time Archives - Versatile Programmer's Diary

VirtualBox is a great virualization solution. I use it to host my website and test it. It helps it me to set up the very same environment as I use in the server. So I dont have to worry whether recent change in the web application will break it. If you are a web developer and not using VirtualBox you should start right now.

Today I faced a weird problem. I could not log in to the web application in the vbox. But I could log in the live server. There was no difference between these two. One is physical server and other was virtual. After observing the http headers carefully I found that php session ids sent from virtual box was not preserved. But for live sever they get preserved. Session id is usually saved in cookie. Its the http clients responsibility to save the cookie and send it back along with successive request. I tested it in curl. it was not saving cooking. Google chrome was also not saving cookie. Only Firefox was saving.

At first I though its a problem of Google Chrome. I was almost submiting a bug to Chrome team. But then I tested in curl and it was not working. Two clients can not have same bug. So this should a problem of my host. I compared all the headers sent by both live server and virtual box server side by side. And guess what I found? The expires time for a cookie sent by virutal server was in past time. So this cookie was expired when generated. It means my virtual box servers time was not in sync. I have to synchronize it with time server. The following command is enough for this.

ntpdate pool.ntp.org

After this everything was working smooth. I always sync the time when i start the vbox server. If you boot your server time will be automatically synchronized. But if you save the state and later resume it you have to synchronise it manually. I never missed synchronizing. Today I forgot it. So I never think about it. I checked last 30 revision from my svn repository to track down the problem.

My suggestion, Always synchronize the time of a vbox server if you resume it. Use the command above for this.

Now a new question arise. Why Firefox used a expired cookie? I’ll verify it later.

If you develop software like me, you may face a scenario where you need to iterate through calendar days. These are specially needed in event management application or any other application that has some sorts of daily tracker. The main point is you want to iterate through dates.

For example, If the current date is “February 28th 2001”, next date will be “March 1st 2001”. Obviously not “February 29th 2001”. If the current date is “February 28th 2004”, next date will be “February 29th 2004”, Not “March 1st 2004”. Yes, that’s quite tricky. So how do you achieve it in programming?

The solution is to use time function families found in standard C header file time.h. The main idea is to first create the time you want to show by mktime(). This is the initial time. To advance to the next day/hour/min/sec/month just convert the resultant value of mktime() to struct tm structure by localtime() or gmtime(). Then add 1 day/hour/min/sec/month to struct tm structure. Use this structure to create time again by mktime().

The above strategy works because of the following lines from manual.

     In addition to computing the calendar time, mktime() normal-
     izes  the  supplied tm structure. The original values of the
     tm_wday and tm_yday components of the structure are ignored,
     and the original values of the other components are not res-
     tricted to the ranges indicated in  the  definition  of  the
     structure

Here is a sample code that iterates through the first 200 dates from epoch.

[code language=”c”]

#include <time.h>;
#include <stdio.h>;

int main(){
struct tm *lt;
int i=200;
time_t t=1; // first second on epoch

while(i–){
lt = localtime(&t);
printf("%s\n", asctime(lt));
lt->tm_hour+=24; // adding 1 day = 24 hours
t = mktime(lt);
}
return 0;
}

[/code]

If you are working with other language you may wonder if this will apply to your language. The good news is most language has wrapper for C routines. May be the name is different. But most probably you have it. Just see the manual. Same technique will work on other language too.

Tag: time

Iterate through each valid calendar days