Archive for January, 2012

Turn off your laptop and leave the server working

If you usually connect to servers via SSH, you have probably had to wait to finish a time consuming task before you could close the console and therefore, your computer. However, there is at least one way for executing the needed commands on the server and going home. The screen command will help you with that.

The first thing you have to do is logging into the SSH server. That’s easy, you know how to do it:


Once you are in, install screen if you don’t have it yet. As easy as this for an Ubuntu server:

sudo apt-get install screen

Now that you have everything you need, execute screen:


This will open another session in the same terminal.

Perform any task you need now. For example, upload a large file to a remote FTP server:

sftp>put a_big_file.tar.gz
Uploading a_big_file.tar.gz to somewhere in your FTP server very slowly
a_big_file.tar.gz 1% 5KB 1.4KB/s 00:05 ETA

That’s going to take long and you have to leave now, so it’s time to detach the session. Press on your keyboard:

Ctrl + a

and then, to definitely detach the session, press:


The server will keep on uploading the file, but now you can close the SSH connection and turn off your computer.

Tomorrow, when you arrive at the office, you might want to know whether the task was finished correctly. Connect to the server and run:

screen -r

This will resume any previous screen sessions, or will show the screens to be resumed if there are more than one.

Reset nondetachable USB devices on your laptop

2012/01/17 1 comment

I’ve had problems with Ubuntu and my laptop’s integrated webcam for quite some time. Because of these problems, a couple of developers started working on some alternative drivers, but the project seems to be abandoned right now (7 months without commits).

With no appropriate drivers, the device displays odd colours and randomly hangs up (specially when using Flash Player) leaving the power led on. This is very unpleasant because it gives you the impression that somebody might be spying on you.

So I googled a bit searching for a way to reset the devices that I can’t detach, and found this great post by Alan Stern in which he gives us a piece of code to do just that.

mkdir usbreset
cd usbreset

Copy the code into usbreset.c:

/* usbreset -- send a USB port reset to a USB device */

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/ioctl.h>

#include <linux/usbdevice_fs.h>

int main(int argc, char **argv)
 const char *filename;
 int fd;
 int rc;

 if (argc != 2) {
 fprintf(stderr, "Usage: usbreset device-filename\n");
 return 1;
 filename = argv[1];

 fd = open(filename, O_WRONLY);
 if (fd < 0) {
 perror("Error opening output file");
 return 1;

 printf("Resetting USB device %s\n", filename);
 rc = ioctl(fd, USBDEVFS_RESET, 0);
 if (rc < 0) {
 perror("Error in ioctl");
 return 1;
 printf("Reset successful\n");

 return 0;

Then build it:

cc  usbreset.c -o usbreset
chmod +x usbreset

Now we have to know which is the bus and device our webcam is attached to:

Bus 002 Device 005: ID xxxx:xxxx (...) Webcam

Last, call usbreset with the path of the device as a parameter:

sudo ./usbreset /dev/bus/usb/002/005

And that’s it. Bus and Device might change after the reset but ID won’t. So, we note down that ID and make a little sh script to avoid the lsusb step (put it in the same place as the usbreset binary). Let’s call it (for originality’s sake):

MATCHES=$(lsusb | sed -n 's/Bus \([0-9]*\) Device \([0-9]*\): ID '$ID'.*/\/dev\/bus\/usb\/\1\/\2/p')
if [ -z ${MATCHES} ]; then
 echo "No match found"
 sudo ./usbreset $MATCHES

And now we can reset our webcam by simply calling ./

USBReset source code:

Build latest ffmpeg from source

2012/01/11 1 comment

FFmpeg logoI use ffmpeg a lot in my work because I need to process lots of multimedia resources programmatically (without human intervention). The prebuilt binaries of ffmpeg usually suffice for your average encoding/decoding tasks (if due to your particular needs you lack certain propietary codecs you can always grab a more codec-rich build such as the one medibuntu offers). But sometimes you need advanced features such as filters (overlays,  scaling, padding…) and since filters are a constantly evolving feature it is interesting to know how to build ffmpeg from source.

Removing old stuff and solving dependencies

First, you need to install git (if you don’t already have it):

sudo apt-get install git

Next, uninstall any previous ffmpeg builds from your system (if you’re building ffmpeg with x264 support like I’m going to do, uninstall x264 as well):

sudo apt-get remove ffmpeg x264 libx264-dev
sudo apt-get autoremove

Now we need to install a bunch of dependencies. This list may vary depending on the ffmpeg configuration you want to use. Don’t worry too much if you forget about some codec or dependency at this point, ffmpeg will tell you if something’s missing in the configuration step.

In my case, I wanted as many codecs as I could remember available to ffmpeg so as you can see the dependency list is quite long:

sudo apt-get install build-essential git-core checkinstall yasm texi2html \
     libfaac-dev libjack-jackd2-dev libmp3lame-dev libopencore-amrnb-dev \
     libopencore-amrwb-dev libsdl1.2-dev libtheora-dev libva-dev libvdpau-dev \
     libvorbis-dev libvpx-dev libx11-dev libxfixes-dev libxvidcore-dev \
     zlib1g-dev librtmp-dev libgsm0710-dev libgsm0710mux-dev libgsm1-dev \
     libgsmme-dev libschroedinger-dev libspeechd-dev libspeex-dev \
     libspeexdsp-dev libspeex-ocaml-dev libdc1394-22-dev

Ok, if you read the dependency list (did you, really?) you’ll have noticed that x264 isn’t among the installed packages. The reason is that I’ll also be building x264 from source because the prebuilt binaries (I’m talking about the ones in Ubuntu’s repository) seem to be too old for the ffmpeg we’re about to build.

Building x264 as a shared library

First, clone x264‘s git repository to grab the latest version of the code. Then, use the –enable-shared flag when configuring to build it as a shared library, otherways ffmpeg won’t be able to use it.

git clone git://
cd x264
./configure --enable-shared
sudo make install

Optionally, you can use checkinstall to build a .deb package and thus make the binaries redistributable:

sudo checkinstall --pkgname=libx264 \
    --pkgversion="2:0.$(grep X264_BUILD x264.h -m1 | \
    cut -d' ' -f3).$(git rev-list HEAD | wc -l)+git$(git rev-list HEAD -n 1 | \
    head -c 7)" --backup=no --deldoc=yes \
    --fstrans=no --default

Well, now that we’ve got all the libraries we need it’s time to build our customized ffmpeg.

Building ffmpeg

Since  January 2011 ffmpeg no longer uses svn to host the code, you should keep this in mind when you read other ffmpeg tutorials (they may be outdated).

git clone git://
cd ffmpeg
./configure --enable-avfilter --enable-vdpau --enable-bzlib \
    --enable-libgsm --enable-libschroedinger --enable-libspeex \
    --enable-pthreads --enable-zlib --enable-libvpx \
    --disable-stripping --enable-runtime-cpudetect \
    --enable-vaapi --enable-swscale --enable-libdc1394 \
    --enable-shared --disable-static --enable-librtmp \
    --enable-gpl --enable-version3 --enable-nonfree \
    --enable-postproc --enable-libfaac --enable-libmp3lame \
    --enable-libopencore-amrnb --enable-libopencore-amrwb \
    --enable-libtheora --enable-libvorbis --enable-libx264 \
    --enable-libxvid --enable-x11grab --enable-filter=movie
sudo make install

Building ffmpeg takes quite some time, be patient. When everything’s done, call ffmpeg without parameters to see if it works.

In my case it didn’t, so I had to use strace to find out what was wrong.

Fixing runtime problems

sudo strace ffmpeg

Reveals the following:

access("/etc/", F_OK)      = -1 ENOENT (No such file or directory)
access("/etc/", R_OK)      = -1 ENOENT (No such file or directory)

It seems ffmpeg is trying to access two files that don’t exist. I’ll create them and see if that works.

sudo touch /etc/
sudo touch /etc/

And… that actually worked!

Now you have a fully functional customized ffmpeg build. Congratulations.

If you want to know more about the latest features and examples of ffmpeg filters, please check out the libavfilter documentation.

Color picker for mobile devices with Flex 4.5

2012/01/02 8 comments

I don’t have much experience with Flex. I’ve just participated in the Babelium Project, a web application for language practising developed in Flex, and now I’ve been asked to port an Adobe AIR desktop application to android devices. In this last work, I came across some problems replacing the unsupported MX components by Spark components and some of them where a pain in the neck.

The ColorPicker was a tricky one to substitute. I found this SparkColorPicker, but I couldn’t make it work correctly on a tablet PC (I told you, I’m far from being an expert). The problem seems to be in the ComboBox which extends, as combo boxes and drop down lists are discouraged in AIR for mobile development. When clicking the button, the DropDown was opened and immediately closed making it impossible to choose a color.

After several attempts to fix the issue, I decided to implement my own color picker using the Callout class. This is how it looks like:

You can see the source code in this github repository:

Categories: Programming Tags: , ,