The timerfd mechanism in Linux (man timerfd_create(2)) allows one to create different types of timers that communicate expiry events through a file descriptor. The standard read(), select(), poll() functions can then be used to detect and process the expiration notifications. As for the clock granularity, with CLOCK_REALTIME and with High Resolution Timer (HRT) support, I get very respectable 1.000000e-09 seconds on my system. To see this, you can compile (using the –lrt flag) and run the following :

#include 
#include 
#include 
 
int main(void)
{
    printf("System Clock Granularity %ld\n", sysconf(_SC_CLK_TCK));
 
    struct timespec gran;
    if (clock_getres(CLOCK_MONOTONIC, &gran) < 0) {
        perror("clock_getres()");
        return -1;
    }
    printf("CLOCK_MONOTONIC granularity %ld %ld\n", gran.tv_sec, gran.tv_nsec);
 
    return 0;
}

Both periodic and non-periodic timers can be created a timer obtained through timerfd_create(). This behavior is configurable via the itermerspec structure passed as an argument to the timerfd_settime() function:

    struct itimerspec timerSpec;
    memset(&timerSpec, 0, sizeof(timerSpec));
 
    timerSpec.it_value.tv_sec = sec;
    timerSpec.it_value.tv_nsec = nsec;
 
    timerSpec.it_interval.tv_sec = intsec;
    timerSpec.it_interval.tv_nsec = intnsec;

Whenever the it_value field has both its tv_sec and tv_nsec fields set to 0, the timer is effectively disarmed and no future timer expiry events should take place.

Once a timer has been set, read() will report the number of timer expirations events that have occurred since the last call.

Epoll

Having the ability to generate timer events, wouldn’t be nice if we could only register a callback function that would get call automatically from the outside to handle any processing that would have to be done ?

Linux provides the epoll functionality which appears much more elegant and easy to use that its traditional select() counterpart. Even better maybe, it is said to be O(1) versus O(n) : this claim is reported here then taken to Wikipedia and discussed on Stackoverflow.

One cool thing for sure is its ability to deliver event notifications on the set of file descriptors that it watches under either edge or level-triggered modes. In the first case, epoll_wait() (normally a blocking call that you would place in a mainloop) will return as long as there is remaining data on one of its file descriptor. By contrast, when one sets the EPOLLONESHOT flag, epoll_wait() will generate only one event after which the associated file descriptor will be disabled.

Cooking up the final solution

Before going any further : yes libevent exists and also uses epoll. The point here is not to use it.

Back to the initial goal of generating timer expiry notifications asynchronously via callbacks, we only have to register the file descriptor associated with our instantiated timerfd elements to epoll with epoll_ctl() and the EPOLL_CTL_ADD flag. Then run the epoll_wait() in a loop and you almost have a complete solution.

Two problems then remain : how do you keep track of which callback function (together with the arguments to pass) goes with a given timer, and finally how do you avoid blocking you application on epoll_wait() ? The latter can be easily solved by wrapping our mainloop in a thread (with the necessary thread-safety precautions): DONE. For the former, epoll_ctl() delights us with its epoll_data_t union member of the epoll_event structure.

    typedef union epoll_data {
        void        *ptr;
        int          fd;
        uint32_t     u32;
        uint64_t     u64;
    } epoll_data_t;
 
    struct epoll_event {
        uint32_t     events;      /* Epoll events */
        epoll_data_t data;        /* User data variable */
    };

“void *ptr” you said ? Yep that’s it : here’s our function pointer. Since epoll_data is a union, we will probably want to record more than one element under ptr. To do so, we can wrap all the information we need under a structure such as :

typedef struct {
    int tfd;
    void (*timed_action_handler)(void*);
    void* arg;
} timed_action_t;

Now when epoll_wait() unblocks we will be able to get access to the associated epoll_event and execute the callback passing its arg argument from the timed_action_t structure.

GitHub

You can checkout a sketch of the approach explained above at https://github.com/pierrelux/timedaction

The trick that I’m presenting here lies in the use of the num2cell function.

In the example below, we add a 1x4 vector to every row of a matrix A.

>> A = magic(4)
A =
 
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1
 
>> y = [1 2 3 4];
>> B = cellfun(@(x)(x + y), num2cell(A, 2), ‘Uni­for­mOut­put’, false)
 
B = 
 
    [1x4 double]
    [1x4 double]
    [1x4 double]
    [1x4 double]
 
>> cell2mat(B)
 
ans =
 
    17     4     6    17
     6    13    13    12
    10     9     9    16
     5    16    18     5

If we wanted to do a similar operation but this time over the columns, we would write num2cell(A, 1) : “1” instead of “2”.

I have came across this problem very often, but always ended up solving it in a different way each time. Here is what looks to me to be the most concise way of doing it.

Indexing the diagonal

>> A = magic(4)
A =
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1
 
>> A(1:length(A)+1:numel(A))
ans =
    16    11     6     1
 
>> A(1:length(A)+1:numel(A)) = 1
A =
     1     2     3    13
     5     1    10     8
     9     7     1    12
     4    14    15     1

Selecting off-diagonal entries

>> A = magic(4)
A =
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1
 
>> A(setdiff(1:numel(A), 1:length(A)+1:numel(A))) = 0
A =
    16     0     0     0
     0    11     0     0
     0     0     6     0
     0     0     0     1
>>

http://sourceforge.net/projects/pidgin-latex/

Pidgin-latex lets you write Latex code normally in your chat conversation and renders the result in real time.

For both parties to be able to see the result, they will have to install this plugin or any other one that is handling the mime type latex.

To switch to math mode, it suffices to enclose the content between “$$” symbols.

The install process is straightforward. No trick needed. Just follow the one line instruction from the README.

A few weeks ago, I stumbled upon Tilda which turned out to be at least equally, and so far, much more usable and interesting to use than Guake. My point behind this is that I loved the fact that the GUI is kept just as clean as I like it: a black border-less window.

Currently on my system, Tilda is launched at startup (System->Preferences->Startup Applications) and is invoked with SUPER-A on the keyboard.

It’s good to have an unobtrusive terminal at the tip of our fingers anytime you need it.

Here is a little screencast to give you  a sense of how it can interact on your desktop:

Let p(y) be the probability generating function for Y.

Expected Value of the Poisson Distribution

gs –dBATCH –dNOPAUSE –q –sDEVICE=pdfwrite –sOutputFile=outputMerged.pdf file1.pdf file2.pdf

jPdf Tweak, the “Swiss Army Knife for PDF files”, is also a great tool and run out of the box with Java but it might be faster just using the above command rather than browsing through jpdftweak’s GUI.

1. Download, extract and create a directory under /usr/share/texmf-texlive/tex/latex/ if necessary. Copy the entire directory, or the single .sty file to that location. (You need to be root)
2. Run “sudo mktexlsr”

From the man pages:

mktexlsr is used to generate the ls-R databases used by the kpathsea library.  Kpathsea: A library for path searching.

Je ne sais trop comment je suis abouti à la bonne configuration. Dans tout les cas, il faut se procurer les codecs appropriés dans les bons canaux d’abord (restricted), et ensuite installer Mplayer et Mplayer-plugin.

Ensuite, on peut lancer Mplayer directement en ligne de commande, ou en créant un “lanceur” sur l’espace de bureau de Gnome. C’est ce que j’ai fini par faire puisque j’oubliais toujours l’adresse. Alors, la voilà, justement:

mplayer -playlist http://www.radio-canada.ca/util/endirect/rdidirect.asx

Un logo svg de Radio-Canada peut être trouvé ici http://upload.wikimedia.org/wikipedia/fr/thumb/9/92/Logo_Television_de_Radio-Canada.svg/617px-Logo_Television_de_Radio-Canada.svg.png

pour servir d’icone au lanceur.

RDI en direct avec Mplayer

Network Overview

Open Network

/etc/config/wireless

config wifi-device  wl0
        option type     broadcom
        option channel  5
        option disabled '0'

config wifi-iface
        option device   wl0
        option network  lan
        option mode   "sta"
        option ssid openNetwork
        option network "lan"

Wep Encryption
/etc/config/wireless

config wifi-device  wl0
        option type     broadcom
        option channel  5
        option disabled '0'

config wifi-iface
        option device   wl0
        option network  lan
        option mode   "sta"
        option ssid wepNetwork
        option network "lan"
        option encryption wep
        option key1 'mywepkeythatyouwontguess'

My intent was at first to setup a WPA2 bridge, but I finally gave up because neither nas or wpa_supplicant (xsupplicant as well) did the job. nas, the proprietary tool for setting up the broadcom chipset, is also a nightmare to debug with its cryptic error messages (or should I say its absence of any error message ?). The configuration file that is shown above is pretty much self-explanatory. Using Wep 128 is a pretty good compromise on security that I would have better avoided, but I think it worth it considering that MAC filtering can be done and that I always have access to a VPN server.

An important thing that you should also not forget finally is to disable the DHCP server on the OpenWrt router because it might conflict with the other router since we are working in a bridged environment.