Rich Felker
8cca79a72c
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.
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/