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Notes on the OpenBSD threaded C library (-lc_r)
================================================
Sources
The main bulk of this library came from:
. FreeBSD's libc_r (John Birrell)
- The scheduler and locking code in the uthread directory.
- Some of the manual pages in the man directory.
- <pthreads.h>
. MIT pthreads (Chris Provenzano)
- The test code in the regress/lib/libc_r directory
I'm mainly tracking changes in FreeBSD's libc_r and integrating
them as I can. The major changes are outlined at the end of this
file.
Standards
This implementation has also been mindful of:
. Posix Threads[1]
. Single Unix Specification[2]
Conformance
Only the absolutely required re-entrant functions have been added
to the C library interface headers (in /usr/src/include).
The conformance goals used were: minimal implementation; strict
conformance with standard; provide reasonable utility when standards
are spineless (e.g., our asynchronous cancel is eager even though
POSIX says async cancels may be acted upon at any time -- ie
never.)
The widely used, but non-standard, gethostbyname_r() has been
half-heartily added (but not prototyped in a header - see source
for details). Please contact me if you have pointers to
standards/comments about this function.
Re-entrant functions added for the standards have also been made
available to libc (i.e. without the need to define _POSIX_THREADS).
Change strategy in libc
The approach taken in making the libc functions re-entrant was to
develop some macros that handle:
. file locking
. monitors (thread-shared data structure locking)
. per-thread private data structure allocation
These macros were used to avoid copious amounts of #ifdef
statements[3].
In the non-threaded libc, file locking and the monitors are no-ops;
the per-thread private data structures were previously declared
static and the macros maintain this.
In the threaded libc_r, file locking is as per the FreeBSD file
descriptor locking, and the monitors are pthread_mutex operations.
Per-thread private data structures use the pthread_specific
functions to dynamically allocate memory on their first use,
initialising them from the static (and hidden) per-compilation-unit
data structures. In this way, each thread appears to have its
very own private libc state.
Errno
Unlike the FreeBSD and MIT pthreads package, errno is not a macro
like __error(); instead it is part of the per-thread context:
saved and restored like a register. This has several advantages
over the errno-as-a-macro method:
- The syscall/cerror code does not have to be re-written
- Libraries compiled without thread support will still work
- Easier to port old packages that use errno, but don't include
<errno.h> to get the macro
- No need to go through all sources and find where errno is
used as a field name or formal parameter name.
The overhead of saving and restoring an integer was considered
too tiny to worry about in comparison to the huge penalty hit of
handling a signal and restoring the rest of a thread's context.
It has been pointed out that this technique will not work in a
mutiprocessor environment, and this is quite true. However the
following reasons are quite persuasive:
- OpenBSD does not do MP (yet)
- This (FreeBSD uthread) implementation will not work in an MP
environment anyway because _thread_run is not a cpu-local
variable.
- There is a lot of coupling between binary ports, the 'old'
libc and other standard libraries. It is predicted that it
will be a huge headache to come up with a scheme that when
someone tries to upgrade, everything won't break in a spectacular
fashion, leaving bits of crippled ports, executables and
shattered libraries everywhere.
Compiler support
The in-tree gcc had its config/openbsd.h modified to support a
`-pthread' switch. Using this switch defines _POSIX_THREADS for
cpp, and replaces the normal -lc linker option with -lc_r.
The objective-C component was also made aware of posix threads,
via the configure switch --enable-threads=posix. {This has not
been well tested though.}
Debugger support
The in-tree gdb has been augmented to recognise when a executable
linked with -lc_r is being debugged. All the documented gdb threads
commands will then work. (Except you can't resume/step execution
in a thread other than the 'current' one.)
To get detailed state information, issue:
(gdb) call _thread_dump_info()
This will show what mutexes, condvars or file locks each process
is waiting on. If you use _flockfile_debug() instead of flockfile()
in your programs, source code references will appear in here too.
(see <stdio.h>)
Changes to FreeBSD uthreads
Although this implementation tracks the FreeBSD libc_r tree to
a large extent, the following significant changes have been made
to John Birrell's uthreads implementation that have yet to appear
(or may never appear) in FreeBSD's implementation:
- architecture independent code generalised and moved out of
#ifdef's[3] (now support for sparc, mips, m68k, powerpc added
to i386, alpha)
- the SIGINFO handler generates much more friendly/useful output
- an implementation of poll() {probably bogus, but X11 seems to
work}
- an implementation of pthread_cancel()
- an implementation of pthread_[gs]etscope() {bogus}
- removal of the freebsd's support for init(8) {i.e., pid == 1}
Caveats
This library is not 100% standards compliant - yet. It is certainly
moving in that direction though. There are still some unresolved
isseues, and the interested reader is directed towards the TODO
file.
Standard disclaimer
This software is made available by the author to the public for
free and "as is". All users of this free software are solely
and entirely responsible for their own choice and use of this
software for their own purposes. By using this software, each
user agrees that the author shall not be liable for damages of
any kind in relation to its use or performance.
Some parts of this software bear their own copyright which is
different to the above disclaimer.
References
[1] P1003.1c/D10 IEEE Draft Standard for Information Technology--
Portable Operating System Interface (POSIX) -- Part 1: System
Application Program Interface (API) -- Ammendment 2: Threads
Extension [C Language]. IEEE Standards, September 1994.
[2] T912, The Single UNIX(R) Specification, Version 2. The Open
Group, February 1997. http://www.opengroup.org/pubs/catalog/t912.htm
[3] #ifdef Considered Harmful, or Portability Experience with C
News. H. Spencer and G. Collyer, Proc. of the Summer 1992
USENIX Conference, San Antionio, Texas, 1992. pp. 185-198
Acknowledgements
Lots of kudos to Chris Provenzano et al. for the original
MIT-pthreads implementation that I still read to get ideas; and
also to John Birrell and the other FreeBSD developers who
consistently write high quality code. The help rendered from the
OpenBSD developers was greatly appreciated - especially from
Todd Fries.
David Leonard <leonard@csee.uq.edu.au>
$OpenBSD: NOTES,v 1.4 2001/08/15 14:43:15 fgsch Exp $
|
