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++.

   Experimental wrapper support:

   If the linuxthreads library has been compiled with
   LINUXTHREADS_WRAP_API defined in the ports makefile then
   the API functions are internally prefixed with linuxthreads_
   to avoid conflict with native threads.  Weak symbols without
   that prefix points to the wrapper functions (internally
   prefixed by _) that makes the linuxthreads functions available
   with the native threads API.  Applications including the
   linuxthreads version of pthread.h will try to call the
   prefixed methods (e.g. linuxthreads_pthread_create) while 
   applications including the native pthread.h will use the
   wrapper functions.  This allows for some level of coexistence
   of libraries compiled with linuxthreads header file and
   libraries compiled with native threads header files as long
   as none of the pthread data types leaks out as function arguments	
   or structure members.  

5) 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.

6) If the rate of sched_yield() calls is very high (due to 
   malloc / spinlock interaction) then 
   LINUXTHREADS_NO_POSIX_PRIORITY_SCHEDULING can be defined when
   compiling the linuxthreads library.  This has the same effect
   as if the kernel was compiled without the _KPOSIX_PRIORITY_SCHEDULING
   option, but limited to the linuxthread library.

7) libraries using SIGUSR2 or SIGUSR1 will conflict with linuxthreads.
   SIGUSR1 is hardcoded in the FreeBSD kernel (cf. RFLINUXTHPN flag
   passed to rfork()), but the linuxthreads library can be changed to use
   another signal instead of SIGUSR2 by defining LINUXTHREADS_ALT_RESTARTSIG
   to the alternate signal number (e.g. 32).

8) By default, the maximum thread stack size is 2 MB, and the maximum
   number of threads is 1024.  Layout of stacks are based on the
   maximum thread stack size.  This means that 2 GB of the address space
   can be used by thread stacks, limiting what is left for other
   purposes (text, data, bss, heap, mmap).  To shrink the address range
   used for thread stacks, the maximum thread stack size can be changed
   by defining LINUXTHREADS_MAX_STACK_SIZE to a suitable value, e.g.
   1048576.  The maximum number of threads can be changed by defining
   LINUXTHREADS_MAX_THREADS to a suitable value, e.g. 2048.
   The product of the maximum thread stack size and the maximum number
   of threads should not exceed 2 GB.  The maximum stack size must be
   a power of 2.

9) The complete FreeBSD source code matching the installed userland
   must be installed in /usr/src before building linuxthreads.