Best of Technical Support

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Best of Technical Support

128-Bit Precision with GCC

I am using the command xlC to compile C++ programs on a UNIX platform. When I need to increase the precision of my mathematical calculations from 64b-128b, the following command is helpful:

xlC128 -qldbl128 <filename> [-lm]

where -lm is used to include mathematical libraries, if required. Now, I want to perform the same calculation with the same precision on Linux. Please guide me as to how I should proceed. If any mathematical libraries are required, please also indicate the source from where I can obtain them.


Get GMP (Gnu Math Precision), a free library for arbitrary precision arithmetic, operating on signed integers, rational numbers and floating-point numbers. You can find it at

—Paul Christensen,

Upgrade Causes Weekly Crash

After I upgraded one of our systems to Red Hat 7.1 from 7.0, the system crashes frequently, something like once a week. The crashes always occur soon after 4:00 A.M. (after cron.daily is executed). Kernel output is

Unable to handle kernel NULL pointer dereference
at virtual address 00000000.

I upgraded kernel to 2.4.3-12, but there's not much improvement.

—Atsuko Crum,

I had similar problems with a different distribution but a 2.4.x kernel. It turned out that a BIOS upgrade for the motherboard took care of most of my problems. Although I still have similar occurrences, they are much less frequent.

—David Brown,

While hardware problems can happen at any time, if this started after you upgraded, you should try downgrading the kernel to the version you had on Red Hat 7.0, or if you don't mind rebuilding a kernel, you can try getting the latest 2.4 available (2.4.10 at the time of this writing). Many bugs were fixed in the first versions of the 2.4 kernel. If you want to rule out hardware issues, you can try an excellent stress tester, Cerberus, which you can get from SourceForge at

—Marc Merlin,

Insufficient Space for Install

Slackware kept prompting that I had no space left for the installation to proceed. I could not believe it because I had 10GB reserved. I partitioned my hard drive like this:

5GB WinNT 4.0512MB for root512MB for swap4GB for usr4GB for home


After formatting your partitions and setting up their mountpoints, switch to a second virtual console (Alt-F2) and verify, using df or mount, that they actually are mounted. If not, you will be installing Slackware on your 512MB root partition. That size may be too small if you are installing X or other large applications.

—Chad Robinson,

I am betting here that you are mislabeling the / (root) with a /root label. Try labeling your partitions like this: 5GB WinNT 4.0512MB /512MB swap4GB /usr4GB /home That should give you adequate space for the installation.

—Paul Christensen,

PAM on Slackware?

As I install more applications on my Slackware server for my users, I find that many require authentication, but there are a few that don't read data from the passwd file. The result is that users need to change their password in multiple locations.

There seems to be a movement toward PAM and LDAP so that if I could switch to support them, users could change their password to all of the services, including Samba, e-mail, pppd and phpgroupware from one location (such as a web-based password change application). However, Slackware doesn't support PAM, and I can't find a HOWTO to install PAM.

—Brian Johnson,

It is possible to install PAM on a non-PAM-aware distribution, but it involves, among other things, replacing all your applications that do authentication with PAM-aware ones (and if your distribution doesn't provide them, you have to get the source, find PAM patches if they're not integrated and recompile everything with the right options for your system). It is a lot of work, and unless you're looking for a special challenge, I recommend you switch to a modern distribution like Debian Linux or Red Hat Linux (which both support PAM natively). Those are only two examples; many other distributions support PAM too.

—Marc Merlin,