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2-Cent Tips

2-cent tip - rpath

Oscar Laycock [oscar_laycock at yahoo.co.uk]


Thu, 8 Oct 2009 02:17:44 -0700 (PDT)

This all started when I wanted to run the latest version of Firefox. I decided to build my own from source. But my kernel, assembler, compiler and C library were too old - in fact, nine years old. So I built new ones under the /usr/local directory. I used the Linux From Scratch book as a guide.

Now when I build new programs, I set the GCC compiler's "rpath" option to point to the libraries in /usr/local rather than in the usual /lib and /usr/lib. The rpath is a list of libraries at the start of a program that can tell Linux where to look for shared libraries when Linux runs a program. A program called the "dynamic linker" does the job. On my system it is "/lib/ld-linux.so.2". You can see a program's rpath by running a command such as "readelf -a /bin/ls". Of course, normally there isn't one. Also you can watch the dynamic linker at work using the "ldd" command. I set GCC's rpath by including it in the CFLAGS environment variable when configuring programs before building them. (You typically type "configure", "make" and "make install" to build a program.) I found a small number of programs ignore CFLAGS, so I made the gcc program a shell script, which then calls the real gcc with the right rpath option.

So I can now run old commands such as "ls" and "find" alongside new programs such as the KDE suite. The now eleven-year-old commands run fine on top of the recent kernel. I also put /usr/local/bin at the start of my path. This may be a security risk but my PC is not connected to the internet or a network.

There is a bit more too it. So here is the CFLAGS setting I used only few days ago:

export CFLAGS="-O2 -I. -I.. -I/usr/local/myglibc27/include -I/usr/local/include
-L/usr/local/myglibc27/lib -L/usr/local/lib -L/usr/local/lib/gcc/i686-pc-linux-gnu/4.2.3 -Wl,-rpath=/usr/local/myglibc27/lib:/usr/local/lib:/usr/local/lib/gcc/i686-pc-linux-gnu/4.2.3,-dynamic-linker=/usr/local/myglibc27/lib/ld-linux.so.2 -specs=/home/oscar/tmp/glibc/myspecs08scr -march=pentium2"

I also similarly set these environment variables: LDFLAGS, CXXFLAGS, CPPFLAGS, LIBCFLAGS, LIBCXXFLAGS. You can see that the include file path (-I's) and libraries path (-L's) match the rpath. The "-I. -I.." is there because some programs need to look at the header files in the build directory first - a bit of a quick fix. Notice how I now have two separate dynamic linkers on my PC. I had to edit the compiler specs file a little. Here is a section to really confuse you:

*startfile:
%{!shared: %{pg|p|profile:/usr/local/myglibc27/lib/gcrt1.o%s;pie:/usr/local/myglibc27/lib/Scrt1.o%s;:/usr/local/myglibc27/lib/crt1.o%s}} /usr/local/myglibc27/lib/crti.o%s %{static:/usr/local/lib/gcc/i686-pc-linux-gnu/4.3.2/crtbeginT.o%s;shared|pie:/usr/local/lib/gcc/i686-pc-linux-gnu/4.3.2/crtbeginS.o%s;:/usr/local/lib/gcc/i686-pc-linux-gnu/4.3.2/crtbegin.o%s}

I think this is choosing which C runtime code to put at the start of the program.

And here is the shell script that stands in for gcc:

[ ... ]

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Two-cent Tip: bash script to create animated rotating mark

Mulyadi Santosa [mulyadi.santosa at gmail.com]


Sat, 24 Oct 2009 16:34:04 +0700

During my boring saturday, I was thinking to create simple animated cycling mark. Here's the script:

$ while(true); do for a in \\ \| \/ -; do echo -n $a; sleep 1 ; echo
-n -e \\r ; done; done

Notice the usage of escaped "\r" (carriage return) and "-n" option to display continous marks at the same line and at the same column

-- 
regards,
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com

[ Thread continues here (5 messages/5.54kB) ]


2-cent Tip: Load Python modules at Startup

Amit Saha [amitsaha.in at gmail.com]


Thu, 22 Oct 2009 18:49:49 +0530

Hello:

I have been using CPython as a calculator, while I do all those number crunching in C. SO, 'import math' is a must.

This is what I did:

- Create a file: .pythonrc in my $HOME and place this line:

   import math

- Now in your BASH, .bashrc or similar: export PYTHONSTARTUP= $HOME/.pythonrc

Everytime you start Python interactively, you should have the 'math' module already imported.

$ python
Python 2.6.4rc1 (r264rc1:75270, Oct 10 2009, 02:40:56)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.

>>> math.pi
3.1415926535897931

Hope this helps.

-- 
Journal: http://amitksaha.wordpress.com,
-blog: http://twitter.com/amitsaha


Two-cent Tip: Detexify

Jimmy O'Regan [joregan at gmail.com]


Wed, 14 Oct 2009 11:45:32 +0100

http://detexify.kirelabs.org/classify.html

Nifty website: draw the TeX symbol you were thinking of, and it tells you which one it (probably) is. Source (MIT license) here: http://github.com/kirel/detexify

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2-cent Tip: Piping to GNU Plot from C

Amit Saha [amitsaha.in at gmail.com]


Sun, 4 Oct 2009 16:03:05 +0530

Hello TAG:

Can this be a possible 2-cent tip?

Couple of things first up:

* GNU plot supports piping, So, echo "plot sin(x)" | gnuplot will plot the sin(x) function.

* However, the plot disappears even before you could see it. For that echo "plot sin(x)" | gnuplot -persist , is useful. It persists the GNU plot main window

The usefulness of the second point is that, if you have a "pipe descriptor" describing a pipe to the open GNU plot instance , you can plot more plots on the first plot, without opening a new GNU plot instance. We shall be using this idea in our code.

#include <stdio.h>
#define GNUPLOT "gnuplot -persist"
 
int main(int argc, char **argv)
{
        FILE *gp;
        gp = popen(GNUPLOT,"w"); /* 'gp' is the pipe descriptor */
        if (gp==NULL)
           {
             printf("Error opening pipe to GNU plot. Check if you have it! \n");
             exit(0);
           }
 
        fprintf(gp, "set samples 2000\n");
        fprintf(gp, "plot abs(sin(x))\n");
        fprintf(gp, "rep abs(cos(x))\n");
        fclose(gp);
 
return 0;
}

The above code will produce a comparative plot of absolute value of sin(x) and cos(x) on the same plot. The popen function call is documented at http://www.opengroup.org/pubs/online/7908799/xsh/popen.html. This code/idea should work on GCC and Linux and any other language and OS that supports piping.

Utility: If you have a application which is continuously generating some data, which you will finally plot, then you can plot the data for every new set of data- that gives a nice visualization about how the data is changing with the iterations of your application. This is a perfect way to demonstrate convergence to the best solutions in Evolutionary Algorithms, such as Genetic Algorithms.

Best, Amit

-- 
Journal: http://amitksaha.wordpress.com,
-blog: http://twitter.com/amitsaha

[ Thread continues here (4 messages/7.26kB) ]



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Published in Issue 168 of Linux Gazette, November 2009

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