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like mutexes and semaphores, rwlocks suffered from a race condition where the unlock operation could access the lock memory after another thread successfully obtained the lock (and possibly destroyed or unmapped the object). this has been fixed in the same way it was fixed for other lock types. in addition, the previous implementation favored writers over readers. in the absence of other considerations, that is the best behavior for rwlocks, and posix explicitly allows it. however posix also requires read locks to be recursive. if writers are favored, any attempt to obtain a read lock while a writer is waiting for the lock will fail, causing "recursive" read locks to deadlock. this can be avoided by keeping track of which threads already hold read locks, but doing so requires unbounded memory usage, and there must be a fallback case that favors readers in case memory allocation failed. and all of this must be synchronized. the cost, complexity, and risk of errors in getting it right is too great, so we simply favor readers. tracking of the owner of write locks has been removed, as it was not useful for anything. it could allow deadlock detection, but it's not clear to me that returning EDEADLK (which a buggy program is likely to ignore) is better than deadlocking; at least the latter behavior prevents further data corruption. a correct program cannot invoke this situation anyway. the reader count and write lock state, as well as the "last minute" waiter flag have all been combined into a single atomic lock. this means all state transitions for the lock are atomic compare-and-swap operations. this makes establishing correctness much easier and may improve performance. finally, some code duplication has been cleaned up. more is called for, especially the standard __timedwait idiom repeated in all locks.
musl libc - a new standard library to power a new generation of
Linux-based devices. musl is lightweight, fast, simple, free, and
strives to be correct in the sense of standards-conformance and
safety.
musl is an alternative to glibc, eglibc, uClibc, dietlibc, and klibc.
For reasons why one might prefer musl, please see the FAQ and libc
comparison chart on the project website,
http://www.etalabs.net/musl/
For installation instructions, see the INSTALL file.
Please refer to the COPYRIGHT file for details on the copyright status
of code included in musl, and the COPYING file for the license (LGPL)
under which the library as a whole is distributed.
Greetings libc hackers!
This package is an _alpha_ release of musl, intended for the curious
and the adventurous. While it can be used to build a complete small
Linux system (musl is self-hosted on the system I use to develop it),
at this point doing so requires a lot of manual effort. Nonetheless, I
hope low-level Linux enthusiasts will try out building some compact
static binaries with musl using the provided gcc wrapper (which allows
you to link programs with musl on a "standard" glibc Linux system),
find whatever embarassing bugs I've let slip through, and provide
feedback on issues encountered building various software against musl.
For bug reports, support requests, or to get involved in development,
please visit #musl on Freenode IRC or subscribe to the musl mailing
list by sending a blank email to musl-subscribe AT lists DOT openwall
DOT com.
Thank you for trying out musl.
Cheers,
Rich Felker / dalias