Floating point numbers ...

Floating point numbers have some unexpected properties.

Try this in your favourite language (if it has real floats)

Python:

>>> print 0.1 + 0.2 == 0.3

False

 C:

#include <stdio.h>

int main() {  printf("%s\n", 0.1 + 0.2 == 0.3 ? "True" : "False");

} 

$ gcc float.c -o float && ./float 

False 

Installing a specific branch of a python package with pip

pip install --user 'git+git://github.com/llvmpy/llvmpy@llvm-3.4'

Parallela16 temperature

Reading the temperature for the Xilinx Zynq chip is done as described in http://forums.parallella.org/viewtopic.php?t=930

with

python -c "T=open('/sys/bus/iio/devices/iio:device0/in_temp0_raw').read(); T = (float(T) / 4096. * 503.975) - 273.15; print T"

ARM debugging with Qemu and gdb

To set up a pty to capture the serial port info, use socat

socat -d -d pty  pty

which prints out which pty to connect Qemu's virtual serial port to, for instance /dev/pts/1, and the other pty, for instance /dev/pty/6, to which one may for instance attach a GNU Screen

screen /dev/pty/6

Now run Qemu

QEMU_AUDIO_DRV=none qemu-system-arm \
-cpu cortex-a15 -machine vexpress-a15 \
-kernel /tmp/vmlinuz-3.10 -append "earlyprintk console=tty1 console=ttyAMA0" \
-nographic \
-gdb tcp:localhost:1234 -S -serial /dev/pts/1

which tells Qemu to start a gdb-server at port 1234 and wait (-S) until a gdb server attaches.

Make sure to have a gdb configured for arm targets, compiled with

./configure --target=arm-linux-gnueabi
make

and run it.  Connect to the qemu gdb-server with

target remote :1234 

To see the machine code, type

C-x a

to enable the terminal interface that shows code cursor, and enable asm with

layout asm

 

Enabling a bridge interface when you're on nfs root

When you enable a bridge interface with brctl from bridge-utils you loose network connectivity for a short while. This means trouble if you use nfs root or something that needs to have network to load the next command,  or acces other files or directories.

Here's what to do.  Warning!  Understand what you do. If you are working logged in remotely and it screws up, you'll have to reboot the machine, because it has lost its access to the nfs root file system!

Anyway, here we go:

We must create a small rootfs on a directory that is not nfsrooted such as a tmpfs (a ram-based file system), there is often one mounted already. Look for one with mount.

I have a tmpfs mounted at /run.  I notice that it is mounted as 'noexec' which means that I can't run commands from it.  To solve that, remount it with the exec option set.
Then make a root dir called /run/root  for instance.
Then copy over /lib and /bin  (we don't need all of it, but we're lazy, remember!).
Put the stuff we need to do in a script file in the temp root, and then execute it from a chroot with busybox (or sh if you have put everything you need into the chroot).  Make sure that you actually have busybox.

Here is what I have in my script (https://gist.github.com/anonymous/5480567):

Pure python sin

Pure python sin up to 6 decimals (nine degree Taylor expansion, no recursion, 11 mult, 1 div)



"""
f(x) = f(x)+f'(x)*x+f''(x)/2!*(x**2)+...

if f(x) = sin(x)

around 0 is
f(x) = sin(0)+cos(0)*x-sin(0)/2*x2-cos(0)/3!*x3+sin(0)/4!*x4+cos(0)/5!*x5+...
 =  x-x**3/6.+x**5/120.+...
"""

# constants
pi = 3.14159265358979
pi2 = pi / 2
k1 = 1. / (2 * 3)
k2 = 1. / (4 * 5)
k3 = 1. / (6 * 7)
k4 = 1. / (8 * 9)

def sin(x):
    s = 1.0
    if x < 0.0:
        s = -s
        x = -x
    if x > pi2:
        n = int(x / pi2)
        x = x - pi2 * float(n)
        if n & 1: x = pi2 - x
        if n & 2: x = -x
    x2 = x * x
    return s*(x*(1.-k1*x2*(1.-k2*x2*(1.-k3*x2*(1.-k4*x2)))))

Set LC_ALL once and for all...

To get rid of the annoying messages in ubuntu complaining about things like


perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = (unset),
        LC_ALL = (unset),
        LC_CTYPE = "UTF-8",
        LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory

you should do

sudo locale-gen en_US.UTF-8
sudo dpkg-reconfigure locales

Thanks to this thread http://ubuntuforums.org/showthread.php?t=1720356 !!!