This version of Slackware contains support in glibc for NPTL (the Native 
POSIX Thread Library).  NPTL works with newer kernels (meaning 2.6.x, or 
a 2.4 kernel that is patched to support NPTL, but not an unmodified 
"vanilla" 2.4 kernel such as Slackware uses) to provide improved 
performance for threads.  This difference can be quite dramatic in some 
situations.  For example, a benchmark test mentioned on Wikipedia 
started 100,000 threads simultaneously in about 2 seconds on a system 
using NPTL.  The same test using the old Linuxthreads glibc thread 
support took around 15 minutes to run!  For most applications that do 
not start large numbers of threads the difference here will not be so 
large, but for high traffic servers, databases, or anything that runs 
large numbers of threads, NPTL should bring big improvements in 
scalability and performance.  For compatibility, the regular 
(linuxthreads) libraries are installed in /lib, and the new NPTL 
versions are installed in /lib/tls.  Which versions are used depends on 
the kernel you're using.  If it's newer than 2.6.4, then the NPTL 
libraries in /lib/tls will be used.  TLS stands for "thread-local 
storage", and the directory name /lib/tls is a little bit misleading 
since now both the linuxthreads and NPTL versions of glibc are compiled 
with TLS support included (this is needed to produce versions of tools 
such as ldconfig that can run under either kind of system).

Getting all the kinks out of the build script to be able to get this to 
work with either 2.4 or 2.6 kernels and be able to switch back and forth 
without issues was quite a challenge, to say the least, and would have 
been much harder without all the good advice and help folks sent in to 
help me along and give me important hints.  A special thanks goes to 
Chad Corkrum for sending in some ./configure options that really helped 
get the ball rolling here.

Here's some information about compiling things using these libraries -- 
by default, if you compile something the headers and shared libraries 
used to compile and link the binary will be the linuxthreads versions, 
but when you go to run the binary it will link to the NPTL library 
versions (and you'll get the NPTL speed improvements) if you are running 
an NPTL capable kernel.  In rare cases you may find that an old binary 
doesn't work right when run against the NPTL libs, and in this case you 
can force it to run against the linuxthreads versions by setting the 
LD_ASSUME_KERNEL variable to assume the use of a 2.4.x (non-NPTL) kernel 
so that NPTL will not be used.  An easy way to see the effect of this is 
to try something like the following while using an NPTL enabled kernel:
  
  volkerdi@tree:~$ ldd /bin/bash
        linux-gate.so.1 =>  (0xffffe000)
        libtermcap.so.2 => /lib/libtermcap.so.2 (0xb7fcf000)
        libdl.so.2 => /lib/tls/libdl.so.2 (0xb7fcb000)
        libc.so.6 => /lib/tls/libc.so.6 (0xb7eaf000)
        /lib/ld-linux.so.2 (0xb7feb000)

Note that in the example above, the binary is running against the NPTL 
libraries in /lib/tls.  Now, let's try setting LD_ASSUME_KERNEL:

  volkerdi@tree:~$ LD_ASSUME_KERNEL=2.4.30 ldd /bin/bash
        linux-gate.so.1 =>  (0xffffe000)
        libtermcap.so.2 => /lib/libtermcap.so.2 (0xb7fcf000)
        libdl.so.2 => /lib/libdl.so.2 (0xb7fcb000)
        libc.so.6 => /lib/libc.so.6 (0xb7eb2000)
        /lib/ld-linux.so.2 (0xb7feb000)

As you can see, now the binary is running against the linuxthreads 
version of glibc in /lib.  If you find old things that won't work with 
NPTL (which should be rare), this is the method you'll want to use to 
work around it.

Now for a little note about compiling things.  In most cases it will be 
just fine to compile against linuxthreads and run against NPTL, and this 
approach will produce the most flexible binaries (ones that will run 
against either linuxthreads or NPTL.)  However, in some cases you might 
want to use some of the new functions that are only available in NPTL, 
and to do that you'll need to use the NPTL versions of pthread.h and 
other headers that are different and link against the NPTL versions of 
the glibc libraries.  To do this you'll need to add these compile flags 
to your build in an appropriate spot:

    -I/usr/include/nptl -L/usr/lib/nptl
    (and link with -lpthread, of course)

Have fun, and report any problems to volkerdi@slackware.com.

Pat