burden for users/distros.

while it looks like most of the non-portable usage in applications
could be fixed given sufficient effort, at least some of it seems to
occur in language runtimes which are exposing the ability to run
unrestricted code in the child as part of the contract with the
programmer. any attempt at fixing such contracts is not just a
technical problem but a social one, and is probably not tractable.

this patch extends the fork function to take locks for all libc
singletons in the parent, and release or reset those locks in the
child, so that when the underlying fork operation takes place, the
state protected by these locks is consistent and ready for the child
to use. locking is skipped in the case where the parent is
single-threaded so as not to interfere with legacy AS-safety property
of fork in single-threaded programs. lock order is mostly arbitrary,
but the malloc locks (including bump allocator in case it's used) must
be taken after the locks on any subsystems that might use malloc, and
non-AS-safe locks cannot be taken while the thread list lock is held,
imposing a requirement that it be taken last.
2020-11-11 15:55:30 -05:00
arch fix vector types in aarch64 register file structures 2020-11-11 10:54:58 -05:00
compat/time32 fix null pointer dereference in setitimer time32 compat shim 2019-12-08 10:35:04 -05:00
crt remove unnecessary and problematic _Noreturn from crt/ldso startup 2019-06-25 19:05:40 -04:00
dist add another example option to dist/config.mak 2012-04-24 16:49:11 -04:00
include add support for SIGEV_THREAD_ID timers 2020-10-28 23:00:08 -04:00
ldso lift child restrictions after multi-threaded fork 2020-11-11 15:55:30 -05:00
src lift child restrictions after multi-threaded fork 2020-11-11 15:55:30 -05:00
tools fix incorrect escaping in add-cfi.*.awk scripts 2020-01-20 15:57:29 -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 warnings by default 2020-08-27 20:43:47 -04:00
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 document mips r6 in INSTALL file 2019-09-27 00:22:48 -04:00
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 1.2.1 2020-08-04 00:21:09 -04:00
WHATSNEW release 1.2.1 2020-08-04 00:21:09 -04:00

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/