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