freebsd-ports/devel/linuxthreads/files

Some brief notes:

1) This package is intended to run on FreeBSD 5.0-current or FreeBSD 4.X, i386
processors only.

Do not use libc_r with the linuxthreads port, and do not compile/link with the
-pthread option (which pulls in libc_r).  Rather, link with libc (which you will
get by default).

Compile your applications that use the linuxthreads port with the following
command line options:

  -D_THREAD_SAFE -I/usr/local/include/pthread/linuxthreads -llthread -llgcc_r

Note that the include (-I<path>) directive shown here should appear before any
other include directive that would cause the compiler to find the FreeBSD file
/usr/include/pthread.h.  Using the FreeBSD pthread.h instead of the linuxthreads
pthread.h will result in an app that fails in many odd and maybe spectacular
ways.

In order to facilitate porting applications which expect a libpthread, you can
create the following symlinks if you want:

  ln -s /usr/local/lib/liblthread.a /usr/lib/libpthread.a
  ln -s /usr/local/lib/liblthread_p.a /usr/lib/libpthread_p.a
  ln -s /usr/local/lib/liblthread.so.2 /usr/lib/libpthread.so.2
  ln -s /usr/local/lib/liblthread.so.2 /usr/lib/libpthread.so
  /sbin/ldconfig -m /usr/lib

If you do this, you can instead use:

  -D_THREAD_SAFE -I/usr/local/include/pthread/linuxthreads -lpthread -llgcc_r

Another option is to create a custom gcc specs file that tells the linker which
version of libgcc to use.  To do this, create a file with the following in it:

--- (/foo/specs) cut here ---
*libgcc:
/usr/local/lib/liblgcc_r.a
--- (/foo/specs) cut here ---

Then use the following command line options:

  -B/foo/ -D_THREAD_SAFE -I/usr/local/include/pthread/linuxthreads -llthread

or if you created symlinks:

  -B/foo/ -D_THREAD_SAFE -I/usr/local/include/pthread/linuxthreads -lpthread

2) If you plan on having lots of threads, check the sysctl value of
kern.maxproc.  Each kernel thread counts against maxproc.  You can increase
maxproc by changing the MAXUSERS value in your kernel config file.  maxproc is
set at 20 + 16 * MAXUSERS.

3) Be aware of the following libc issues:

  a) Not all libc calls are thread safe.  In particular gmtime, localtime, etc
  are not thread safe.  In general, where the pthreads spec calls for "_r"
  functions, these are either not provided, or if provided are not thread safe
  (in most cases) and the related libc calls are not thread safe.  This differs
  somewhat from the FreeBSD libc_r library, where some, but not all, of these
  functions are both thread safe and have "_r" versions.

  b) Not all of the libc calls that are supposed to be cancellation points are
  implemented as such.  While linux threads has the cancel functions
  implemented, deferred cancellation will not work as required by POSIX
  1003.1c-1995, since the co-operation needed from libc is not complete.

  c) The mutex wrapper functions only provide standard linuxthreads mutexes
  (i.e. non-recursive mutexes).  This might lead to deadlocks if libc
  depends on recursive mutexes.

4) Be aware of the following libgcc issue:

   FreeBSD 4.* (gcc 2.*):

   __register_frame_info() and __get_eh_info() from libgcc.a are linked
   into shared libraries that use exceptions, e.g. libstdc++.  Those
   functions are not compatible with linuxthreads due to pthread_mutex_t
   and pthread_once_t having different sizes and static initializers.
   Linking the shared linuxthreads library before any such library causes
   the liblgcc_r.a version of those functions to be used.

   FreeBSD 5.* (gcc 3.*):

   __register_frame_info() and __frame_state_for() from libgcc.a are
   linked into shared libraries that use exceptions, e.g. libstdc++.
   Those functions are not compatible with linuxthreads due to
   pthread_mutex_t and pthread_once_t having different sizes and static
   initializers.  Linking the shared linuxthreads library before any such
   library causes the liblgcc_r.a version of those functions to be used.
   Use liblstdc++ and liblsupc++.

4) Exit handling is broken.

   If the linuxthreads library has been compiled with
   LINUXTHREADS_DETECT_UNSAFE_EXIT defined in the ports makefile then
   the library tries to avoid further calls to functions registered
   with atexit if not called from the main thread or if other threads
   were active.  Since this implicitly indicates a failure to do
   proper cleanup, the exit code is then changed to 1.

   If the linuxthreads library has been compiled without
   LINUXTHREADS_DETECT_UNSAFE_EXIT, then calls to exit() has a
   slightly higher probability of crashing or hanging the program when
   other threads are active.  If another thread than the main thread
   performs the exit call, the exit code will appear to be 0.

   If multiple threads calls exit then the application will likely
   crash.

   If other threads has been joined by the main thread before it calls
   exit then exit handling should be fairly safe and the correct exit
   code can be detected by the parent process.